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Coming to Grips with Ragweed

Deeply cut leaves, reddish stems, narrow green flower spikes? Yep, that’s ragweed (Ambrosia artemisiifolia)!

If you sneeze more often than normal between the beginning of August and the end of September, if your eyes sting, if you’re short of breath and your energy level is close to zero, you are probably suffering from an allergy to ragweed pollen. Hay fever you may call it, although doctors prefer the term seasonal rhinitis. And you’re not alone: in North America, about 10% of the population suffers from hay fever. And ragweed is by far the most allergenic plant on that continent, accounting for almost half of all seasonal allergies.

There are many species of ragweed, always in the genus Ambrosia (Greek for “food of the gods”, a name that was given long before it was understood it caused hay fever!), but it’s mostly common ragweed, also called short ragweed (A. artemisiifolia) that’s the culprit. After all, it’s by far the most common species.

Common ragweed is an annual native to central Canada and the United States, but it spread throughout North America when native forests were cut and replaced with fields of crops, creating an environment similar to the prairies, the plant’s original home. It’s found almost everywhere except in the far north, although populations are moderate on the West Coast and in Atlantic Canada.

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Distribution of ragweed in Europe in 2012. Photo: European Aeroallergen Network

There are no native ragweeds in Europe, but common ragweed has naturalized there as well, probably brought over in contaminated forage seed. It’s been around since the 19th century and is now spreading rapidly and causing serious symptoms in many areas. It’s believed that it will have spread to essentially the entire European continent except the extreme north within the next 35 years.

Small But Harmful

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Common ragweed: the leaves look a lot like those of wild carrot, but are a yellowish green. Photo: Harry Rose, Wikimedia Commons

Common ragweed is a rather insignificant plant from 4 to 38 inches (10 to 70 cm) in height. Its pinnate foliage is deeply cut, much like that of wild carrot or Queen Anne’s lace (Daucus carota), but is yellowish green in color rather than dark green of the latter. Also, it’s usually covered in fine hairs, especially underneath the leaf. Its cylindrical stems are reddish or brownish green, also usually covered in fine hairs. Its flower spike, found at the top of the plant, resembles a very narrow green pagoda … but you won’t see it until the end of summer. The tiny flowers are green or yellowish green too and are not very showy.

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Common ragweed flowers are easily recognizable, but not very striking. Photo: Meneerke bloem,

Ragweed flowers don’t need to have flamboyant colors, as they don’t have to attract insect lr bird pollinators. They are pollinated by the wind. So clouds of ragweed pollen fill the air – and human nostrils – at the end of summer. Each plant can produce up to one billion pollen grains per year, pollen that can travel up to 400 miles (640 km).

The Blame Game

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No, goldenrod doesn’t cause hay fever in spite of beliefs to the contrary. Photo: Olivier Pichard, Wikimedia Commons

For a long time, North Americans blamed goldenrod (Solidago canadensis) for late-summer hay fever. I know my father did: any goldenrod that sprouted on our lot was rapidly put to death! This plant, with bright golden yellow flowers, blooms at the same time as does ragweed and, since the first late summer hay fever symptoms corresponded with goldenrod coming into it’s very visible bloom, it was believed to be the cause of the disease.

Sadly, many people still destroy goldenrod on the pretext that it causes allergies, but in fact it is essentially non-allergenic: its pollen is too heavy to be transported by the wind. Blame instead its inconspicuous cousin, ragweed (both belong to the same plant family, the Asteraceae), whose light-weight pollen is easily carried on the slightest breeze.

Where Does Ragweed Hang Out?

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Ragweed likes bare or nearly bare soil and tolerates poor soils where few other plants will grow. Photo: R.A. Nonenmacher, Wikimedia Commons

Common ragweed is an annual. It has to start from scratch each spring, sprouting from fallen seeds. Unfortunately, its seeds can remain viable for 40 years or even longer. But to germinate, they need sun. As a result, ragweed won’t tolerate competition from taller plants that are already well-established. It instead settles in spots where the vegetation is either low, sparse or absent, thus allowing the sun to reach the ground.

In addition, ragweed is very tolerant of saline soils. In many yards, the place to look for it is along sidewalks, roads and driveways, where grass lawns grow poorly due to applications of road salt over the winter. They kill the lawn or impede its growth, allowing salt-tolerant ragweed to proliferate. You’ll also find it on vacant lots (landfill is not conducive to dense growth of other plants, but ragweed does fine there) and along roads and railways, in parking lots, etc.

You won’t often find it in flower beds or in wooded areas, because the dense vegetation there keeps ragweed from germinating. The use of mulch will eliminate it completely: it simply can’t germinate in a mulched bed.

How to Control Ragweed

If you want to control ragweed, it’s best to start early, in late spring or early summer. Don’t wait until it starts to bloom in late summer.

You could always treat it with herbicides … except that ragweed has become resistant to many common herbicides, such as glyphosate (RoundUp). And obviously, if you garden organically, you won’t want to use herbicides at all. Fortunately, there are other methods.

Simply mowing ragweed regularly will keep it from producing pollen-bearing flowers.

Since for many homeowners, ragweed is linked to weak or dying lawns near roadways, the best temporary solution is to mow the area frequently. No, this will not kill ragweed plants already present (they’ll sprout again from the base), but at least it will prevent them from flowering. Since common ragweed is an annual, that means it will die with the first hard frosts. Unfortunately, others will sprout in the spring. That’s why mowing can only really a be temporary solution.

For more permanent control, replace weak sod near roadways with either fresh, healthy sod or replace the contaminated soil found there with fresh soil and oversow with quality grass seed. Also, get in the habit, at snow melt, of leaching the soil near the road, letting clear water run for a few minutes to dissolve and carry away salt deposits. That will allow the lawn to grow more densely. And when grass grows densely, ragweed won’t be able to germinate.

Learn to recognize ragweed when it germinates, as above, and you’ll really be ahead of the game! Photo:

If ragweed appears elsewhere on your lot, the easiest action is to pull it out. It doesn’t have a highly-developed root system and is usually easy to yank out. Wear gloves if possible: touching ragweed will not normally provoke dermatitis as long as it’s an occasional thing, but people who regularly handle it can develop contact dermatitis. Yes, another form of allergic reaction!

After you’ve got it out, cover the soil with at least 2 inches (5 cm) of mulch to prevent the seeds from germinating next year or by planting dense vegetation: ground covers for example.

Can You Compost Ragweed?

If the ragweed you pulled out isn’t in bloom, you can add it to your compost bin. But if it is carrying its narrow green flower spikes, be forewarned that the flowers will often mature and even produce viable seed even as the mother plant is dying. Therefore, it’s better to put flowering ragweed plants in the garbage rather than in the compost.

Ragweed should have no place in our gardens.. Make it your duty as a citizen to eradicate it from your own lot… and encourage your neighbors to do so as well!

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