Horsetail is a perennial fern (yes, although once called a “fern relative,” horsetail is now accepted as a true fern). It is almost cosmopolitan in distribution, found in temperate climates worldwide. It produces two types of stems: sterile stems and fertile stems.
Sterile stems emerge in the late spring and persist until frost. They are thin and green, measuring 8 to 20 inches (20 to 50 cm) high. They are abundantly provided with thin green branches placed in whorls all around the stalk, giving the plant the appearance of a small Christmas tree … or of a horse’s tail.
In the spring, at least under conditions of high humidity, it may also produce fertile stems. They are upright, thicker, unbranching, and come in various shades of cream to to orange or gray. This is because they lack chlorophyll (the green pigment that gives the sterile stems their coloration). Each fertile stem carries an upright cone at its tip. The cone produces spores which are carried away by the wind.
Despite the presence of wind-borne spores, common horsetail only rarely spreads by spores. In most garden settings, it instead sprouts from wandering underground rhizomes: stems that grow horizontally and sucker profusely. It also produces underground tubers that can also help in its proliferation.
Because of its ability to expand almost indefinitely by rhizomes, and despite many medicinal and practical uses (campers know it can be used as a scouring pad to clean pots, for example), horsetail is generally considered a weed.