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Common Ragweed
The plant actually responsible for late-summer hay fever, and the reason goldenrod gets blamed unfairly. A native annual of bare ground with drab green flower spikes and about a billion wind-borne pollen grains per plant - but an annual with a shallow taproot, so pulling it in spring genuinely works.
Ragweed's hazard is its pollen, not its sap. One plant releases roughly a billion wind-borne pollen grains between August and the first hard frost, and ragweed is the leading cause of late-summer hay fever in North America. Pollen travels for hundreds of miles, so clearing your own yard helps your neighbourhood more than your own sinuses. Handling it can also cause an itchy contact rash, so wear gloves and long sleeves, and cut it BEFORE it flowers rather than while it is shedding.
Overview
Common ragweed (Ambrosia artemisiifolia) is the plant behind your late-summer misery. It is an unremarkable-looking annual, 1 to 4 feet tall, with hairy stems and deeply cut, fern-like leaves, growing on bare and disturbed ground - roadsides, field edges, driveways, thin lawns, construction scrapes, and neglected vegetable beds. Its flowers are tiny, green, and petal-less, carried in slender spikes at the stem tips, and almost nobody notices them.
That anonymity is the whole problem. Ragweed is wind-pollinated, so it spends nothing on petals and everything on pollen: a single plant releases on the order of a billion pollen grains in a season, grains light enough to travel hundreds of miles and to be found miles up in the atmosphere. Ragweed pollen is the leading cause of late-summer allergic rhinitis in North America, affecting something like one American in six, from about mid-August until the first hard frost.
Meanwhile goldenrod blooms in the same ditches at the same moment with big showy yellow plumes, and takes the blame every year. Goldenrod is insect-pollinated: its pollen is heavy and sticky and stays on the flower, and it almost never reaches anyone's nose. If you are sneezing in September while looking at goldenrod, the culprit is the drab green plant standing next to it.
One piece of genuinely good news: unlike almost every other weed on this list, ragweed is an annual with a modest taproot - no rhizomes, no creeping roots, no regenerating fragments. Pull it in spring and that plant is gone forever. The catch is the seed bank, which lasts decades, so the job is a yearly one. Nobody plants it, and the app will never offer it; this page is here so you can recognize it and remove it in time.
Quick facts
For identification. Not a growing guide.
- Life cycle
- Annual
- Where it grows
- Full sun · Part sun, well-drained soil
- Height
- 36 in12 to 72
- Spread
- 18 in8 to 36
- Cold
- Killed by frostdamaged below 30°F
- Heat
- Struggles above 105°F
- Self-sows
- Yes. Never let it set seed.
Through the year
How to recognize it in each season.
Spring seedlings (the cheapest time to remove it)
Mar to May, every year- Recognize the seedling: fern-like, deeply cut leavesRagweed comes up early in spring on bare ground, with the first true leaves already showing the plant's signature: deeply cut, fern-like or carrot-top-like foliage, hairy, dull green, and roughly the same colour on both sides. Look for it in the places bare soil sits in full sun - garden edges, driveway strips, thin lawn, construction scrapes, and last year's vegetable beds.
- Pull it now - it is an annual with a shallow taprootThis is the great advantage of ragweed over most weeds on this list: it is an ANNUAL with a modest taproot and no rhizomes, no creeping roots, and no fragments to worry about. A seedling pulls out cleanly and never comes back. Ten minutes in April is worth a whole miserable September, and unlike bindweed or thistle you are not fighting an underground network.
- Cover the bare ground it needsRagweed seed needs light and bare soil to germinate. Mulch, cover crops, dense planting, and a thick lawn all shut it out - it cannot establish in closed vegetation. Covering the ground is the only measure that reduces next year's crop as well as this year's.
Key signs- Fern-like, deeply cut hairy seedlings on bare ground
- Bare, sunny, disturbed soil - the only place it establishes
- Confusion with mugwort seedlings, which have silvery-white leaf undersides
Bushy growth (the last easy window)
May to Jul, every year- Get it out before the last week of JulyCommon ragweed grows into a bushy, hairy plant 1-4 feet tall through early summer. Pull, hoe, or mow it out before flower spikes appear - in most of the country that means finishing by late July or the first days of August. Everything you remove now is pollen that never enters the air, and the plant is still easy to pull.
- Confirm the ID against mugwort and giant ragweedMugwort (Artemisia vulgaris) has similar dissected leaves but distinctly SILVERY-WHITE undersides and a strong sage-like smell when crushed; ragweed leaves are dull green on both sides and only faintly aromatic. Giant ragweed (Ambrosia trifida) is the other ragweed - much taller, to 12 feet or more, with big leaves in three or five broad lobes rather than fern-like dissection. Both giant and common ragweed are serious pollen producers.
Key signs- Bushy hairy plants 1-4 feet tall by midsummer
- Late July - the deadline for removal before flowering
- Giant ragweed towering nearby, with three- to five-lobed leaves
Pollen season (August to first frost)
Aug to Oct, every year- Drab green spikes - and why goldenrod gets the blameRagweed flowers are tiny, green, and petal-less, carried in slender upright spikes at the stem tips. They are wind-pollinated, so they invest nothing in looking attractive, and most people never notice them. Goldenrod blooms at exactly the same time in exactly the same ditches with big showy yellow plumes, and gets blamed - but goldenrod pollen is heavy and sticky and carried by insects, and it almost never reaches your nose.
