Plant Profile: Ragweeds

 Common ragweed, Ambrosia artemisiifolia, and great ragweed, A. trifida.

Common ragweed, Ambrosia artemisiifolia, flowering in late August. 

For people with seasonal allergies, ragweeds are beasts.

Pollen from these plants, also called hay fever weeds, cause much of the sneezing, watering eyes, coughing, wheezing and other symptoms that torment allergy and asthma sufferers in late summer and early fall.

Both great ragweed and common ragweed, the two species frequently found here, are native annuals. They’re often found along roadsides, in abandoned lots, along field edges and in other disturbed places. Most seeds germinate in early spring, but some may germinate as late as July. Flowering peaks in August and September and lasts until the first frost.

Common ragweed plants are 1-3 feet tall at maturity. Leaves are opposite below and alternate above, divided and deeply lobed, to 6 inches long and 4 inches wide at the base.

Great ragweed is 3-12 feet tall at maturity. Leaves are opposite, the lower ones three-lobed and the upper ones simple and ellpitical. Largest leaves grow up to 12 inches long and 8 inches wide.

Ragweeds produce separate staminate (male, or pollen-producing) and pistillate (female, or seed-producing) flower heads on spike-like racemes. Both kinds of flowers are found on the same plant; in other words, the plants are monoecious (mo-NEE-shus). Staminate flowers are grouped into stalked, downward-facing heads on the upper part of each raceme. Pistillate flowers are clustered below, often nestled in leaf axils.

After pollination, pistillate flowers develop small diamond- or top-shaped fruits with a central “beak” surrounded by ridges, each ridge ending in a short spine. The fruits look like miniature crowns, so ragweeds are also called crown weeds. Each fruit contains a single seed, and an individual plant of either species can produce thousands of seeds each season. Common ragweed seeds are viable in soil for two to three years and up to 40 years (3). Giant ragweed seeds are less durable; most lose viability after one year (4).

 

Left: Great ragweed racemes are 3-8 inches long. Right: Closer view of ragweed flower heads. Staminate heads are stalked and face downward. Pistillate heads contain only one flower. The one at the arrow has been pollinated and a young, green fruit is developing. Common ragweed racemes are shorter but otherwise similar.

Both types of flowers are small and simple; they have no large, colorful petals. That’s because the plants are primarily wind-pollinated and therefore don’t invest in structures needed to attract insects. Typical of wind-pollinated plants, the staminate flowers produce tremendous amounts of pollen. Many sources state that a single plant can release up to 10 million pollen grains a day and up to 1 billion grains a year.

It’s unclear where those numbers come from, but recent studies confirm similarly large amounts. In France, where ragweed is introduced and invasive, researchers found that a single common ragweed (A. artemisiifolia) produces from 100 million to 3 billion pollen grains per season (1). A study of intact vs. mowed common ragweed in Quebec found that an intact plant produces more than 100 million pollen grains per season (2).

These great ragweed leaves are dusted with yellow pollen.

Those millions of grains, multiplied by the number of plants that can densely fill an optimal habitat, present a serious health threat to people with ragweed allergies. The plants do have some ecological benefits, however. As colonizers of disturbed places, they can hold soils in place as other plants succeed them. In addition, their protein- and oil-rich seeds are eaten by migrating and winter-resident song birds and game birds, as well as by chipmunks, voles, and other rodents.

Beastly or beneficial, ragweeds are an enduring part of our landscape. Maybe that’s why Linnaeus put them in the genus Ambrosia, Greek for “immortal,” “divine,” or “food of the gods.” Given the seeminly unending symptoms ragweed pollen can cause, the first meaning, immortal, seems to fit. The last two, though, are hard to fathom. Ragweeds are indeed persistent. But for allergy sufferers, they are anything but divine.

References 

1. Boris Fumanal, Bruno Chauvel, François Bretagnolle. 2007. Estimation of the pollen and seed production of common ragweed in Europe. Annals of Agricultural and Environmental Medicine (AAEM) 14 (2), pp. 233-236.

2. Simard M.J., and Benoit, D.L. 2011. Effect of repetitive mowing on common ragweed (Ambrosia 
artemisiifolia L.) pollen and seed production. Annals of Agricultural and Environmental Medicine (AAEM)18 (1), pp. 55–62.

3. Cornell College of Agriculture and Life Sciences. Common ragweed. Website accessed 9/5/25.

4. The Ohio State University. College of Food, Agricultural, and Environmental Sciences. Giant ragweed: A weed of extremes. 9/27/16.