Common ragweed is instantly recognizable by its ornate, 2–3 times pinnately lobed, hairy leaves. It’s very common in Missouri, and you’ve probably seen it many times. It is an annual plant growing from a taproot, often growing in colonies in disturbed places such as roadsides and old fields. The leaves are quite distinctive.
The flowers are very similar to those of giant ragweed. They 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 flowerhead encloses just one floret, with one beak; these form below the male heads and are nearly hidden in the leaf axils. Blooms July–November.
The leaves are opposite at the base of the plant but alternate toward the top, hairy, ornate, the blades about 6 inches long, oval in general outline, 2–3 times pinnately, deeply lobed with more than 5 primary lobes, the ultimate lobes lance-shaped to narrowly oblong, sometimes with a few teeth. The crushed leaves have a strong scent.
Similar species: Six species of Ambrosia have been recorded in Missouri:
- In addition to common ragweed, the commonly encountered members of the genus in Missouri are giant ragweed (A. trifida), a tall plant with rough, finely-toothed, palmately 3- to 5-divided leaves with pointed lobes; 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 rail yards.
Height: 1–4 feet; usually about 2½ feet.
Common nearly statewide.
Habitat and Conservation
Very common and widespread, usually found in sunny, typically dry habitats, especially in disturbed locations.
Among natural habitats, common ragweed occurs in a wide variety of sunny habitats: upland prairies, savannas, glades, tops of bluffs, banks of streams, rivers, and spring branches, marshes, margins of ponds, lakes, and sinkhole ponds, and openings of bottomland forests ranging to upland forests.
Common ragweed is well adapted to a wide variety of disturbed habitats, such as pastures, old fields, fallow fields, crop fields, levees, ditches, farmyards, railroads, roadsides, and other open, disturbed areas.
Native annual forb.
Ragweeds, including this species, are a leading cause of hay fever in late summer. Because ragweed 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.
Common ragweed is a problem agricultural weed, especially in soybeans, where it can reduce yields by 30 percent. Apparently, some strains of common ragweed have evolved a resistance to glyphosate-based herbicides.
Cattle and other livestock tend to bypass the foliage, since grazing on it can cause nausea.
Common ragweed is native to North and South America, but it has been introduced elsewhere in the world. In many places where it has been introduced, it has become invasive, expanding its range and displacing native species despite efforts to contain it or limit its spread. It is also invasive in parts of California and the American Southwest, where it is not native. People have introduced two species of North American ragweed leaf beetles to try to control the plant overseas: Ophraella communa in Europe, and Zygogramma suturalis in Russia and China.
The species name, artemisiifolia, means “Artemisia-leaved.” Artemisia is another genus in the sunflower family; it includes sagebrush, mugwort, and wormwood species, which are notable for their finely dissected leaves.
It's not clear why the genus name Ambrosia got applied to the ragweeds and bursages: a 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:
- Beetle larvae of many groups, such as longhorn beetles, weevils, and tumbling flower beetles. Adult and larval leaf beetles also chew the leaves; a few of these species are specialized just for eating ragweeds.
- The larvae of several types of leaf-mining flies eat tunnels through the middle tissues of the leaves.
- A wide variety of true bugs suck the juices, include plant bugs, stink bugs, aphids, planthoppers, leafhoppers, treehoppers, mealybugs, and aphids.
- The caterpillars of certain types of moths eat common ragweed. The caterpillars of the ragweed flower moth (Schinia rivulosa), a noctuid species, eat the flowers and developing flowerheads, a fact that should cheer up allergy sufferers. Other moth caterpillars that eat ragweed include a number of other noctuids, the Virginian tiger moth (Spilosoma virginica), several types of twirler moths (gelechilids) and tortrix moths (tortricids), the wavy-lined emerald (Synchlora aerata, a geometrid), the ragweed plume moth (Adaina ambrosiae), and more.
- Several species of grasshoppers chew the foliage of common ragweed, too.
With all the insects that eat common ragweed, colonies of this plant typically attract insect-eating songbirds, especially during fall migration, when ragweeds and their insect communities are at their height.
The seeds are rich in oils and are eaten by northern bobwhite, wild turkey, sparrows, doves, finches, and other wildlife, including several types of mice, ground squirrels, and voles. The seeds can be an important wintertime food for birds and mammals.
Herbivorous mammals tend to bypass this bitter plant. However, mammals do appreciate the heavy cover created by the dense jungles of giant ragweed: think of rabbits darting into ragweed thickets!