GROWTH HABIT: Horseweed is a winter or summer annual, 1 to 5 feet tall.
LEAVES: Leaves are alternate, crowded on the stem, simple, bristly with hairs and sessile or short petioled. Lower leaves are spatulate and sparingly or coarsely toothed, while upper leaves are lance-shaped to linear.
STEMS: Stems are erect, unbranched below but often branched above.
FLOWER: The inflorescence is branched with slender flower stalks, flowers are inconspicuous with small white ray and yellow disk flowers.
SEEDS: Seeds are numerous, small, flattened, about 1/16 inch long, with a white bristly pappus.
OTHER: A native to North American grasslands, horseweed is common in pastures, meadows, cultivated fields, along roadsides and in waste areas. Mowing infested meadows or pasture when the plants are in the bud stage will prevent seed production. The leaves and flowers contain a terpene which is particularly irritating in the nostrils of horses. Flowering and seed production occur from late June to September.