Cultivation of Giant Horsetails
(see also www.equisetum.org for general discussion of horsetail cultivation)


Contents
:

1.  Introduction

2.  Plants currently in cultivation

Cultivation practices
A.    Collection and propagation
B.    Growing substrate
C.    Moisture
D.    Containers
E.    Mineral nutrition
F.    Light
G.   Humidity
H.   Temperature
I.    Stem support

Other Considerations
1. Pests

2. Do giant horsetails grow too large for indoor cultivation?
3. Will giant horsetails become invasive weeds?


 Introduction

    There are important scientific, conservation, educational, and aesthetic reasons for introducing highly unusual and underappreciated plants, such as giant horsetails, into cultivation.  From a scientific standpoint, such unusual plants are likely have unusual physiological and morphological characteristics that can be experimentally studied and manipulated in cultivation to give us a better understanding of the Plant Kingdom.  From a conservation standpoint, plants brought into cultivation, especially those from unique and isolated wild populations, constitute a "hedge" against possible destruction of the wild parent population.  This helps to ensure that the genetic resources of the original population will not be completely lost to humankind should this population disappear.  Furthermore, cultivation of plants of different provenances can provide much information about the functional genetic diversity (i.e. diversity in form and physiology) within a species and can further guard against loss of wild populations due to human encroachment or environmental stochasticity.  With their extraordinarily primeval appearance, giant horsetails also have great potential to inspire children and adults to learn about the past and present diversity of plant life.   Aesthetically, the giant horsetails, and especially exceptionally robust and regularly- branched giant horsetails like those from northern Chile, have a strikingly beautiful symmetry.

    Unfortunately, many of the giant horsetail clones currently in cultivation lack provenance information and do not appear to be from the most impressive wild populations (Chad Husby, personal observation). Hopefully the information provided on this website will encourage botanical gardens, researchers, and horticulturists to to seek out exceptional populations of giant Equisetum for study and cultivation and to record accurate provenance information for these understudied plants. 

    I have been growing giant horsetails indoors under lights and in greenhouses for several years.  I currently have two positively identified clones of Equisetum giganteum (a clone from Jamaica, collected by Richard Moyroud, and several clones I recently collected in northern Chile), two clones of E. myriochaetum (one from the University of California Botanical Garden , obtained by them from a nursery in Veracruz State, Mexico and one from the Royal Botanic Garden Edinburgh).  I also have several clones that I have not yet positively identified.  Growing these plants has been a very rewarding experience both horticulturally and scientifically.  My experiences have allowed me to make several rather interesting observations about the biology of these remarkable plants.  Unfortunately, because my growing space has been very limited until recently (both in area and height) my plants have not been able to reach large size or normal upright form.  However, I currently have transplanted divisions of my plants to larger containers in the research greenhouse at Florida International Univeristy.  I would be very interested to hear from others who can share their experiences and observations with growing these plants.



Plants currently in cultivation

    Several botanical gardens and university teaching greenhouses in the temperate northern hemisphere also grow giant horsetails for their botanical uniqueness and primeval appearance.  I am less familiar with the extent to which giant horsetails are grown in  the tropics and the southern hemisphere.   Equisetum myriochaetum appears to be by far the most common species in cultivation in the temperate northern hemisphere.  Although many plants in cultivation are labeled E. giganteum , these often turn out to be misidentified E. myriochaetum ( Moyroud, 1991 ; Husby, personal observation).  For example, many of the plants labeled E. giganteum in U.S. botanical gardens appear to be divisions of a clone of E. myriochaetum that was collected in Ecuador in 1987 and brought to the University of California Botanic Garden .  Until recently, this clone was labeled as E. giganteum .  In early 2001, Dr. Alan R. Smith determined that the clone is actually E. myriochaetum . (Holly Forbes, 2002, personal communication).  For more discussion of misidentification of giant horsetails, please refer to the Notes and Observations page.  Until recently, the only positively identified E. giganteum clone being cultivated in the northern hemisphere was a clone collected in Jamaica by Richard Moyroud ( 1991 ).  I have recently brough Equisetum giganteum from northern Chile into cultivation for research.  The botanical gardens growing positively identified E. myriochaetum clones include the New York Botanical Garden and The University of California Botanic Garden 1 .  There is a clone being grown at the Royal Botanic Garden Edinburgh that was collected in Peru and is labeled as E. giganteum.  However, I received a division of this clone and I am fairly confident that it is actually E. x schaffneri (the plant has stomata in bands of 1-2 and irregular branch ridge tubercles).  The Missouri Botanical Garden once had a clone of E. x schaffneri in cultivation that was collected by Dr. Richard Hauke (presumably in Costa Rica).  However, this clone eventually died.  There was a clone on the campus of the University of Hawaii labelled E. x schaffneri , but this plant may no longer exist (Dr. Gerald D. Carr, personal communication).


    The plants currently in cultivation exhibit a considerable range in vigor and regularity of branching.  Richard Moyroud ( 1991 ) cultivated four clones of E. myriochaetum from botanical gardens (two of which were originally labeled E. giganteum )  in South Florida.  He found that only two clones grew well, one of which was not very tall or regularly branched. In contrast, a Mexican clone of E. myriochaetum that is being grown by the University of California Botanical Garden (and now myself) exhibits very regular branching in cultivation (see photo of a stem of this plant).  Mr. Moyroud has grown a clone of E. giganteum collected in Jamaica and found the clone to have relatively little vigor and irregular branching habit.  I am currently growing this Jamaican E. giganetum  clone outdoors and in a greenhouse and have found that, although it can grow relatively well under the right conidtions (though not as vigorously as some other giant horsetail clones), its branching habit is markedly irregular.  During my investigations of herbarium collections, I have seen both well-branched and poorly-branched specimens from Jamaica, so there are probably other Jamaican E. giganteum populations likely exhibit more regular branching habits.  The E. giganteum (presumably) populations along the road between Moyobamba and Tarapoto Peru exhibit highly regular branching along almost the whole length of their stems, as do the more robust populations in northern Chile .  Many of the most impressive E. myriochaetum herbarium specimens that I have seen have been from Chiapas State, Mexico and Guatemala.  The most impressive E. giganteum specimens and photos I have seen have been from northern Chile .  Unfortunately, the exceptional plants from most of these areas are not in cultivation, although I have recently brought plants from Chile into cultivation.  Hopefully, more giant horsetails with exceptionally large, vigorous, and regularly branched stems will eventually be introduced into cultivation so that we can learn more about the biology of these amazing plants and enjoy their strikingly beautiful and primeval symmetry.  


