Ragweed (Ambrosia spp.) is a common weed that is unwelcome because of its fine pollen, which causes severe allergic symptoms in sensitive individuals.
Native Americans cultivated the weed as a source of nutrition, and today it is used to absorb pollution from the soil. Despite that, it’s rare that someone welcomes this plant in their garden.
By late summer, the pollen from ragweed is wreaking havoc on people’s sinuses, causing mild, moderate, or severe seasonal allergic rhinitis, more commonly known as allergies or hay fever. This results in sneezing, itchy, watery eyes, and oral allergies.
Controlling ragweed helps alleviate these symptoms, though the pollen can travel for miles on the wind, so unless neighbors take care, you may still experience symptoms.
There are over 50 species in the Ambrosia genus, with 17 species in the US. A majority of them grow in the midwest and the eastern part of the country. It’s native to North America but unwelcome because of its allergenic properties.
Western ragweed (A. psilostachya), common ragweed (A. artemisiifolia), lanceleaf ragweed (A. bidentata), coastal ragweed (A. hispida) and giant ragweed (A. trifida) are the most widespread in the US. The plant can be found in every state including Hawai’i and not all species cause severe allergies.
When people talk about ragweed, they’re generally referring to the common species.
Ragweed isn’t a problem because it outcompetes other plants. It’s a problem because it causes severe allergies in so much of the population. It is even known to cause life-threatening symptoms in those with asthma or an extreme sensitivity to pollen.
You’ll find ragweed everywhere, from urban gardens to rural fields, but it tends to be concentrated in rural areas. Anywhere with disturbed soil that isn’t colonized by tougher plants, like roadsides, vacant lots, fallow fields, or canal sides, can be a hospitable home for ragweed.
The plant prefers dry, warm areas.
The plant is a member of the daisy (Asteraceae) family. It’s an annual or a perennial forb with a shallow root system, though a few species have a rhizomatous root system.
Unlike some weeds, which are readily identifiable thanks to bright flowers or colorful berries, ragweed is nondescript.
The common species is upright and has double compound leaves, which are leaves made up of smaller leaflets that look like copies in miniature of the full-sized leaf. Essentially, it’s a leaf made up of multiple tiny leaves. The foliage is bright green with occasional purple spots.
Giant ragweed is upright and the leaves are palmate (palm-like) with finely serrated margins.
When in flower, both have long racemes with inconspicuous, monoecious yellow flowers. Once the blossoms fade, they are followed by tiny brown, single-seeded fruits called achene.
Goldenrod (Solidago spp.) looks similar, but the flowers are bright yellow when mature, while ragweed flowers are more of a greenish-yellow. The pollen is finer on ragweed, and the flower stalks emerge from multi-branching stems.
Goldenrod, on the other hand, has a single stem. It’s a perennial that returns each year.
Mugwort (Artemisia vulgaris) is related to ragwort, but mugwort leaves are more finely lobed, and if you turn them over, you’ll find fine white hairs.
Ragweed flowering is triggered by daylight hours. When the days start to become shorter, the plant is triggered to start blossoming. It will continue blooming until fall.
A single plant can release up to one billion pollen grains, and the pollen season has been increasing as the climate warms. In areas like Canada, the ragweed pollen season is nearly a month longer than it was 30 years ago.
Once the seeds mature, they can land in the soil and lie dormant for up to 80 years.
Contact with the pollen can cause dermatitis, so wear protective clothing when removing it. The easiest way to kill ragweed is to mow it regularly every three weeks. This will eventually deprive the roots of nutrients.
If you decide to dig the plants by hand, take care to remove them before they’re able to flower. A broad-spectrum herbicide such as glyphosate can also be used, but remember that these are indiscriminate and will kill anything they come in contact with. They also have negative environmental and health impacts, so they should be considered a last resort.
In addition to hand removal or mowing, keeping surrounding plants healthy enables them to push out ragweed.