Giant Ragweed, Allergy-Causing Weed Par Excellence

Tall Weed Packed With Allergens

Giant ragweed

Melissa McMasters / Flickr / CC BY 2.0

Taxonomy and Botany of Giant Ragweed

Plant taxonomy classifies giant ragweed as Ambrosia trifida. Plants in this genus are members of the sunflower (or "aster") family, but non-botanists will be hard-pressed to find many similarities between this weed and the plants responsible for those pricey sunflower seeds that you put in your bird feeder for jays, cardinals, grosbeaks, etc.


Although this noxious weed is a very different-looking plant from its smaller relative (Ambrosia artemisiifolia), both are responsible for fall allergies.

This allergy-causing weed is a broadleaf annual plant.

Weed Characteristics

Under the right conditions, there is one characteristic of giant ragweed that you can't miss: namely, its height. 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.

Help With Identification

The shapes of the individual leaves from this plant can vary greatly. On the very same plant, some leaves can be three-lobed (its most common leaf-type), at the same time that others are five-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 United States except Nevada, and it 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 This Weed

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 will 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 have 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 three-lobed. 

Culprits Responsible for 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.