Cultivation practices & considerations

A.    Collection and propagation  (see also the discussion by Piggott, 2001 )

1.  Vegetative Propagation

    Although I have not personally collected giant horsetails from the wild for cultivation purposes, I have experience propagating cultivated plants and I have successfully collected living material of several temperate Equisetum species.  It is generally easy to propagate container-grown Equisetum species by simple division.  Obtaining sufficiently long rhizome pieces  from wild populations for propagation can be more difficult.

a.)  Rhizome divisions

    Fortunately, because Equiseta are rhizomatous plants, one can usually collect propagules from a wild population without harming it. When one carefully collects a few modest rhizome sections from even a relatively small clone, there generally remains much more rhizome for continued survival and growth.  Large clones will be virtually unaffected by removal of a few rhizome divisions.  It is best to refrain from collecting from a very small clone.  I have found rhizomes (preferably with some visible buds at some nodes) to be the most reliable propagative material to collect.  However, hard or rocky soil and the deep-seated rhizomes of many species can make it difficult to collect adequate lengths of rhizome.  If possible, it is generally easiest to collect rhizomes from soft, water-saturated soil using a spade and/or a long knife to sever rhizome pieces.  Rhizomes exposed by river erosion or growing out of the side of a bank are especially convenient for collection.  

    Longer rhizome sections with multiple nodes are preferable because they have more buds and more energy reserves for new growth than do shorter pieces.  In addition, rhizomes growing in non-saturated soil appear to be harder and more resistant to decay than those growing in saturated soil.  Therefore, rhizomes growing above the saturated soil layers are generally more promising propagules than rhizomes growing submerged in water or saturated soil.

    I generally cut off the tops of any stems attached to the rhizomes so that only part of the unbranched lower portions of the stems remain attached.  If too much stem is attached to the rhizome, this could have an excessive drying effect (via transipration from the stem) on the rhizomes before new roots develop.  Aboveground and belowground stem segments can also be used for propagation ( see below ).

    It is crucial that collected rhizomes and stems not be allowed to dry or overheat.  For collecting containers, I like to use either ziploc bags or rigid plasic containers containing moist paper towels.  The containers should have some openings large enough to allow air circulation (and prevent excessive heat buildup in the sun), but at the same time maintain high humidity around the propagules.  The collecting containers should be kept shaded as much as possible to prevent heat buildup.  As with most vegetative propagules, Equisetum rhizome cuttings should be kept relatively cool, if possible, to reduce respiration rates and thus conserve their internal energy reserves.

    Of course, it is best to plant propagules as soon as possible after collection.   I  plant rhizomes in relatively deep containers filled with a substrate that holds much moisture, has high capillary wicking capacity, and is well aerated (I find commercial soilless sphagum peat/perlite/vermiculte mixes, such as ProMix® or Sunshine Mix®, to work well).  I plant the rhizomes horizontally  ~1-2 cm deep in the substrate (with ~10-12 cm of substrate beneath the planted rhizome) to ensure ample moisture and aeration around the rhizomes.  (Please see advantages under "Moisture" below for discussion of why adequate aeration is important even for  these wetland plants.)  Crucially, the substrate must be be kept constantly moist (but preferably not saturated) around the rhizomes while they sprout new roots and stems.   Below I discuss three possible methods for maintaining moisture levels.

  b.)  Aboveground stem cuttings

    Aboveground stem cuttings (either from the main stems or from  branches) of giant horsetails can also be used to start new plants.  This is because all Equiseta have preformed root and shoot bud primordia ( photo of an emerging adventitious root below a branch bud of E. giganteum ) at each node ( Gifford and Foster, 1988 ).  Indeed, many temperate Equisetum species have been successfully propagated by stem and branch cuttings (Schaffner, 1931 ; Praeger, 1934 ).  Although I have successfully propagated E. giganteum, E. myriochaetum, and E. x schaffneri from aboveground stem cuttings, I have found aboveground stem propagules to be  less reliable than rhizome propagules, aboveground/belowground stem cuttings, or layering.   Equisetum myriochaetum and E. x schaffneri appear to be perhaps easier to propagate via this method than E. giganteum .  However, my experience so far is limited to a few clones. Cuttings from mature stems nearer the stem base appear more satisfactory because more dormant stem buds are present in this region, not having already developed into lateral branches.  Longer stem cuttings with multiple nodes are more likely to successfully establish.  Lateral branches can also be used, but I have found that these thinner cuttings may have a greater tendency to rot before becoming established.  Stem cuttings  retain a definite polarity after being detached from the parent plant in that new roots and stems extend more readily from proximal nodes than from distal nodes.

