Equisetum spp. – June 9th 2025 – Horsetails

The horsetails are living fossils, with ancestors living in the Late Devonian 370 to 360 million years ago. Horsetails resemble Calamites, a plant that was 30-50 metres tall, forming part of the coal-swamps in the Carboniferous period (360-300 million years ago). Modern Equisetum appeared in the Jurassic (201 million to 145 million years ago).

Artist’s impression of a Jurassic scene with Equisetum in the foreground and on the left; Araucaria in the distance and branches of Ginkgo are depicted in the top-left foreground. Attribution: 0920 Jurassic Forest Scene, see https://bizleyart.com/contact-us where other good examples are on sale.

Horsetails are allied to the ferns¹, being spore-bearing vascular plants, but they look nothing like ferns. Instead of having broad leaves for photosynthesis, the ‘leaves’ are merely scales and the green photosynthetic tissues are located within the stiff ridged stems and the whorls of stem-branches. Rather than producing spores on their leaves like ferns do, they have cone-like structures called strobili² held aloft at the tip of the plant.

All horsetails are perennial plants, arising from persistent creeping rhizomes, spreading by extending these organs into new soil and sprouting aerial shoots at intervals. Like ferns, they are dispersed as airborne spores, which germinate on wet soil to form tiny pieces of heart-shaped green tissue (‘prothallus’) which produce male (‘antheridia’) and female (‘archegonia’) sex organs. Following fertilisation, a new plant is formed.

Field horsetail, Equisetum arvense. Left, excavated plants removed from cultivated soil, showing the tough brown rhizomes. Right, close-up showing the tiny leaves (‘microphylls’) appearing as scales where the branches emerge from the stem, and also the ridges on the stem. The number and shape of the ridges are useful characters for identification of species. Photos: John Grace.

Horsetails and silica

Horsetails accumulate the element silicon (Si) in their stems as silica (SiO₂), which can be as much as 22 percent of their dry weight and gives the stems their stiffness. Other plants also accumulate silicon, especially grasses, but never to such an extent. Silica is of course what sand, glass and quartz are made of, and I have often wondered about how the silica can possibly enter the plant’s root system because silica does not dissolve in water (we depend on glass to not dissolve every time we have a drink). However, it turns out that silica is actually slightly soluble in water because of this reaction:

SiO₂ + H₂O -> H₂SiO₃

and H₂SiO₃  is a weak acid, dissociating into H⁺ and various silicate anions which are swept into the plant along with all the usual essential nutrients. Si is not usually listed in textbooks as a micronutrient but, without it, horsetails and grasses would not grow properly. I found one research paper in which Equisetum arvense was harvested as a silica fertilizer.

Equisetum arvense breaking through asphalt…this is a really tough plant that truly deserves the title ‘one of the world’s worst weeds’. Image: Chris Jeffree.

Horsetails and dinosaurs

The sauropod dinosaurs were the largest animals that ever lived, and they were Jurassic herbivores. The vegetation then included Equisetum, Araucaria, Ginkgo and ferns. Flowering plants had not yet diversified and so there were no grasses to chew. Researchers at the University of Bonn tested the digestibility of plant material in an artificial rumen, using the digestive juices from sheep. To their surprise they found that Equisetum was the easiest to digest, ranking alongside grasses in their digestibility tests. Examination of the animals’ jaws and teeth suggested that the animals did not chew very much, but simply gulped their food down in much the same way as birds do today. (Birds are now considered by some to be living dinosaurs). 

Equisetum for food

If horsetails were such good food for dinosaurs, were they ever food for humans? Yes they were, according to wikipedia:

People have regularly consumed horsetails. The fertile stems bearing strobili of some species can be cooked and eaten like asparagus (a dish called tsukushi in Japan).  Indigenous nations across Cascadia consume and use horsetails in a variety of ways, with the Squamish calling them sx̱ém’x̱em and the Lushootseed  using gʷəɫik, or horsetail roots, for cedar root baskets. The young plants are eaten cooked or raw, but considerable care must be taken.

In the interests of science, I tried some yesterday – just the tender young tips. It’s not as tasty as asparagus and it feels rather fibrous in the mouth, but not too bad. The wikipedia article warns against over-consumption, as there is some danger of liver damage, especially to children and pregnant women. Equisetum arvense is toxic to livestock, particularly horses.

Other uses

Because of the high silica content, horsetails are somewhat abrasive and have been used to clean pewter and other metal objects; also they make quite good scouring pads for washing tableware. Also, horsetails have been used in traditional medicine. When I googled ‘herbalist equisetum’ I found that many health benefits are claimed: tinctures and extracts,  remedies for bladder and kidney issues, wound healing,  herbal teas and infusions.  

British Horsetails

Equisetum is the only surviving genus of the horsetails, with only 18 species in the whole world, of which Britain has nine. Here, we focus on just six.

The field horsetail, Equisetum arvense. Left: growing with Pilosella officinale; right: cone bearing shoots – note they have no chlorophyll, in contrast to the vegetative shoots. Images: Chris Jeffree.

The field horsetail, Equisetum arvense, is considered one of the world’s ‘worse weeds’ (Holm et al. 1991). It is common, with an underground system of creeping wiry rhizomes, up to 2 metres deep, which gardeners find impossible to remove completely from the soil. Small fragments produced by digging or hoeing can sprout green shoots. The shoots are covered with wax and herbicides such as glyphosate simply run off. The quite rare cone-bearing shoots of this species lack chorophyll, giving them a somewhat ghostly appearance. E. arvense is native in all countries of the Northern Hemisphere and occurs as an accidental introduction in Australasia, South America and even a few locations in Africa.

