Giant ragweed is a much-branched annual, often growing by the thousands in bottomlands and disturbed areas. Its giant colonies cause late-summer misery in the form of hay fever for many Missourians.
The flowers lack apparent petals and sepals and are grouped into drooping clusters that are arranged in spikes. The male (staminate) flowerheads are quite small, green, and in loose, terminal racemes; they lack petal-like ray flowers, so it's hard to tell they are fully open, or even that they're flowers. The flowerheads face downward on their stalks, and pollen, which causes allergic reactions in people, is released to the wind. The female flowerheads form below the male heads and are nearly hidden in the leaf axils. Blooms July–September.
The leaves are opposite, palmately 3- to 5-divided (or at times undivided), the lobes pointed with fine teeth, rough, hairy (scabrous).
Similar species: Six species of Ambrosia have been recorded in Missouri:
- In addition to giant ragweed, the other commonly encountered members of the genus in Missouri 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.
- The two other species of Ambrosia that have been recorded for Missouri are rarely encountered: perennial bursage (A. tomentosa) and annual bursage (A. acanthicarpa). Both are uncommon, introduced, and known from only limited locations in Missouri. Annual bursage is known only from historical collections in Jackson and Wayne counties; perennial bursage is recorded only from a persistent colony in the St. Louis railyards.
Height: to 6 feet or more.
Common nearly statewide.
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.
Native Missouri herbaceous annual plant. Usually considered a weed. A major contributor to late-summer pollen allergies.
Giant ragweed is a leading cause of hay fever in late summer. Because its flowers are not showy, and because goldenrod, which blooms about the same time, is very showy, goldenrod is the plant that often gets blamed for late-season pollen allergies. But goldenrod's pollen is sticky and forms clumps — perfect for clinging to its insect pollinators' bodies. It isn't blown in the wind like ragweed's pollen. It's not fair to goldenrod that it gets blamed for the allergies caused mostly by ragweeds.
Giant ragweed 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. Collections of giant ragweed fruits (seeds) have been found in archaeological sites.
It's not clear why the genus name Ambrosia got applied to this group of weedy, bitter, allergy-causing plants. In Greek mythology, ambrosia was the food of the gods, and people who ate it became immortal or at least lived very long lives. It seems to have as much to do with this group of plants as does the sweet, creamy fruit salad that also shares the name. The genus was named by Carl Linneaus, the 18th-century Swedish naturalist who originated the system for scientific names. You have to wonder: What was he thinking?
Few pollinating insects visit the flowers of this wind-pollinated plant.
Many insects eat the plant or suck its juices. Some of these insects are specialized to eat only this genus, or even only this species. Insects that eat giant ragweed include:
- Beetles such as the ragweed leaf beetle (Zygogramma suturalis), a round, dark beetle with natty stripes, plus certain species of longhorned beetles, tumbling flower beetles, and weevils, whose larvae bore into the stems of even the leaf stems.
- True bugs, which suck the juices, include the green stink bug (Chinavia hilaris), the ragweed plant bug (Chlamydatus associatus), and aphids, including the brown ambrosia aphid (Uroleucon ambrosiae), which apparently is limited to only this species in the eastern United States. Other true bugs feeding on giant ragweed include lacebugs that feed from the undersides of the leaves, and leafhoppers and treehoppers, such as the buffalo treehopper (Stictocephala bisonia).
- Grasshoppers chew the leaves.
- The larvae of leaf-mining flies eat tunnels through the middle tissues of the leaves.
- The caterpillars of certain types of moths eat giant ragweed. The caterpillars of Thoreau's flower moth (Schinia thoreaui), a noctuid species, eat the flowers and developing flowerheads, a fact that should cheer up allergy sufferers. Other moth caterpillars include the Virginian tiger moth (Spilosoma virginica), several types of twirler moths (gelechilids), a number of other noctuids, the soft-lined wave (Scopula inductata, a geometrid), and more.
- The caterpillars of the gorgone checkerspot (Chlosyne gorgone), an ornate orange-and-black brush-footed butterfly, eat giant ragweed, in addition to several other members of the sunflower family.
With all the insects that eat giant ragweed, the giant colonies of this plant typically attract insect-eating songbirds, especially during fall migration, when giant ragweed, and its insect communities, are at their height.
Herbivorous mammals tend to bypass this bitter plant, although meadow voles have been recorded eating the young shoots. However, mammals do appreciate the heavy cover created by the dense jungles of giant ragweed: think of rabbits darting into ragweed thickets!
Some birds, such as northern bobwhite, mallard, and greater prairie-chicken, eat the seed.
The tough seed coats probably prevent digestion by many animals. Still, many types of mice have been recorded eating the seeds. Most animals that consume the seeds probably serve to distribute them.
They're not very ornamental, but this and other vigorously growing, colony-forming lowland plants help to bind soils. Together, this jungly community prevents erosion and filters contaminants out of the water running off the land, protecting waterways from fertilizers, pesticides, and silt runoff.