Giant Ragweed and Ragweed Allergy

Plant taxonomy classifies giant ragweed as Ambrosia trifida. Although giant ragweed is a very different-looking plant from common ragweed (Ambrosia artemisiifolia), both are responsible for ragweed allergy.

Giant ragweed is an herbaceous, broadleaf annual weed.

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 giant ragweed 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, giant ragweed is otherwise unremarkable, even as weeds go. It bears inconspicuous yellow flowers; nor does its foliage offer much interest.

Click on the picture at right to open the mini-photo gallery, where you'll see a photo of two individual leaves from a giant ragweed plant. Notice that one of the leaves is 3-lobed (its most common leaf-type), while the other is 5-lobed.

Giant ragweed 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 source of the notorious ragweed allergy thrives in disturbed soils and is frequently found along roadsides.

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 ragweed allergy -- 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 instances of resistance in giant ragweed have been reported.

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

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 "ragweed allergy." Together, common ragweed and giant ragweed account for most of the hay fever experienced in North America in the fall. Symptoms of ragweed allergy are sneezing and runny nose, along with itchy eyes.

Goldenrod, another weed, is commonly blamed for causing this "hay fever." But goldenrod is merely a victim of circumstance: it just happens to bloom at the same time of year (late summer-early fall) as ragweed. Being by far the more conspicuous of the two, goldenrod has become the scapegoat for ragweed allergy. 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!