- Cutting now still helps, but do it defendedRemoving flowering ragweed still cuts off the rest of the season's pollen, and it stops seed set. But you will be working in a cloud of the exact thing you are allergic to: wear gloves, long sleeves, and a well-fitted mask, choose a still or damp morning rather than a dry breezy afternoon, and shower and change clothes afterwards. Mowing works too, though a mown plant will branch and re-flower lower down, so repeat every three weeks.
- Stop the seed - it lasts decades in the soilA single common ragweed plant can produce a few thousand seeds, and buried ragweed seed has stayed viable for 30 to 40 years in long-running germination trials. You cannot clear that bank, but you can stop feeding it. Cut or pull flowering plants before the seed hardens and browns, and never let a patch stand through to fall unmanaged.
Key signs- Slender green flower spikes at the stem tips - pollen is already moving
- Peak symptoms mid-August through September in most of the country
- Seed hardening and browning - remove plants before this
First hard frost (pollen season ends)
Oct to Feb, every year- The frost that ends the season - and what it does not endThe first hard frost kills common ragweed outright and ends pollen season, which is why hay fever sufferers watch the forecast in October. But the plant has already shed its seed into the soil, where it will stay viable for decades. Nothing about the plant survives winter; everything about the problem does.
- Cover bare ground before springThe whole battle is decided by whether next spring's soil is bare and sunny. Sow a cover crop, mulch beds, overseed thin lawn, and leave as little exposed soil as you can over winter. This is far more effective than anything you can do next August.
Key signs- First hard frost - the end of pollen season
- Bare ground going into winter, which is next year's ragweed crop
Hazards
Decades-Long Seed BankSeverity High DisordernowSeedlings appearing every spring on the same bare, sunny patches; seed hardening and browning on plants left standing into fall.
Seedlings appearing every spring on the same bare, sunny patches; seed hardening and browning on plants left standing into fall.
Pull seedlings in spring while they are small; as an annual with a modest taproot and no rhizomes, every plant removed before flowering is permanently gone. Expect several years of declining numbers rather than an immediate clearance.
Never let a plant set seed - remove or cut flowering plants before the seed hardens. Keep the ground covered with mulch, cover crops, dense planting, or thick turf, because the seed needs light at the surface to germinate.
A single common ragweed plant sets a few thousand seeds, and buried-seed trials have recovered viable ragweed after 30 to 40 years in the soil. That bank is why ragweed reappears on the same bare ground every summer regardless of how completely it was cleared the year before, and why control is an annual habit rather than a one-time job.
Ragweed Pollen (late-summer hay fever)Severity High DisordernowRagweed allergy commonly cross-reacts with melon, banana, cucumber, zucchini, sunflower seed and chamomile tea (oral allergy syndrome), and with mugwort pollen.
Slender green petal-less flower spikes at the stem tips from early August; symptoms peaking on dry, breezy mid-mornings and easing after rain and after the first hard frost.
Cutting during pollen season still stops the rest of the season's pollen and prevents seed set; wear gloves, long sleeves and a well-fitted mask, work on a still or damp morning, and shower and change clothes afterwards. For symptoms, track local pollen counts, keep windows shut in peak weeks, and speak to a clinician about antihistamines, nasal steroids or immunotherapy - this is general information, not medical advice.
Remove ragweed before flower spikes appear - the working deadline is late July across most of the country - and keep bare ground covered so fewer plants germinate each spring.
Common ragweed is the leading cause of late-summer allergic rhinitis in North America, affecting roughly one American in six. A single plant releases on the order of a billion wind-borne pollen grains between early August and the first hard frost, and the grains are light enough to travel hundreds of miles - so local removal helps the neighbourhood more than the individual sufferer. Goldenrod, which blooms alongside it with showy yellow plumes, is insect-pollinated and almost never responsible.
Contact Rash from HandlingSeverity Medium HazardnowAn itchy, spreading rash appearing a day or two after weeding, on the areas that contacted the foliage.
An itchy, spreading rash appearing a day or two after weeding, on the areas that contacted the foliage.
Wash the area promptly and treat as you would any contact dermatitis; see a clinician if it is widespread or persistent. Sensitized people should have someone else do the removal.
Wear gloves and long sleeves when pulling or cutting ragweed, and wash exposed skin and clothing afterwards.
Separate from hay fever, ragweed foliage contains sesquiterpene lactones that cause an itchy contact dermatitis in sensitive people - a delayed, eczema-like rash on the hands, forearms, and any exposed skin that brushed the plant. It is not an urgent hazard, but it surprises people who assume ragweed is only an airborne problem.
Herbicide-Resistant PopulationsSeverity Medium DisorderA treated stand that shrugs off an application which worked in previous years, or that thins only where plants were also cut.
A treated stand that shrugs off an application which worked in previous years, or that thins only where plants were also cut.
If you use a herbicide, choose a product whose label lists the site you are treating (turf, ornamental bed and food garden all have different labels), follow that label exactly since it is legally binding, and do not repeat the same product season after season.
Use physical removal as the primary method - it is fully effective on an annual with no rhizomes - and reserve any herbicide for situations where hand removal is genuinely impractical.
Common ragweed populations across US cropland have developed documented resistance to several herbicide groups, so repeated reliance on a single product is a poor strategy and a common reason chemical control appears to fail. This matters far less for a homeowner, since ragweed is an annual that can simply be pulled.
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