- Pest Type: Weed
- Crops Affected: Wheat, Cotton, Potatoes, Soybeans, Corn
- Scientific Name: Conyza canadensis
- Pest Order: Asteraceae
Horseweed (marestail) is a winter or summer annual. Seedlings develop into a basal rosette. Mature plants produce an erect central stem (0.3-2 m tall) with a terminal panicle of inconspicuous flowers.
Reproduction is by seeds which germinate in spring or late summer. Late summer germination results in overwintering rosettes.
Cotyledons are oval, 2-3 mm long. Young leaves are egg-shaped, with toothed margins. Seedling leaves form a basal rosette and are covered with spreading to ascending short hairs.
Lower leaves are on prominent petioles. The upper leaves are narrower and taper to the stalk. When growing as a winter annual, the basal rosette is produced in late summer. After the stem elongates, the basal leaves deteriorate. Stem leaves are lanceolate to linear, with nearly entire margins.
Stems are erect, bristly hairy, with many small flowering branches in the upper portions. Leaves (about 10 cm long by 10 mm wide) are hairy, alternate, numerous, and crowed along the stem. Blades are sessile, linear to elliptic, broadest at the apex and tapering at the base. Leaf margins can be entire but are usually toothed. Leaves become gradually smaller up the stem.
Flowers are present from July through October. Branches from the main stem produce dense panicles consisting of numerous small (5 mm in diameter) flower heads. Ray flowers (25-50) are white or somewhat pinkish, disk flowers (7-12) are yellow. Seeds are enclosed in a 1 mm long achene. Pappus consists of whitish bristles that facilitate wind dispersal.
Plants turn brown before fruit dispersal. Dried woody stems persist through winter. The main stem often breaks from the weight of the inflorescence. Many seedlings can be found close to the parent plant.
This is primarily a weed of nursery crops, orchards, other perennial crops, waste areas, fallow fields, fence rows, and reduced-tillage row crops.
Horseweed (marestail) is common throughout North America.
Annual fleabane (Erigeron annuus) is similar, but has more prominent toothing on the leaves, and the flower heads are larger with showy white to pinkish ray flowers. Rough fleabane has flowers similar to annual fleabane, but the leaves are more spatulate than horseweed leaves, and the leaf hairs are shorter and appressed.