Horseweed (Canada Fleabane, Hog Weed)

Photo of horseweed flowers
Scientific Name
Conyza canadensis (formerly Erigeron canadensis)
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.
Where To Find
image of Horseweed Canada Fleabane Hog Weed distribution map
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.
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.
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.
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About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!