Horseweed (Canada Fleabane, Hog Weed)

Conyza canadensis (formerly Erigeron canadensis)


Photo of horseweed flowers
Mary Ellen (Mel) Harte,

Asteraceae (daisies)


A flowering annual with variable height. Large specimens may be densely branched in the top half of the plant. Flowerheads many, in a dense, elongate, terminal inflorescence. Heads tiny, about ³⁄₁₆ inch across, the cream-colored rays so small that they are seen only with a magnifying lens. Blooms June–November. Leaves many, hairy, elongated, alternate around the single, hairy stem.


Height: commonly 2–7 feet, but quite variable. Some may be only a few inches tall.

Habitat and conservation

Occurs in fields, roadsides, waste places, gardens, banks of streams and rivers, upland and sand prairies, glades, openings in upland forests, and open, disturbed places. In Missouri it is known especially from disturbed areas. It is a serious crop weed, especially of corn and soybeans. Like many other weeds, this one has developed herbicide-resistant strains.

image of Horseweed Canada Fleabane Hog Weed distribution map
Distribution in Missouri


Human connections

Good farmers naturally want to rid their fields of weeds. Missouri, an agricultural state, is home to one of the world's leading producers of herbicides. Another proposed solution to the weed problem is genetically engineering crop plants that are more resistant to increasingly strong herbicides.

Ecosystem connections

The flowers are visited by insects, especially wasps and flies, which drink nectar. Other insects eat the leaves. Mammals generally don't eat this plant because of bitter-tasting chemicals in the leaves.