Horseweed (marestail)

August 10, 2015

Conyza canadensis (L.) Cronq.

Life cycle

Erect winter or summer annual.

Leaves

Cotyledons are egg- to spatula-shaped and hairless. Seedling leaves initially develop from a basal rosette; mature leaves are alternate and numerous, and crowded along an erect central stem. Leaves are hairy and linear to lance-shaped with smooth to toothed margins, gradually becoming smaller toward the top of the plant.

Horseweed rosette.

Stems

Erect, hairy central stem arises from a basal rosette, then branches to flower, reaching up to 7 feet tall. Flowering branches resemble a horse’s tail.

horseweed plant
Erect central stem of horseweed.

Flowers and fruit

Numerous clusters of small, white flower heads are found on many short branches near the top. Each seedhead is capable of producing thousands of small seeds; each seed is enclosed in a single-seeded, wind-disseminated fruit.

horseweed new flowerhead
Horseweed flowers (left), and seedhead (right).

Reproduction

Seeds.

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MSU Extension Integrated Pest Management