What Is Ragweed? How to Identify It and Where It Grows
Ragweed is a wind-pollinated annual weed in the genus Ambrosia, and it is the main cause of late-summer hay fever in North America. Two species account for nearly all of it: common ragweed, which grows knee to waist high, and giant ragweed, which can reach fifteen feet. Both have small green flowers arranged on upright spikes, and neither looks like the yellow plant most people blame.
Ragweed is unremarkable to look at, which is most of the problem. It has no showy flower, it grows in places nobody tends, and it is easy to walk past for years without registering it. Meanwhile a single plant can release up to a billion pollen grains in one season, and those grains have been measured 400 miles out to sea and two miles up in the atmosphere.
This page covers what ragweed looks like, how to tell the two species apart, where it grows, and how to distinguish it from the plants it gets confused with. If you are looking for the season calendar and how to reduce your exposure, that is in our guide to ragweed season.
What does ragweed look like?
Ragweed is easier to identify by its flowers than its leaves, because the leaf shape varies between species and even between plants.
Small, green, on spikes
Ragweed flowers grow in narrow upright clusters at the top of the stem, like small green bottle brushes. They are not showy, they have no petals worth the name, and they fade into the surrounding foliage. If you are looking for something colorful you will not find it.
Deeply divided, fern-like
Common ragweed has finely dissected leaves that look almost like a fern or a carrot top. Giant ragweed has broad leaves with three or five distinct lobes, more like a maple. Both are rough to the touch and arranged in pairs low on the plant.
Hairy and branching
Stems are covered in fine hairs and branch as the plant matures. Common ragweed branches low and spreads. Giant ragweed sends up a single dominant stem that can grow thicker than a broom handle.
The reliable tell is the flower spike. If you see small green flower clusters standing upright at the top of a rough, hairy, branching plant in August or September, that is ragweed.
The two species that matter
There are more than a dozen Ambrosia species in North America. Two produce the overwhelming majority of the pollen people react to.
| Common ragweed Ambrosia artemisiifolia |
Giant ragweed Ambrosia trifida |
|
|---|---|---|
| Height | 1 to 4 feet, occasionally 6 | 3 to 12 feet, sometimes 15 or more |
| Leaves | Finely divided, fern-like, greyish-green underneath | Broad with three or five lobes, like a large maple leaf |
| Stem | Branching, hairy, often reddish at the base | Thick, coarse, single dominant stem |
| Where it grows | Roadsides, fields, vacant lots, gardens, disturbed soil | Riverbanks, floodplains, field edges, damp ground |
| Range | Nearly all of the lower 48 | East of the Rockies, most abundant in the Midwest |
| Pollen | The larger overall source, simply because there is more of it | Produces more per plant, but grows in fewer places |
Both flower in response to shortening days in late summer, which is why the season starts at roughly the same time every year regardless of how the weather has run.
Where does ragweed grow?
Ragweed is a colonizer. It takes hold in disturbed soil where other plants have not established, which puts it in exactly the places nobody maintains.
- Roadsides and highway verges. Regularly disturbed, poor soil, no competition. Prime ragweed habitat.
- Vacant lots and construction sites. Bare ground is an invitation.
- Field margins and fence lines. Especially around row crops.
- Riverbanks and floodplains. Giant ragweed in particular.
- Gardens and lawn edges. Common ragweed will grow in a flower bed given the chance.
Geographically, ragweed is present across nearly all of the lower 48. It is heaviest through the Midwest, the Ohio Valley, and the Mid-Atlantic, and least common west of the Rockies. If you are in the Pacific Northwest and reacting in September, ragweed is probably not your problem. Our state-by-state pollen count guide covers where the heaviest loads are.
One thing worth knowing: pulling ragweed out of your own yard will not meaningfully help your symptoms. The pollen travels hundreds of miles. Your exposure is set by the regional load, not by your property.
Plants ragweed gets confused with
Goldenrod
The most common mistake by a wide margin. Goldenrod blooms at the same time, grows in the same places, and is bright yellow and impossible to miss. Ragweed is green and invisible. So goldenrod takes the blame.