The water horsetail, Equisetum fluviatile. Left: growing in a lake with the water-lily Nymphoides peltata; right, tips of fertile shoots, showing cones. Images: Chris Jeffree.

The water horsetail, Equisetum fluviatile has a similar geographical distribution to field horsetail, mostly in the Northern Hemisphere. It forms dense partially-submerged colonies by freshwater shores, and some people consider it invasive. The stems are completely hollow, a feature that presumably provides buoyancy and assists transfer of oxygen to the rhizomes, which are usually growing in mud. It has been used to feed reindeer and other livestock in Scandinavia,

The wood horsetail, Equisetum sylvaticum. Left: showing the ‘drooping’ branches; right, fertile shoots with little chlorophyll in their stems. Images: Chris Jeffree.

The wood horsetail, Equisetum sylvaticum has secondary branches (i.e. branches on its branches); these branches droop down and so, overall, this horsetail looks quite pretty. It is found on mildly acid peaty soils and it flourishes on damp soil in hilly districts. As suggested by its name, it occurs in forests and open woodlands.  

Variegated Horsetail, Equisetum variegatum. The stems are prostrate, more or less at random, growing here with water pennywort and silverweed. Images: Chris Jeffree.

Variegated Horsetail, Equisetum variegatum is an evergreen species with (usually) no branches. The common form of this one is prostrate with very slender stems, but in Ireland a more robust, taller form is found (up to 60 cm). When prostrate it can be easily overlooked by recorders and it may be more common than the distribution map suggests. It likes calcium-rich substrates and is found in dune slacks, river shingle and quarries. It tolerates flooding.

The great horsetail, Equisetum telmateia. The logarithmic progression of the internodes may have inspired Napier to invent logarithms. On the right, the much shorter fertile shoots which lack chlorophyll. Images: Chris Jeffree.

The great horsetail, Equisetum telmateia is the tallest one, reaching a majestic 1.5 m or more by the end of summer. It forms rather large colonies in wet places, including rich clay soils and ‘flushes’. I see it in South Ayrshire near the coast, and in Scotland it does appear to be somewhat coastal. Elsewhere in the UK it appears by roads and railways. Some gardeners like its architectural appearance and I have found that plants can be purchased from nurseries. But it may be hard to control in the garden.

The marsh horsetail, Equisetum palustre. Note the short branches. Image: Chris Jeffree.

The marsh horsetail, Equisetum palustre grows to 60 cm and is found in nutrient rich wet meadows and wet areas in hilly regions. It can also be found in dune-slacks and by roads and tracks. It is widespread in Britain.

Readers wishing to identify Equisetums may find these images useful, but the drawings in Rich and Jermy (1998) should provide confirmation of the identity of any dubious specimens. Hybrids often occur in Equisetum. As many as 24 are described in great detail in Stace et al. (2016) Hybrid Flora of the British Isles.

Distribution of five of the British species, from BSBI Maps.

Some people in Scotland refer to horsetails as ’mares-tails’. This is wrong: mares-tail is the aquatic plant Hippuris vulgaris,  unrelated to Equisetum.

Notes

1. Horsetails are placed in the Family Equisetaceae, which is included within the ferns on the basis of the structure of the spermatozoids (the male gametes which swim in the water films around the prothallus), and also similarity of their roots.
2. The term ‘cone’ strictly applies only to conifers. The strobilus of Equisetum is cone-like only in its shape, but here we say ‘cone’ to make easy-reading.

References

Gee CT (2011). Dietary options for the sauropod dinosaurs from an integrated botanical and paleobotanical perspective. Understanding the life of giants, pp.34-56 in Klein et al. The Biology of Sauropod Dinosaurs, Google Books.

Greger M and Landberg T (2024). Equisetum arvense as a silica fertilizer. Plant Physiology and Biochemistry, 210, 108606.

Holm LG et al. (1991). The world’s worst weeds – distribution and biology. Florida, Krieger.

Hummel J, Gee CT, Südekum KH et al. (2008). In vitro digestibility of fern and gymnosperm foliage: implications for sauropod feeding ecology and diet selection. Proceedings of the Royal Society B: Biological Sciences, 275(1638), pp.1015-1021.

Rich TCG and Jermy AC (1998) Plant Crib. Botanical Society of the British Isles.

Sundue M (2009) Silica bodies and their systematic implications in Pteridaceae (Pteridophyta). Botanical Journal of the Linnean Society 161, no. 4 (2009): 422-435.

Williams VS, Barrett PM, Purnell MA (2009) Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding, Proc. Natl. Acad. Sci. U.S.A. 106 (27) 11194-11199, https://doi.org/10.1073/pnas.0812631106.

Yamanaka, Shigeru, Kanna Sato, Fuyu Ito, Satoshi Komatsubara, Hiroshi Ohata, and Katsumi Yoshino. (2012). Roles of silica and lignin in horsetail (Equisetum hyemale), with special reference to mechanical properties. Journal of applied physics 111, no. 4

https://herbaria.plants.ox.ac.uk/bol/plants400/profiles/EF/Equisetum

©John Grace