The giant horsetails are tall, evergreen, distinctly primeval looking plants that range from central Mexico to central Argentina and Chile. These marvelous plants are the largest members of the small (only 15 species worldwide) and very unique genus Equisetum. The giant Equiseta are non-flowering plants and, like ferns, reproduce via spores rather than seeds 1 . There are two giant horsetail species, Equisetum giganteum and E. myriochaetum , and a hybrid between them, E. x schaffneri (Hauke, 1963 ). The only other Equisetum species that ranges into the southern hemisphere is the diminutive E. bogotense (see an illustration and photograph of this species, which is found in both Central and South America).
As for all Equiseta,the giant horsetails spread vegetatively via extensive rhizome systems, often forming large clones. The rhizomes give rise to erect, determinate, aerial stems that produce regular whorls of side branches, giving the stems a remarkably precise radial symmetry . Colonies of such stems have a remarkably ancient appearance, as the 19th century botanist Richard Spruce remarked upon seeing a grove of giant horsetails for the first time:
"I could almost fancy myself in some primeval forest of Calamites2 and if some gigantic Saurian had suddenly appeared, crushing its way among the succulent stems, my surprise could hardly have been increased."
- from Notes of a Botanist on the Amazon and Andes , 1908
Giant horsetails inhabit elevations between 150 and 3000 meters and their distributions tend to follow mountain ranges in the tropics (but not at the southern end of the range of E. giganteum, which reaches temperate latitudes). Like other Equisetum species, giant horsetails grow in in areas with ample groundwater supply, often along rivers and in wetlands ( Hauke 1963 ; 1969a ).
Although giant horsetails are of considerable botanical interest, belonging to what may be the oldest surviving vascular plant genus in the world ( Hauke, 1963 ), botanists know relatively little about these remarkable plants beyond their taxonomy and anatomy. Indeed, the only two published papers that have dealt specifically with giant horsetail ecology were limited to some qualitative (but intriguing) observations on their natural history ( Hauke, 1969a ; Álvarez de Zayas, 1982 ). One study has investigated the biomechanics of aerial stems in E. giganteum3 ( Spatz et al., 1998 ). I discuss what is known of giant horsetail ecophysiology in another section of this website.
The most extensively studied aspect of the giant horsetails has been their medicinal properties . The medicinal use of giant horsetails has a history that reaches back to the pre-conquest Inca of Peru ( Tryon, 1959 ) and the plants are currently used in medicine (often as diuretics, but also for many other medicinal purposes) throughout Latin America (Morton, 1981 ; Hauke, 1967 ; Murillo, 1983 ; Gorzalczany et al., 1999 ). Investigators using animal models have found that giant horsetail extracts have diuretic ( Gutiérrez et al., 1985 ) and hypoglycemic (Cetto et al., 2000 ) effects and have "nerve growth factor(NGF)-potentiating activity" ( Li et al., 1999 ). Furthermore, a controlled study by Revilla et al. (2002) showed that traditionally prepared Equisetum myriochaetum extract had significant hypglycemic effect (not resulting from increased insulin secretion) in type 2 diabetics.
Although giant horsetails appear to have potential as remarkably primeval looking ornamental plants, their cultivation is mostly confined to a few botanical gardens and university teaching greenhouses. Fortunately, cultivation of these plants is relatively straightforward as long as they are never allowed to dry out and their requirements for micronutrients and silicon are met (see section on cultivation ). Giant horsetails adapt well to indoor and greenhouse cultivation in containers. However, because are essentially tropical plants, they are likely to survive outdoors only in warmer areas. The range of Equisetum giganteum extends to the more temperate areas of central Chile and Argentina, so plants from those areas may be more hardy.
I believe that the giant horsetails
are extraordinary plants. It is my hope that this website will help
to catalyze interest in their scientific and horticultural value.
1. For information
on the basic biology of the genus Equisetum , I recommend
Dr. David Webb's excellent pages on
Equisetum biology.
2. Calamites is an extinct genus of tree-sized horsetails that lived during the Carboniferous period.