    Stem cuttings must be kept constantly moist and are extrememly sensitive to any degree of water shortage. However, cuttings are also very sensitive to lack of oxygen and stagnant water.   My experiments suggest that Aero-hydroponics can be a useful system for propagation of giant Equisetum stem cuttings because it provides maximally aerated water for the cuttings.  This method helps to retard stem rotting (even for small branch cuttings) and to stimulate rapid emergence of adventitous roots and new shoots at the cutting nodes.  More simply, one can root the stems by floating them horizontally in water, but it is important that the water be circulated and aerated to maintain adequate oxygen levels and prevent stagnation because this often leads to rotting (airstones or an aquarium pump can be used).  In addition, I find it beneficial to use opaque-sided containers and grow aquatic plants, (especially floating plants like Lemna , Salvinia , Pistia, and Azolla )  in the water to discourage algae growth.  This "floating stem" propagation technique was inspired by Wagner and Hammitt ( 1970 ), who observed natural proliferation of severed stems of Equisetum hyemale floating on a pond.  I suspect that partially immersing stems vertically in containers of water (preferably with aeration and circulation) would also work well because this technique was successfuly used by Schaffner (1938 ) to root E. hyemale stem cuttings.    Miguel Porto (2000, personal communication) has successfully propagated giant horsetail stem cuttings by keeping them between pieces of moist paper under fluorescent lights.  This method helps to maintain moisture and aeration but requires special vigilance because the paper must be regularly misted to prevent it from drying out.  Alternatively, one can plant stem cuttings horizontally in a moist and well-aerated solid substrate (like a soilless mix of peat and perlite).  After one has stimulated stem cuttings to form new roots and shoots by one of these methods, the cuttings should be planted in moist, well-aerated, substrate in a brightly lit location.

c.)  Aboveground/belowground stem cuttings

    My experiences with propagating Equisetum giganteum from northern Chile have led me to conclude that one of the most effective propagation methods is to collect aboveground/belowground stem cuttings.  These cuttings involve making a cutting composed of the unbranched lower portion of the abovebround stem along with as much of the dark colored (and of distinctly hardened texture) underground portion of the stem.  Please refer to this photo to see the distinction between these aboveground and belowground portions of an upright stem.  The tough belowground portion of the aerial stem seems quite resistant to decay and appears to contain many reserves for supporting new root and stem growth.  All my cuttings of E. giganteum of this type have succeeded whereas many of my cuttings comprising aboveground stems alone have failed.

d.)  Layering

    Giant horsetails can be successfully propagated via simple layering.  I have observed simple layering in E. giganteum in northern Chile and I have had success with this method in cultivation.  I suspect that modified air-layering (without any wounding of the stem) may also prove to be a viable stem propagation method.   Miguel Porto and I have noticed that the preformed adventitious roots of E. giganteum stems will begin to emerge even on intact aerial stems when the plants are grown in a high humidity environment.  Air-layering one or more nodes of an aerial stem would provide such a high relative humidity environment and likely stimulate root and shoot emergence.

2.  Sexual propagation (spores)

    Giant Equiseta can be successfully propagated via spores under laboratory conditions (using special nutrient agar, petri dishes, controlled temperature, etc.), as reported by Hauke ( 1963 , 1969b ).  However, the green spores of Equisetum species rapidly lose their ability to germinate (after only 5-17 days) and must be kept in a high humidity environment to prevent dessication ( Hauke, 1963 ).  Proper temperature and light are also crucial for germination ( Hauke, 1963 ).  In addition, the gametophytes are especially sensitive to fungal ( Piggott, 2001 ), algal ( Hauke, 1963 ), and insect larvae ( Hauke, 1969a ) contamination .  These exacting requirements limit the utility of using spores for propagation unless it is possible to sow the spores quickly under sterile conditions in a favorable environment.  Furthermore, ripe cones may not be present in a given population when a collector visits it.  I did not find any coning stems in the population of E. giganteum that I observed in Panama .  I have not yet attempted spore propagation, but I hope to do so eventually.



 
B.    Growing substrate (see also related discussion of containers below)

    I have had good success growing giant horsetails and other Equisetum species in a standard peat/perlite/vermiculite soilless mix (I currently use Promix® BX) formulated for indoor, nursery, and greenhouse plants.  This substrate provides excellent water holding capacity, ample ability to wick adequate moisture to all levels of  the 28 cm (~11 inch) deep containers I use. Furthermore, sphagnum peat has the advantage of providing a substantial buffering capacity for nutrients and moisture ( Whitcomb, 1988 ).  I have experimented with coconut coir with two Equisetum species (E. telmateia and E. variegatum) and found this substrate to give very poor results for even though it was of premium quality from the purported best world source (Sri Lanka).  Now, I am beginning experiments with growing plants in the inert expanded clay aggregates commonly used in hydroponics.  When growing Equiseta in soilless mixes, I have found that micronutrient supplementation is often very beneficial (especially for E. giganteum ) (see discussion of mineral nutrition below).

    Richard Moyroud, a botanist and nurseryman in Florida, successfully cultivates giant horsetails in containers and in the ground using a strongly acidic Florida clay.  He has had good long-term success growing an E. giganteum clone he collected in Jamaica and an E. myriochaetum clone from the Royal Botanic Garden Edinburgh in this substrate with no additional fertilization.

    Anthony Pigott, who maintains the United Kingdom's National Collection of Equisetum , uses a mix of peat substitue, loam, and clay in approximately 5:5:1 proportion.  His cultivation methods are described on his website .

    It is important to note, however, that soil and clay vary greatly in composition and quality from one place to another.  Furthermore, soil may carry undesirable insects or disease organisms.  Hence, one needs to be careful about the source and preparation of any soil to be used in a growing substrate.  Some types of soil may not have adequate micronutrients or silicon for giant horsetails or may otherwise have properties not conducive to Equisetum growth.



