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Photo of a giant ragweed plant.
Giant ragweed forms large stands in disturbed areas, causing late-summer misery in the form of hay fever.
Chris Evans, Illinois Wildlife Action Plan, Bugwood.org
Other Common Name
Horse Weed; Great Ragweed; Buffalo Weed
Family

Asteraceae (daisies, sunflowers)

Description

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.

Size

Height: to 6 feet or more.

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Photo of giant ragweed staminate, male flower, clusters.
Giant Ragweed (Male Flower Clusters)
Giant ragweed is causes hay fever in late summer. The male flowerheads lack petal-like ray florets, so it's hard to tell these are open flowers.

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Photo of a giant ragweed colony.
Giant Ragweed Colony
Giant ragweed forms extensive colonies in disturbed bottomland and agricultural areas.

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Photo of a giant ragweed as a young plant not more than about 1 foot tall.
Giant Ragweed (Young Plant)
Giant ragweed is a problem weed in crop fields, especially soybean fields, and herbicide-resistant strains seem to be evolving.

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Photo of giant ragweed spike of flower clusters.
Giant Ragweed Flowers
The flowers of giant ragweed lack petals and are grouped in drooping clusters that are arranged in spikes.

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Photo of giant ragweed seeds seen from different angles.
Giant Ragweed Seeds
Some birds, including quail, eat the seed of giant ragweed.
Habitat and conservation

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.

Image of Giant Ragweed Horse Weed Great Ragweed Buffalo Weed  Distribution Map
Distribution in Missouri

Common nearly statewide.

Human connections

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.

Ecosystem connections

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.