Goldenrod is insect-pollinated. Its pollen is heavy and sticky and stays on the plant waiting for a bee, which means it rarely becomes airborne and rarely causes hay fever. Ragweed is wind-pollinated and built to travel. If you are sneezing in September, it is not the yellow plant.
Mugwort
Similar height, similar leaf shape, similar habitat, and it also causes allergies. The tell is the underside of the leaf: mugwort is distinctly silver-white underneath, where common ragweed is a duller greyish-green. Mugwort also has a strong sage-like smell when crushed.
Carrot tops and yarrow
Both have finely divided leaves that resemble common ragweed at a glance. Neither has ragweed's upright green flower spikes, and both are smoother to the touch.
Why identifying it matters, and where it does not
Knowing what ragweed looks like is useful for two things: understanding what is growing on your property, and confirming that the plant you are about to blame is the right one.
It changes very little about managing symptoms. Because the pollen travels so far, removing ragweed from your own yard has a marginal effect at best. What actually reduces your exposure is limiting the pollen that gets indoors and removing what does. Ragweed pollen runs roughly 17 to 23 microns, which is comfortably within the range a HEPA filter captures, so a correctly sized purifier in the bedroom does more for your September nights than any amount of weeding.
Our guide to ragweed season covers the timing and the room-by-room approach, and our air purifiers for allergies are sized for the job.
Ragweed identification FAQ
What is ragweed?
Ragweed is a wind-pollinated annual weed in the genus Ambrosia and the main cause of late-summer hay fever in North America. Two species account for nearly all of it: common ragweed, which grows one to four feet tall with finely divided fern-like leaves, and giant ragweed, which can reach fifteen feet with broad three-lobed leaves. Both have small green flowers on upright spikes.
What does ragweed look like?
Look for small green flower clusters standing upright at the top of a rough, hairy, branching plant in August or September. Common ragweed has finely dissected fern-like leaves and grows one to four feet. Giant ragweed has broad leaves with three or five lobes and can reach fifteen feet. Neither has showy or colorful flowers, which is why ragweed is so often overlooked.
What is ragweed pollen?
Ragweed pollen is the wind-carried reproductive material released by ragweed plants in late summer. A single plant can produce up to a billion grains in one season, and the grains are light enough to have been measured 400 miles out to sea and two miles up in the atmosphere. Individual grains run roughly 17 to 23 microns, well within the range a HEPA filter captures.
What are the types of ragweed?
There are more than a dozen Ambrosia species in North America, but two produce the overwhelming majority of allergenic pollen. Common ragweed, Ambrosia artemisiifolia, is the more widespread and grows across nearly all of the lower 48. Giant ragweed, Ambrosia trifida, produces more pollen per plant but grows in fewer places, mainly east of the Rockies along riverbanks and floodplains.
Where does ragweed grow?
Ragweed colonizes disturbed soil, which puts it on roadsides, in vacant lots, along field margins and fence lines, on riverbanks, and in unmaintained garden edges. It is present across nearly all of the lower 48, heaviest through the Midwest, Ohio Valley and Mid-Atlantic, and least common west of the Rockies.
Is goldenrod the same as ragweed?
No. Goldenrod is insect-pollinated, so its heavy sticky pollen rarely becomes airborne and rarely causes hay fever. It takes the blame because it blooms alongside ragweed and is far more visible with its bright yellow plumes. Ragweed has small green flowers on spikes and is wind-pollinated. Ragweed is the one causing symptoms.
Will removing ragweed from my yard help my allergies?
Only marginally. Ragweed pollen travels hundreds of miles, so your exposure is set by the regional pollen load rather than by what grows on your property. Reducing the pollen that gets indoors and filtering what does is far more effective than weeding.
How tall does ragweed get?
Common ragweed typically grows one to four feet, occasionally six. Giant ragweed usually reaches three to twelve feet and can exceed fifteen in good conditions, which is where its name comes from.
