Leaf:  Mostly opposite, simple; broadly ovate to orbiculate, unlobed or deeply 3-lobed (sometimes 5-lobed), pointed tip, symmetrical base, lower leaves up to 30 cm across and 35 cm wide, upper leaves smaller; margin finely serrate; upper surface dark green, rough, sparsely covered with tiny, stiff hairs; on long petioles. 

Flower:  Small, greenish heads of staminate (male) or pilstillate (female) disk flowers, separate from each other on the same plant, occurring in long, slender, nodding racemes at the ends of branches (male) or in the axils of the upper leaves (female) below the spikes of male flowers; each flower small, greenish; staminate flowers being cup-like with the 3 longest lobes blackish along the midveins, pistillate flowers enclosing a single ovary; blooming August through October; wind-pollinated.

Fruit:  Bur-like, warty, single achene, stoutly blunt-beaked (beak 1 mm long) at tip, beak surrounded by crown of 5 to 8 short, thick, blunt teeth; achene 6 to 12 mm long; brown to gray; with a single small seed. 

Stem:  Erect, single or branching above, coarse, woody at the base, with lengthwise black lines, sometimes pubescent (covered with soft to bristly hairs) or reddish; 1 to 4 m tall. 

Form:  Annual, many-branched, rangy, rough-looking forb. 
 

Discussion

The genus name Ambrosia is Greek for "the food of the gods".  This name doesn't seem to fit because the plant is bitter-tasting enough to be avoided by most species of grazers.  The species name trifida means "divided into 3 parts", referring to the large 3-lobed leaves.  The common names kinghead and crownweed refer to the crown-shaped nature of the tiny fruits.  

Deer will eat the leaves of giant ragweed; songbirds, pheasants, and quail consume the seeds.  Seeds are high in nutrition with 47% crude protein and 38% crude fat, making a valuable food source for rodents.  Stands of giant ragweed provide shelter for much small wildlife.  A small patch of giant ragweed may grow into a huge stand of thousands of plants covering several acres.

Native Americans collected the fruit of this plant for food. They used the fibers from the stem to make thread, and chewed on the roots to drive away fear at night. 

An 1898 publication (cited below) states that giant ragweed was used for treatment a variety of ailments from sore mouths, ulcers, and fevers to gonorrhea.  

Horses are the only livestock that can consume the entire plant, and farmers of the past sometimes cut giant ragweed and laid it aside to dry for horse feed during the winter.  So, one of its common names is horseweed.   

Giant ragweed is a major contributor to hay fever, as it sheds massive quantities of irritating pollen.  Many people blame goldenrod for their allergies, but goldenrod is only guilty of blooming at the same time as ragweed.  Some people have allergic reactions to touching this plant. 

Distinguishing Characteristics

Giant ragweed is tall and rough-looking with large, 3-lobed leaves and crown-shaped achenes.

Associated Plants

Common ragweed (Ambrosia artemisiifolia), common milkweed (Asclepias syriaca), goosefoot (Chenopodium album), and curly dock (Rumex crispus). 

Distribution: 

Giant ragweed is usually found in loose, moist soil, growing in ditches and fields near crops such as corn and soybeans, in waste places, along railroads, around farm buildings, and in thickets. This plant can be found in every state of the U.S. except Alaska, Hawaii, and Nevada. 

Images

Click each small picture to see larger image. 

October, 2003  JA	October, 2003  JA	October, 2003  SW
October, 2003 SW	October, 2003  JA	October, 2003  SW

Location

N 39.26902 °    W -89.89103 °
The Ambrosia trifida was found along the west end of the fitness trail, 10 paces west of the balance beam in Loveless Park, Carlinville, Macoupin County, IL. 

Copyright

Photos © 2003 Jim Ashby and Steven Wise, Carlinville High School, linke@carlinvilleschools.net       
           

References

Virginia Tech Weed Identification Guide
http://www.ppws.vt.edu/scott/weed_id/ambtr.htm

Connecticut Botanical Society
http://www.ct-botanical-society.org/galleries/ambrosiatrif.html

Weed Science Society of America
http://www.wssa.net/subpages/weed/larrymitich/ragweeds.html

Harvey Wickes Felter and John Uri Lloyd.  1898.  King's American Dispensatory. http://www.ibiblio.org/herbmed/eclectic/kings/ambrosia-trif.html

Plants Profile, Natural Resources Conservation Service, United States Dept. of Agriculture
http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=AMTRT2

Pest Management and Crop Development Bulletin.  April 12, 2002.  University of Illinois Cooperative Extension
http://www.ag.uiuc.edu/cespubs/pest/articles/200203l.html

Illinois Plant Information Network
http://www.fs.fed.us/ne/delaware/ilpin/121.co

Weed Information, California Department of Food and Agriculture
http://pi.cdfa.ca.gov/weedinfo/AMBROSIA2.htm

Tim Trower and Chris Boerboom.  "Giant Ragweed".  Weed Science, University of Wisconsin.
http://ipcm.wisc.edu/uw_weeds/extension/articles/ragweed.htm

Stubbendieck, James, Geir Y. Friisoe, and Margaret R. Bolick. 1994.  Weeds of Nebraska and the Great Plains.  Nebraska Dept. of Agriculture, Bureau of Plant Industry, Lincoln.  589 pp

Mohlenbrock, Robert H.  1981.  Wildflowers of fields, roadsides, and open habitats of Illinois.  Illinois Dept. of Conservation, Division of Forest Resources and Natural Heritage, Springfield.  226 pp.

 Click here to return to the plant list for Carlinville High School's South-Central Illinois flora studies.