How to Identify Giant Ragweed

Giant ragweed

Melissa McMasters / Flickr / CC BY 2.0

Giant ragweed (Ambrosia trifida) is a member of the sunflower family, but people who aren't botanists will be hard-pressed to find many similarities between this weed and the bright flowers that produce sunflower seeds. Giant ragweed is categorized as a broadleaf weed for its flat and relatively broad leaves. It is an annual plant, meaning it completes its entire life cycle in one growing season. Although this noxious weed looks different from its smaller relative, common ragweed (Ambrosia artemisiifolia), both are responsible for fall allergies.

Learn what giant ragweed looks like and how to get rid of it in your garden.

Giant Ragweed Identification

Giant ragweed is indigenous to North America. It can be found in Canada and every state in the continental United States except Nevada, with a substantial infestation in Ohio. It thrives in disturbed soils along with row crops like corn and is frequently found along roadsides.

Under the right growing conditions, there is one characteristic of giant ragweed that you can't miss: its height. It is not uncommon for this plant to stand 15 feet tall or more. The stalk on such a plant will be thicker than a broom handle, and it might bear many large branches. However, the plant is otherwise unremarkable, even as often-nondescript weeds go. Giant ragweed bears inconspicuous yellow-green flowers, and its foliage doesn't offer much interest.

The shapes of the individual leaves from this plant can vary greatly. On one single plant, some leaves can have three lobes (its most common leaf type) while other leaves can have five lobes. This variation in leaf shape makes identification somewhat tricky.

Giant Ragweed and Pollen

As with many plants considered baneful in the 21st century, giant ragweed once was used medicinally in years past. But when people think of the plant nowadays, one aspect about it commonly comes to mind: fall allergies. Giant ragweed is a primary source of the pollen that causes hay fever.

Goldenrod 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. However, goldenrod pollen is sticky and can be spread only by insects, not by wind. By contrast, ragweed pollen floats off the plants easily on the gentlest of breezes.

Getting Rid of Giant Ragweed

Ragweed spreads via seed, so efforts to eradicate it should focus on preventing seed production. Frequent mowing in the spring and summer in uncultivated areas will help reduce seed formation.

Hand-pulling in the early spring before flowering is the best method to get rid of giant ragweed. Early eradication prevents seeds from forming. The plant's roots are shallow and come out of the ground easily. However, make sure to wear thick gardening gloves and long sleeves and pants, as ragweed sap can cause skin irritation for some people.

Article Sources
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  1. Ragweed Allergy. American College of Allergy, Asthma and Immunology.

  2. Giant Ragweed. Iowa State University Extension and Outreach.