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Giant Ragweed, Allergy-Causing Weed Par Excellence

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"Giant" ragweed is no misnomer, as this picture illustrates. Click the picture to access the mini-photo gallery for more pictures.

David Beaulieu

Taxonomy of Giant Ragweed:

Plant taxonomy classifies giant ragweed as Ambrosia trifida. Although this noxious weed is a very different-looking plant from its smaller relative (Ambrosia artemisiifolia), both are responsible for fall allergies.

Plant Type:

This allergy-causing weed is a broadleaf annual plant.

Characteristics of Giant Ragweed:

Under the right conditions, there's one characteristic of giant ragweed that you can't miss: namely, its height (see picture at right). It is not uncommon for this Goliath to stand 15 feet tall or more! The stalk on such a plant will be thicker than a broom handle and may bear many large branches. However, the plant is otherwise unremarkable, even as weeds go. It bears inconspicuous yellow flowers; nor does its foliage offer much interest.

Identification of Giant Ragweed:

Click on "More Images" under the picture (above, right) to open the mini-photo gallery, where you'll see a photo of two individual leaves from this plant. Notice that one of the leaves is 3-lobed (its most common leaf-type), while the other is 5-lobed. This variation in leaf shape makes identification somewhat tricky for beginners.

Where Giant Ragweed Grows:

This weed is indigenous to North America. It can be found in every state in the continental U.S. except Nevada and is also widespread in Canada. This notorious source of fall allergies thrives in disturbed soils and is frequently found along roadsides.

Fighting Fall Allergies: Getting Rid of Giant Ragweed:

Since it spreads via seed, efforts at getting rid of giant ragweed should focus on preventing seed production. As a bonus, if eradication occurs prior to flowering, you'll minimize your fall allergies -- at least in your own yard. Hand-pulling is the best method of getting rid of giant ragweed for homeowners, as the plants are easy to pull. So put on your garden gloves and do some weeding! Roundup herbicide is sometimes used, but according to the University of Illinois, instances of resistance in giant ragweed have been reported.

Name Origin for Ambrosia Trifida:

The scientific names of plants usually shed light on plant discussions. But in this case, the scientific name, Ambrosia trifida, leaves something to be desired. Ambrosia was the food of the gods in Greek myth; while I've never tasted giant ragweed, my guess is that the gods could have done better (perhaps they were immune to fall allergies!). The word, trifida, refers to the leaf, which is often 3-lobed. But as my photo shows, sometimes the leaf is 5-lobed.

Culprits of Fall Allergies:

As with so many plants considered baneful in the 21st century, giant ragweed was used medicinally by the denizens of tougher eras. But when one thinks of the plant nowadays, one thing comes to mind, and that is fall allergies. Together, common ragweed and its taller cousin account for most of the hay fever experienced in North America in the fall. Symptoms of this fall allergy are sneezing and runny nose, along with itchy eyes.

Goldenrod, another weed (or a type of wildflower, if you prefer), is commonly blamed for causing such "hay fever." But goldenrod is merely a victim of circumstance: it just happens to bloom at the same time of year (late summer to early fall) as ragweed. Being by far the more conspicuous of the two, goldenrod has become the scapegoat for fall allergies. The fact is that goldenrod pollen is sticky and can be spread only by insects, not the wind. By contrast, ragweed pollen floats off easily on the gentlest of breezes -- much to the regret of your sinuses.

Would you like a closer look at this weed (and its little brother), so that you can be certain of what it looks like? View my larger pictures of ragweed.

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