C.    Moisture

    Giant horsetails, like all Equiseta , are wetland plants that absolutely require a constant supply of moisture.  Letting the potting substrate begin to dry will result in dieback of the aerial stems and eventually complete death of the rhizomes.  On one occassion I witnessed the rapid and permanent wilting of a once impressive pot of E. myriochaetum that was allowed to begin drying.  Hauke ( 1966 ) observed that even Equisetum arvense ,  a very vigorous species that appears to be able to grow in somewhat less moist conditions than most others,  required daily watering when grown in pots and would "perish from irreversible wilting"  if not watered during the weekend as well as on week days.   This was despite the fact that E. arvense was the most water-use-efficient of the three Equisetum species ( E. arvense , E. hyemale , and )  studied by Dosdall ( 1919 ).  Maintaining high moisture supply is even more crucial for starting plants from rhizome pieces and stem cuttings.  There are several possible methods for maintaining continually moist substrate conditions and I discuss these below.

Indoors:

1.  The containers can be set in either individual trays or a common basin with ~1-4 cm of water in the bottom so that the potting medium is kept constantly moist by wicking action of the substrate.  This is the method that Anthony Pigott and Richard Moyroud use and requires the least maintenance of the methods I discuss.  This method is suitable so long as the basin is emptied periodically and the pots are occasionally  leached by watering thoroughly from the top.  This leaching is important for preventing excess buildup of fertilizer salts and irrigation water minerals in the substrate from evaportion.  A buildup of salts and other dissolved minerals can be very detrimental to horsetails because this reduces the availability of water in the substrate and may cause toxicities or nutrient uptake imbalances.  Hence, this method is particularly suited to outdoor or greenhouse cultivation where  it is convenient to use a hose to flush out old water in the basins onto the ground.  Introducing floating plants, such as duckweed (Lemna ), Azolla, or Salvinia to the basins of water, can prevent excess algae from growing and also absorb some excess fertilizer salts.  Containers for use with this method must have drainage holes on the bottom that allow water to enter the pots regardless of the basin water level.  Rhizomes will often "escape" through drainage holes when this method is used.  These rhizomes can be used for propagation when they have grown sufficiently long.

2.  I currently use a modification of the semi-hydroponic TM watering method pioneered by Ray Bark of First Ray's Orchids .  This is essentially a compromise between methods 1 and 3 and is the method that I have found most convenient for non-greenhouse indoor cultivation.  I use translucent plastic containers with 2-3 holes (~5 mm diameter) drilled ~4-6 cm high on the side of each container.  I then fill the bottom part of the container (to just above the side drainage holes) with inert clay aggregates (medium grade, ~8-16 mm diameter) commonly used in hydroponics.  On top of the aggregates I place an ~3 cm thick layer of long-grain sphagnum moss or coconut husk chips.  Finally, I fill the rest of the container (~15-20 cm above the sphagnum or coconut husk layer; see discussion of containers ) with the peat/perlite/vermiculite mix that I typically use.  When I water these containers, the part of the container below the drainage holes serves as a water reservoir and the clay aggregates wick water from the reservoir to the upper layers.  I can always check the water level in the containers because they are translucent.  The sphagnum or coconut chip layer prevents the fine peat-based substrate from sifting down into the reservoir.  I set these containers on inverted pots inside large plastic trays to catch overflow from the pot bases during watering.  With this method, it is crucial that water be available in the reservoir at all times, so one needs to replenish the reservoir when the water level becomes low.

Advanges of the modified Semi-hydroponic TM method:

i.)  The main advantage of this method is that the pots can be easily and frequently leached.  To leach the containers, one simply waters so that substantial water overflows from the container reservoir and spills into the catchbasin below.  This prevents buildup of fertilizer salts and tapwater minerals in the substrate and, unlike method #1, does not require changing of the basin water.  I grow floating plants in the catchbasins to prevent algae growth.

ii.)  As with method #1, there is no guesswork involved in deciding when to water.  For both methods, water must be present in the reservoir at all times.

iii.)  Because the clay aggregate layer is the only layer that gets saturated with water, this method avoids anaerobic decay of the substrate by keeping the organic substrate above the water reservoir.  I have found that using organic substrates with method #1 results in a sour smell in the bases of the pots that can be detected during re-potting or by smelling the base of the pots when they are lifted out of the reservoir.  This smell results from the formation of organic acids (such as acetic acid) and is evidence of anaerobic decay of the organic substrate ( Armstrong and Armstrong, 2001 ).  Using the clay aggregate method, the sour smell has completely disappeared from my pots, suggesting that aerobic conditions prevail throughout the substrate.
**Note: the clay pellet layer can also be used with method #1 to prevent souring of the lower substrate.
    Researchers have shown that that organic acids produced by decaying organic matter in saturated soil can reduce growth, or even cause die-back, of wetland plants such as the common reed ( Phragmites australis Cav.) and rice (Oryza sativa L.) ( Armstrong and Armstrong, 2001 ; 1999 ; Armstrong et al., 1996 ).  Furthermore, even wetland plants (that are adapted to flooded soil conditions), grow more slowly under highly reducing conditions caused by decaying organic matter in saturated soil than they do under oxidizing conditions.  This phenomenon has been shown in wetland plants such as rice ( Kludze and DeLaune, 1995 ) cattail (Typha domingensis L.) ( Kludze and DeLaune, 1996 ),   Sawgrass (Cladium jamaicense L.) ( Kludze and DeLaune, 1996 ), and baldcypress (Taxodium distichum L.) (Pezeshki DeLaune, 1998 ).  

    The clay aggregate reservoir method, allows the grower to avoid anaerobic decomposition of the substrate and has the added benefit of preventing buildup of rhizomes in the saturated layer (discussed in iv . below).  Since rhizomes and roots eventually die and decay, it seems desirable to prevent large amounts of such dead decaying organic matter to accumulate in a water saturated zone around healthy rhizomes and roots.

