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Great Ragweed, Buffalo Weed, Horseweed, Giant Ragweed, Tall Ambrosia - Ambrosia trifida


Family: Asteraceae - Aster family Genus Common Name: Ragweed Native Status: NativeDicot Annual Subshrub Herb
Ambrosia trifida - Great Ragweed, Buffalo Weed, Horseweed, Giant Ragweed, Tall Ambrosia. Ambrosia - the nectar of the gods. An unlikely name for the genus of 24 species native to the United States which causes so much discomfort to us mere humans. At least one of these species is found in every state; Ambrosia trifida is found in all but 3 - Alaska, Hawaii, and Nevada. It is the tallest of the Ambrosia species, sometimes growing to more than 10 feet tall.

The ragweeds generally cause much hayfever, allergic reaction, and asthma exacerbation due to their tiny airborne pollen, which can drift and be inhaled far from the source plant. In addition, this highly persistent plant is considered a noxious weed farmers. According to Wikipedia, it can reduce corn and soybean crop yields by more than 50%.

Found in:
AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, IA, ID, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NH, NJ, NM, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY

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Ambrosia trifida

Distribution of Ambrosia trifida in the United States and Canada:
Map unavailable.
Map courtesy of The Biota of North America Program.
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Site: Hamilton County, TN Date: 2010-August-28Photographer: Gerald C. Williamson
Nikon D60
Tamron SP 90MM f/2.8 AF Macro
The flower clusters form not only terminally on the stems, but in the leaf axils as well.
Ambrosia trifida
Site: Walker County, GA Date: 2010-August-28Photographer: Gerald C Williamson
Nikon D60
Tamron SP 90MM f/2.8 AF Macro
The green/yellow flowers of Great Ragweed produce the pollen which so famously causes hay fever.
Click on the photo for a larger image
Ambrosia trifida
Site: Walker County, GA Date: 2010-August-28Photographer: Gerald C Williamson
Nikon D60
Tamron SP 90MM f/2.8 AF Macro
The lower (and sometimes higher) leaves of Great Ragweed give it the species epithet trifida – having 3 lobes, sometimes 4 or 5.
Click on the photo for a larger image
Ambrosia trifida
Site: Walker County, GA Date: 2010-August-28Photographer: Gerald C Williamson
Nikon D60
Tamron SP 90MM f/2.8 AF Macro
The mid- and upper-leaves are usually not lobed, and are slightly serrate. These and the lower leaves are on distinct petioles which are slightly winged and are hairy.
Click on the photo for a larger image
Ambrosia trifida
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Ambrosia trifida initially published on USWildflowers.com 2010-08-29; Updated 2015-06-24

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All content except USDA Plants Database map Copyright Gerald C. Williamson 2024
Photographs Copyright owned by the named photographer



Code Update 20230302