Perennial. Never has flowers or seeds but reproduces by spores and by horizontal underground stems (rhizomes). Distinguished by its ashy-gray, unbranched, leafless shoots tipped with brownish, spore-producing cones in early spring, and later, from late spring or early summer onwards, by its whorls of 6 to 8 green, leafless branches and complete absence of flowers.

Stems & Roots: The rhizomes are dark brown or blackish, spread out for long distances and are often 1 m (3 1/3 ft) below the ground surface. They send up numerous aboveground shoots but of two different types at different times of the year. In early spring, the shoots are ashy-gray to light brown, unbranched, hollow, jointed stems; each node (joint) surround-ed by a toothed (b) sheath (a); and the tip of stem end-ing in a brownish, spore-producing cone (c). After the cones have shed their spores (early May) these whitish to light brown stems wither and die down. At the same time, the second type of shoot emerges from the ground. These are green, slender, erect, hollow stems, leafless but with whorls of 6 to 8 branches at nearly every node; each branch may branch again with whorls of smaller branches; stems and branches surrounded by a small, toothed sheath (d) at each node but never end in a spore-producing cone. Both kinds of stems are easily pulled apart at the nodes and can be fitted back together like sections of a stove pipe.

Flower and Fruit: The cones shed their spores in early May.

Habitat: Field horsetail occurs in all parts of Ontario in depressional areas with poorly drained soils, as well as in sandy or gravelly soils with good drainage such as railroad embankments and roadsides. An intense competitor, it can severely suppress crops and other plants.

Caution: It contains a substance which destroys vitamin B in animals. It is especially poisonous to young horses. Hay containing this weed may be more poisonous than fresh plants in the field.