    Although giant horsetails can clearly tolerate saturated anaerobic conditions in the substrate (probably via oxygen diffusion through the large air canals in stems and rhizomes), I think that they are likely to do best when provided  with both constant moisture and aerobic conditions throughout most or all of the substrate.  Indeed, my horsetails are growing more vigorously than ever using the Semi-hydroponic TM method.  Page ( 2002 ) has observed that Equisetum species in Britain and Ireland vary with regard to their ability to tolerate lack of substrate water movement (and hence presumably greater anaerobiosis in the substrate).  Therefore, it seems prudent to "err on the safe side" by providing considerable aeration along withample  moisture availability.  The species requiring the most substrate water movment (hence, presumably, most aerobic conditions) in the British Isles is Equisetum telmateia (Page, 2002 ).  I am having very good success growing this species using the modified semi-hydroponic TM method, so this method appears to provide sufficient aeration for one of the species least tolerant of anaerobiosis.  This semi-hydroponic TM method is also proving very satisfactory for growth of my E. giganteum, E. myriochaetum, and E. x schaffneri clones as well as other Equisetum species ( E. diffusum , E. variegatum , and E. laevigatum ).  At this time it is not clear how the anaerobiosis tolerances of the giant horsetails compare to those of other species in the genus.  This would make a very interesting research question (see more detailed discussion on the Ecology and Physiology page).

    When a grower uses a substrate with little or no organic matter (such as the acid clay used by Richard Moyroud), the problem of anaerobic substrate decay would not be such an important issue.   Anaerobic decay  would also likely be less of an issue for mixes that include large proportions of mineral soil and/or clay in addition to organic matter (such as the mix favored by Anthony Pigott )

iv.)  A fourth advantage is that the layer of clay aggregates at the bottom can discourage rhizomes from accumulating and circling around the bottom outside edge of the pot.  Once the rhizomes reach the hard clay aggregates, they tend to be deflected in a different direction or sometimes simply become wedged between the aggregates and cease downward growth (this probably stimulates branching of the rhizome above the wedged end).  Premilinary observations suggest that this encourages the rhizomes to grow more evenly throughout the growing media and encourages a more even filling of the container with aerial stems than would otherwise occur ( Anthony Pigott discusses the problem of rhizomes and shoots eventually accumulating around the outside edges of pots).  

v.)  Another advantage of this method is that the translucent pots allow observation of rhizome and root growth, which is quite interesting.  (of course, one could also use translucent pots with methods #1 and #3.)
       
Disadvanges of the modified Semi-hydroponic TM method
 i.)  The main disadvantage that I have experienced so far is that more frequent watering is required for this method than for method #1, especialy when plants begin to fill their pots.

ii.)  If the sides of the translucent containers receive enough light exposure then algae will grow on the outer layer of the potting media.  This appears to be mainly an aesthetic concern.  Because I grow my plants under overhead fluorescent lights, this has not been much of an issue for me.  However, it is more prevalent in a greenhouse with abundant light. Using translucent containers that are tinted blue might solve the problem by minimizing transmission of the photosynthetically active wavelengths (see. Soffer, 1986 ).
3.  The containers can be made without any water reservoir so long as they are watered frequently enough that the substrate remains continually moist.  This can be accomplished either by regular hand watering or by an automated system (such as a drip system or sprinker system).  Of course, this method leaves little margin for error.

Outdoors:

    If one lives in a warm enough area to attempt growing giant horsetails outdoors then the plants may be grown in the ground or in containers.  If the horsetails are to be grown in the soil, then they should be planted in a low wet area, preferably near a stream, marsh, or pond.  If no natural wet area exists on the property, then a bog can be constructed for the giant horsetails using a special liner to hold water in the upper soil layers.  If the giant horsetails are to be grown in containers, then the same methods as those used for indoor container cultivation can be used.


D.    Containers

i.)  Type
    A variety of cotainers can be successfully used for growing giant horsetails.  I favor plastic translucent containers because they are compatible with the semi-hydroponic TM method described above and because the translucence allows one to observe rhizome and root growth.  I prefer to buy containers without holes so that I can drill holes to accomodate the semi-hydroponic TM method.  The semi-hydroponic TM pots offered by First Rays Orchids work well, as do various plastic containers that can be purchased at department stores.

ii.)  Overall size
    In the past, I have successfully grown giant horsetails in pots as small as 6 x 6 inches (15.24 x 15.24 cm).  Of course, in smaller pots, the stems will not get large and successful division will sometimes not be as easy due to the smaller amount of rhizome material available.  Easier and faster propagation of plants grown in larger containers is due to the increased length of rhizomes and hence greater number of buds and greater stored energy present in the resulting larger divisions.  Currently, I am also using rectangular plastic storage containers of various sizes, the smallest of which are ~27 cm (~11 in.) tall, ~33 cm long (~13 in.) long, and ~18 cm (~7 in.) wide.  These containers have proven to be sufficiently large to allow good rhizome growth and very successful division.

iii.)  Depth
    Container depth is a crucial consideration and, along with substrate type, is a principal factor affecting drainage. Contrary to popular belief, adding stones or or other coarse objects to the bottom of a container does not improve drainage, but using a taller container does ( Whitcomb, 1988 ).  This is because a container is watered there is always a perched water table at the bottom of the substrate column.   Hence, adding coarse components at the base of a column of a given type of substrate merely shortens the substrate column and hence raises the perched water table.  Thus, adding coarse drainage aggregates actually decreases the volume of well-aerated substrate, whereas using a taller container increases the volume of well-drained substrate above the perched water table.  I have not yet performed experiments to investigate the effects of container depth on growth of giant horsetails.

    My preferred method for growing horsetails involves providing a substrate environment that is constantly moist and also well-aerated.  Because giant horsetails benefit from having a substantial volume of moist substrate available for rhizome growth, I prefer to use relatively deep (27 cm = approx. 11 in.) containers along with a substrate that has high wicking capability (i.e. high capillarity) and so can wick sufficient moisture throughout the substrate column (drawing moisture from the water reservoir in the base of the pot).  For discussion of the substrate I use, see above .   Using deeper containers creates a longer gradient of substrate moisture and aeration conditions.   This allows Equisetum rhizomes and roots to seek out the most favorable growing conditions.

