Giant Ragweed

Photo of a giant ragweed plant.
Scientific Name
Ambrosia trifida
Asteraceae (daisies, sunflowers)

A much-branched annual, often growing by the thousands in bottomlands. Flowers lack petals and sepals and are grouped into drooping clusters that are arranged in spikes. Male flowerheads quite small, green, in loose, terminal racemes; female flowerheads form below male heads, nearly hidden in leaf axils. Blooms July–September. Leaves opposite, palmately 3- to 5-divided (or at times undivided), the lobes pointed with fine teeth, rough, hairy (scabrous).

Similar species: There are 6 species of Ambrosia in Missouri. The most common are common ragweed (A. artemisiifolia), with ornate, 2–3 times pinnately lobed leaves; lanceleaf ragweed (A. bidentata), with alternate, lanceolate, mostly unlobed leaves; and western ragweed (A. psilostachya), with 1-time pinnately lobed leaves.

Common Name Synonyms
Horse Weed; Great Ragweed; Buffalo Weed
Height: to 6 feet or more.
Where To Find
Image of Giant Ragweed Horse Weed Great Ragweed Buffalo Weed  Distribution Map
Common nearly statewide.
Common in pastures, rich soil in old fields, fallow fields, crop fields, levees, ditches, railroads, roadsides, and open, disturbed areas. Also occurs in prairies, banks of streams and rivers, sloughs, marshes, margins of ponds and lakes, and bottomland forests. It forms extensive colonies in disturbed bottomland and agricultural areas.
Giant ragweed is a leading cause of hay fever in late summer. It is a problem weed in crop fields, especially soybean fields. Herbicide-resistant strains seem to be evolving. Prehistoric Americans cultivated a large-fruiting strain for food and also used ragweed ceremonially and medicinally.
Few insects visit the flowers of this wind-pollinated plant. Several moth species eat the foliage. Mammals tend to avoid this bitter plant. Some birds, such as quail, eat the seed, but the tough seed coats probably prevent digestion by many animals.
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Similar Species
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!