This plant is a winter annual that germinates in the fall or early spring. It is not a problem in fields where tillage is used to prepare a seedbed since tillage destroys established plants. Few horseweed seeds will germinate when soils reach temperatures suitable for planting corn or soybean. Horseweed seedlings produce a basal rosette of leaves. The first leaves have smooth leaf margins, and later leaves have a toothed margin.
All leaves have a hairy surface.
Horseweed plants in Ames currently are 1-2 inches in diameter. Later in the spring, typically in May, the stem elongates, resulting in an erect plant that may reach 6 feet in height. Although no controlled studies have evaluated the competitiveness of horseweed with corn or soybean, its growth habit suggests it would require very high populations to impact crop yields. Horseweed should not interfere with harvest since the plant matures and dries down in mid-summer long before harvest. The primary impact of horseweed in soybean is that it detracts from the appearance of the field.