    For more detailed discussion of container depth and substrate type (along with helpful figures), I highly recommend the following horticultural technical bulletins:

http://www.agf.gov.bc.ca/croplive/plant/horticult/floricul/aeration.pdf
http://envhort.ucdavis.edu/Ehweb/Newsltr/1998/GP98winter.pdf
http://www.ces.ncsu.edu/depts/hort/floriculture/plugs/ghsubfert.pdf


E.    Mineral nutrition

    It is important to note that the giant horsetails (and other Equisetum species, for that matter) do not seem responsive to high rates of macronutrient ( N-P-K) fertilization.  In fact, overfertilization is a real danger to Equiseta.  Richard Moyroud has maintained E. myriochaetum and E. giganteum in containers with the same acid clay substrate for about 10 years without any supplemental fertilization.  However, plants that he fertilized heavily were almost killed by the high fertilizer levels.  Similarly, Anthony Pigott has found that the Equiseta he grows in soil-based substrates do not appear to require additional fertilization.  High fertilizer levels may harm giant Equiseta by lowering the osmotic potential of the substrate water or through nutrient imbalances and toxicities.

    On the other hand, I have found that some fertilization (especially with micronutrients) is necessary if one is using a soilless mix.  When I fertilize my Equiseta growing in soilless mixes, I use only a low-strength solution (described below) and I do not fertilize with every watering.  I usually irrigate with a fertilizer solution at 2-4 week intervals and irrigate with plain water in between fertilizations.  I often irrigate the plants to the point that excess water starts to drain out of the side holes in my containers (i.e. I filll the built-in reservoirs to overflowing).  This ensures that excess fertilizer salts (and minerals that might accumulate from evaporation of irrigation water) are periodically flushed from the substrate.   I have also found that slow-release fertilizer sticks that include micronutrients can be effective for use with fast-growing Equiseta in a greenhouse.  I have found that it is wise to avoid using high rates of macronutrient fertilization and to periodically leach the substrate with plain water to avoid overfertilization.   Making sure that adequate levels of micronutrients are available (preferably via a slow release fertilizer or by adding some clay or mineral soil to the substrate) often appears to be a key to successful cultivation of giant horsetails in soilless substrates.

I use three different fertilizers to grow Equisetum species in soilless substrates:

1.  Controlled-release micronutrient fertilization ( key to success with E. giganteum in soilless mixes ):
    For growing giant Equiseta (and several other Equisetum species) in soilless substrate, I have found the addition of a controlled-release micronutrient fertilizer is very important (especially for E. giganteum ).  Whitcomb ( 1988 ) discussed studies showing the benefits of slow-release micronutrient fertilization for growth of many containerized woody plants in soilless mixes and like benefits accrue to horsetails.  I use Micromax ® micronutrient fertilizer (at a low rate:  1/8 teaspoon per volume of substrate that will loosely fill a 5" pot) with excellent success, but other similar controlled-release micronutrient fertilizers should also work well.    Kelp-based fertilizers may also prove to be satisfactory for providing micronutrients (and extra potassium), although I have not yet tried these.  For giant Equisetum growers who use soil in their substrate mixes, micronutrient fertilization may well be unnecessary because adequate micronutrients are likely present in the soil component.  I do not know which micronutrient(s) are most important for giant Equisetum nutrition.  However, Cannon et al. ( 1968 ) found that many Equisetum species are accumulators of zinc, so perhaps they have a high requirement for this nutrient.

2.  Macronutrient fertilization :
    I also fertilize my giant horsetails with a complete liquid hydroponic fertilizer (Dyna-Gro TM 7-9-5) and, in the greenhouse, with Miracle-Gro Plant Food Spikes (6-12-6 with micronutrients).  I usually use the liquid fertilizer at the lowest recommended concentration (1/4 teaspoon/gallon) because in my experience horsetails appear to have rather low absolute N-P-K requirements.  The reason I use a hydroponic fertilizer is because such fertilizers tend to contain a balance of all essential mineral elements, including micronutrients, Calcium, Magnesium, and Sulfur.  A study of the N and K nutritition of Equisetum arvense showed that this species has high K requirements relative to N ( Andersson, 1999b ).  If this finding also holds for giant horsetails, then ensuring adequate K fertilization (relative to N) will be important for maintaining good growth and vigor.

3.  Silicon fertilization :
    Horsetails are the only vascular plants known to require silicon (Si) as an essential mineral element ( Epstein, 1999 ).  The requirement for silicon has been shown for Equisetum arvense ( Chen and Lewin, 1969 ) and for E. hyemale ( Hoffman and Hillson, 1979 ), so this requirement appears to hold for members of both subgenera within Equisetum .  An important function of silica in Equisetum is in maintenance of shoot erectness (Kaufman et al., 1971).  Furthermore, silica content of stems appears to be directly associated with stem longevity (Srinivasan et al., 1979).  Horsetails incorporate much silicon into their stem tissues and external ridges, knobs, and rosettes of silicon give the stems of many species their rough and abrasive character (including the rough-stemmed E. giganteum , but to a lesser extent the smooth-stemmed E. myriochaetum ) ( Gifford and Foster, 1989 ; Hauke, 1963 ).  People have taken advantage of this abrasize quality by using Equisetum stems for washing dishes (hence the common names 'scouring rush' and 'limpiaplata'), polish woodwind reeds, and polish silver (hence the name 'yerba del platero').  The outer layer of silica on Equisetum stems may help explain why horsetails appear to be little bothered by insect feeding or fungal diseases ( Hauke, 1969a ; Kaufman et al., 1971 ).  Gardeners have long used silica rich Equisetum extracts to help protect plants against pathogens and predators ( Quarles, 1995 ).  For discussion of Silicon's role in plant health, see my 1998 paper " The Role of Silicon in Plant Susceptibility to Disease ".

