Horseweed forms a small rosette in early late winter or early spring. Plants develop slowly in early spring. From the rosette, plants grow almost completely vertical. Horseweed often grows as a single vertical shoot up to 7 feet tall, with a large panicle of flowers on top. Sometimes the plant will branch low near the soil, but it rarely branches above. Leaves are dark green, hairy and without petioles.
Horseweed flowers heads are small with yellow disk flowers and barely noticeable white ray flowers. Flowering occurs generally June through September, but can vary slightly. Flower heads occur in large terminal panicles. Seed heads are spherical and about 1 cm in diameter. The pappus is dirty white.
Favorable environment notes:
Horseweed grows in areas of disturbed soil such as orchards, field nurseries, agronomic crops and unmanaged areas such as roadsides and ditches.
Seeds are dispersed mainly by wind. Very small single seeds are (1.0-1.5mm), tipped with small dirty white bristles are caught in wind currents and transported.