Horsetails are allies of ferns. These are ancient plants with close relatives back in the Carboniferous period. Field Horsetail is one of most common Equisetums, but unusual in one respect - it is dimorphic, that is there are separate sterile and fertile aerial stems. The sterile stems usually range from 2 inches to 20 inches in height but can grow to over 30 inches. These are green, much branched, ridged, with a hollow center. The fertile, or spore bearing stems, are shorter, and some write that they rise in early spring before the sterile stems and are often withered away by the time the sterile stems rise, but in central Minnesota I find they both rise at the same time and there there are far fewer fertile stems than sterile. The fertile stems are unbranched, shorter (6 to 10 inches), brown in color and lack the breathing pores (stomates) of the green stems. Where each branch of the sterile stem occurs there is a sheath and when fully elongated, the first internode of the branch is longer than the sheath. The fertile stems have much larger sheaths, but do not develop branches and die back after spore production.
The ascending branches of the sterile stems, which hold the leaves, appear in a whorl around the stem with each branch rising from the base of a sheath. Stems may have up to 20 branch whorls. The sheaths end in darker colored pointed teeth, of which there will be fewer than 14 and which are really an extension of the stem ridge. Small scales on the branches appear at the branch nodes and these are the true leaves.
Spore production: Fertile stems end in a small spore producing cone known as a Strobilus which is composed of whorls of shield shaped sporophylls on whose inner surface are the actual spore bearing sporangia, each sporophyll containing 5 to 10 sporangia. The strobilus is elongated and tapers to an obtuse to rounded tip.
Habitat: Field Horsetail grows from horizontal rhizomes directly under the soil surface. These are branched and have storage tubers. It an be an aggressive spreader. It grows in waste places, roadsides, path-sides particularly where the soil is light and sandy but has a good moisture content. It grows in partial shade to full sun and if in full sun it can dominate other plants.
Names: The genus name, Equisetum, is Latin for "horsetail" and within the genus are held all of the world's horsetails, of which 11 are found in North America. The species name, arvense, is also from the Latin and meaning 'growing in or pertaining to cultivated fields'. The author name for the plant classification, 'L.', refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: The lack of fertile parts on the green stems and instead, on a separate brown stem, that then dies when spore production finishes is a distinguishing characteristic of this horsetail in our geographic area. Other Horsetails also have a separate fertile stem, but after spore production they then develop branches and continue growing. These include Equisetum sylvaticum, Marsh Horsetail, and Equisetum pratense, Meadow Horsetail.
Above: A green sterile stem in spring. These are brownish-green initially and appear after the brown fertile stems.
Below: Development of the sterile stem. The dark brown teeth on the sheaths indicate where the branches will develop. Each tooth is an extension of the stem ridge. 4th photo shows the hollow center core of the stem.
Below: Fertile stems are brown and have larger sheaths, no branches, and develop the spore bearing strobilus at the tip. 1st photo - stem before spore release. 2nd photo - stem sheath.
Below: The mature female strobilus. The strobilus is the spore producing organ that appears on top of fertile stems. The spores are produced in a sporangia, similar to a fern. The sporangia are contained in the small shield-like structures that form whorls around the strobilus.
Below: Mature plants of mid-summer.
Notes: Field Horsetail is indigenous to the Garden. Eloise Butler listed it as one of four species of Equisetum that she had in the early Garden when she submitted an essay of ferns and fern allies to the Agassiz Association in 1919.
The Minnesota DNR states the Field Horsetail is found in all Minnesota counties but one - Stevens. There are 13 different species of Equisetum found in Minnesota. It is found throughout North America except Florida and Louisiana.
Lore and uses: The stems of Horsetails contain much silica and have been used historically in the new and old world for scouring and sanding, although the method by which Horsetails accumulate this silica is mostly unknown. [for technical details see this abstract] There is also some reference for folk medicinal use, principally as a diuretic. Environmentally, they absorb heavy metals form the soil and are therefore quite useful in degraded sites.
Toxicity: There is literature stating that Horsetail is toxic to livestock especially horses. While it has low palatability browsers do eat it and, as to horses, the U S Forest Service reports that concentrations in hay forage need to be above 20% before it can cause scours, paralysis or death in horses.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"