    Since Si levels are low in organic soilless media and many ornamental plants and Equsetum arvense have been shown to benefit from Si fertilization ( Chen et al.,  2000 ( full text )), I add a potassium silicate liquid fertilizer ( Pro-Tekt ®, manufactured by Dyna-Gro TM ) to the my fertilizer solution (at the lowest recommended rate of 1/4 teaspoon/gallon).  In addition to providing silicon, this fertilizer also adds more K to fertilizer the solution and, as discussed above, a higher proportion of K is probably beneficial for Equisetum ( Andersson, 1999b ).  If soil or clay are included in the growing substrate, then additional Si fertilization may be unnecessary.


F.    Light

    Giant horsetails require plenty of light and are shade intolerant ( Hauke, 1969a ; Moyroud, 1991 ).  I have had good success growing them indoors under compact fluorescent lights, which are quite bright.  I am also growing giant horsetails with excellent success in full sun outdoors (in Florida) and in a bright greenhouse.


G.    Humidity

    In my experience, giant horsetails appear to grow well in a wide range of humidity levels.  Botanic gardens often grow giant horsetails in humid conservatories.  However, I am growing these plants both in a humid greenhouse and in a small air-conditioned apartment with relatively low ambient relative humidity (RH) during the day (40-50%) and and higher relative humidity at night (~60-75%).  My giant horsetails do quite well in both environments.  Miguel Porto reports that giant Equiseta grow well outdoors in Portugal where the RH levels are low.  However, giant horsetails can only tolerate low RH levels when provided with constant substrate moisture.  It is also quite possible that horsetail growth may benefit from increased RH.


H.    Temperature

    Little information is available on the temperature tolerances of giant horsetails because most plants are grown in conservatories. Miguel Porto has reported that his giant Equiseta from Costa Rica grew well throughout the winter when temperatures dropped as low as 7ºC at night.   Equisetum. myriochaetum is growing successfully outdoors in San Francisco, California and West Palm Beach Florida, where low temperatures can reach -1ºC.  It seems likely that plants from higher elevation populations or populations of E. giganteum near the southern part of this species' range  (central Chile and Argentina) would be more cold tolerant.  Richard Moyroud successfully grows E. myriochaetum (one clone) and E. giganteum (one clone) outdoors in south Florida (with hot, humid summers and warm nights), so some clones are heat tolerant.  However, several clones of E. myriochaetum grew relatively poorly outdoors in South Florida and hence may have been less heat tolerant (possibly they came from higher elevation populations) ( Moyroud, 1991 ).  Warm summer night temperatures in lowland tropical areas such as South Florida may be problematic for high elevation giant horsetail clones as is the case for some tropical montane bamboos.  However, much experimentation remains to be done with regard to temperature tolerances.


I.    Stem support

    Individual giant horsetail stems up to 2-3 m high and ~ 1 cm in diameter can generally support themselves ( Spatz et al., 1998 ; Moyroud, 1991 ). However, for such slender stem diameters, taller stems require support either from neighboring stems or other vegetation.  The emarkably robust stems of the northern Chile populations of E. giganteum can remain self-supporting to greater heights (Chad Husby, personal observation).

    To encourage giant horsetail stems to remain upright in the garden, it would be helpful to plant them in a location that is sheltered from the wind.  In addition, some sort of support (such as a fence around the plant) can help prevent stems from falling over due to wind or due to having grown too tall.


Other considerations



Pests

    Fortunately, giant horsetails appear to have few problems with herbivorous insects and fungal diseases in nature ( Hauke, 1969a ).  However, on a few occassions I have observed horsetails in greenhouses being attacked by mealybugs, aphids, and scales.  Anthony Pigott has also had insect pests on his horsetails.  Plants in a greenhouse setting may tend to be more susceptible to pests because the stems of giant horsetails appear somewhat less hard when grown in a high relative humidity (RH) environment than when grown in a lower RH environment.  Both Miguel Porto and myself have observed this reduction in stem hardness or "toughness" when grown under higher RH.  Furthermore, Dayanandan et al. (1973) noted that outdoor-grown plants of Equisetum hyemale var. affine tend to have "heavier wax and silica depositions" on the epidermis than do greenhouse grown plants (However, the outdoor plants the authors compared to the greenhouse-grown plants may not have been of the same clone).  This difference may be due to the generally higher humidity levels in greenhouses than outdoors.


Do giant horsetails grow too large for indoor cultivation?


    Although giant horsetails have the potential to grow quite tall under favorable conditions, they can be kept smaller by limiting the size of the growing container.  I have found that plants in a small container will continue to produce relatively small stems.  Periodic division and replanting will also help keep stems small.  If larger stems are desired, then larger containers should be used.  

    Growing these plants under artificial light presents a challenge because most artificial light sources need to be within ~0.5 m of the top of the planting container to be effective.  I get around this problem by simply guiding longer stems around my compact fluorescent light fixtures when the stems begin to grow into the fixtures (otherwise, coming into contact with the bulbs would kill the stem apex).  Once the stems grow upwards beyon the light source, they eventually start to curve to the side and then downward (see discussion of phototropism ).  At least in E. giganteum and E. x schaffneri, the tall stems eventually start to become more flexible and lay on their sides so that the whole stem is once again exposed to light.  Hence, one can grow these plants indoors under lights, but of course this prevents the plants from developing an attractive upright form.


Will giant horsetails become invasive weeds?

    Giant horsetails are early-successional plants and are poor competitors that are sensitive to being shaded by other vegetation ( Hauke, 1969a ; Øllgaard, 2000, personal communication).  In addition, the slender architecture of giant horsetail stems causes them to cast little shade on other plants.  The one giant horsetail clone (of E. myriochaetum ) that has been successfully established outdoors in cultivation in south Florida has spread little from its place of planting more than 10 years ago.  Furthermore, the stems of this clone are quite sparse and clearly do not threaten any native vegetation.  Because reproduction by spores requires very exacting conditions (see discussion of Ecology and Physiology ) and because the spores are short lived ( Hauke, 1963 ), dispersal by this means beyond a cultivation site is unlikely.  Moyroud ( 1991 ) makes the point that E. giganteum spore from the Greater Antilles has probably "rained into South Florida...for millennia" without establishing any lasting presence of this species in Florida.

    I am not aware of any case of a horsetail species being an ecological problem anywhere in the world.  Horsetails in general are poor competitors and do not threaten other vegetation.  However, two species, E. arvense and E. palustre are sometimes agricultural weeds in temperate areas ( Holm et al., 1977 ).  These species, both members of the subgenus Equisetum, can cause health problems in livestock when the animals consume the horsetails stems in forage for extended periods of time ( Holm et al., 1977 ).  The toxin in E. arvense is a thiaminase enzyme that eventually causes thiamine deficiency whereas the toxin in E. palustre is an alkaloid, palustrine ( Holm et al., 1977 ).  Equisetum arvense is sometimes a weed in certain temperate crops, but only reduces yield when present at high densities ( Parsons, 1992 ).  Equisetum palustre is far less frequently reported crops than is E. arvense ( Holm et al., 1977 ).  Equisetum arvense is the only Equisetum species that has become an agricultural weed in areas in which it is not native (such as Argentina, Madagascar, New Zealand, and Australia) ( Parsons and Cuthbertson, 1992 ;   Holm et al., 1977 ).    It is not surprising that E. arvense is the only Equisetum species to become an exotic weed because it is also the most "weedy" member of the genus in its native area.  It is "weedy" in the sense that it is the "most common and widespread species of the genus" (Hauke, 1966 ).  Anthony Pigott maintains the most comprehensive Equisetum collection (including every species and many varieties and hybrids) in the world ( U.K. National Collection ). He has found that "it is only E. arvense which grows vigorously enough to become a problem to the gardener" (see "cultivation" at www.equisetum.org ).  However, he advises caution regarding some of the more vigorous hybrids (such as E. x font queri and E. x bowmanii)  and suggests that these are probably best confined to containers.

    Populations of the giant Equiseta are mostly uncommon and scattered throughout most of their ranges (cf. Hauke, 1969a ), with the notable exceptions (of which I am aware) of the Ecuadorian Andes (Benjamin Øllgaard, personal communication) and the Central Plateau and Northern Highlands of Chiapas State, Mexico ( Smith, 1981 ), and several river valleys in northern Chile (Chad Husby, personal observation).  I have heard no reports of any cultivated giant horsetail escaping cultivation or becoming invasive.  On the contrary, I have heard from several greenhouse curators who have had considerable difficulty growing certain giant horsetail clones.

    Unlike the two weedy members of the subgenus Equisetum , the giant horsetails do not appear to be harmful to livestock.  A recent study revealed no acute toxicity of E. giganteum in mice ( Gorzalczany et al. 1999 ).  Furthermore, ( Hauke, 1969a ) reported that many of the populations he visited were associated with pastures and that "cattle graze upon it with apparent relish".  Indeed, Hauke ( 1969a ) mentions that one campesino "claimed that Cola de Caballo was good for the cattle, preventing some trouble with their blood".  Because cattle ranching is extensive in Central America (Chad Husby, personal observation), it seems unlikely that a toxicity due to giant horsetails would be overlooked by ranchers.  Hence, it seems likely that they pose no threat to livestock or agriculture.

    No species in the subgenus Hippochaete (to which the giant horsetails belong) is an ecological or environmental problem anywhere.  I know of only one documented case of long-distance anthropogenic dispersal of a member of the subgenus Hippochaete .  The European horsetail E. ramossismum was introduced (via rhizome pieces in ship ballast) to sites  in two port cities 2 in the United States more than a century ago (Hauke, 1979 ).  However, the populations have not spread beyond their sites of introduction and one of the populations appears to be declining due to encroachment by woody vegetation ( Hauke, 1992 ).

    Horsetails in general, and the giant horsetails in particular, do not have many associations with herbivorous insects or pathogenic fung ( Hauke, 1969a ).  This suggests that one would not expect giant Equiseta to become unexpectedly vigorous and invasive in a new environment as a result of being "released" from herbivores and diseases that suppressed the plants in their native environments.  Some biologists studying invasive species believe that the "release effect" is often a significant factor in plant invasions. Experience so far has borne out the hypothesis that a "release effect" is not to be expected for horsetails cultivated in new locations.  As discussed above, even after more than 100 years E. ramosissimum has failed to spread from its sites of introduction in the United States and this temperate European species is better adapted to the climate of the United States mainland than are the giant horsetails.  Furthermore, after 10 years outdoors in South Florida E. myriochaetum has shown no propensity to spread from its sites of cultivation or to compete with native vegetation.  Indeed, with the exception of E. arvense , the few Equiseta that have been introduced into new places appear to grow less well than in their native environments.

    The giant horsetails, being memebers of the subgenus Hippochaete , do not appear to be potential invasive ecological or agricultural weeds when cultivated as ornamental plants.  Experience so far suggests that the real challenge will be to find ways to encourage them to grow large and dense in garden settings while preventing other vegetation from choking them out.


1.  Although the  University of  California Botanical Garden lists E. giganteum in  its  collections database,  in 2001 Dr. Alan R. Smith determined that this plant is actually E. myiochaetum  (Holly Forbes, personal communication, 2002).

2.  The cities are:  Pensacola, Florida and Wilmington, Delaware.

If you have any comments or questions, please contact the author, Chad Husby ( chad.husby@fiu.edu or husby.1@osu.edu )

© Chad E. Husby 2003

Last updated March 19, 2003

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