Giant Ragweed, Wild Hemp, Tall Ambrosia, King’s Head, Crown Weed, Buffalo Weed, Bitter Weed, ETC.

Ambrosia trifida

am-BRO-zhuh  TRY-fee-duh

Synonyms of Ambrosia trifida (9) (Updated on 11-29-23 from Plants of the World Online): Ambrosia aptera DC., Ambrosia integrifolia Muhl. ex Willd., Ambrosia trifida var. aptera Kuntze, Ambrosia trifida var. heterophylla Kuntze, Ambrosia trifida f. integrifolia (Muhl.) Fernald, Ambrosia trifida var. integrifolia (Muhl. ex Willd.) Torr. & A.Gray, Ambrosia trifida var. normalis Kuntze, Ambrosia trifida var. polyploidea J.Rousseau, Ambrosia trifida var. texana Scheele

Ambrosia trifida L. is the accepted name for the Giant Ragweed. Both the genus and species were named and described as such in Carl von Linnaeus in the second volume of the first edition of Species Plantarum in 1753.

As of 11-29-23 when this page was last updated, Plants of the World Online lists 46 accepted species in the Ambrosia genus. It is a member of the plant family Asteraceae with 1,702 genera. Those numbers could change as updates are made on POWO. The number of genera fluctuates often.

The above distribution map for Ambrosia trifida is from Plants of the World Online. Areas in green are where the species is native and purple is where it has been introduced. The map on the USDA Plants Database for North America (above Mexico) is the same. 

THERE ARE SEVERAL LINKS AT THE BOTTOM OF THE PAGE FOR FURTHER READING AND TO HELP WITH A BETTER POSITIVE ID.

I am not sure I really need to write lengthy descriptions about this common weed. It grows on the farm from one end to the other. It is found in every state in the United States, almost all of Canada, and many other countries. I suppose if you don’t know what ragweed is for sure and you want to find out, then maybe this page and the photos will help you out.

Besides it being one of the major causes of hay fever, it has allelopathic properties that can inhibit the growth and development of surrounding plants.

Ambrosia trifida is a summer annual forb that can grow as tall as 15’ and can form HUGE colonies. It prefers fertile, moist loamy soil but can grow in most soil types. 

They can grow in full sun to part shade. This species can be found along wooded areas, fields, fence rows, back roads, meadows, near rivers, abandoned lots, waste areas, ETC.

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The strong green stems can grow up to 15 feet tall under ideal conditions. The stems have a rough texture because of their short hair. 

The above photo shows how they branch out at leaf nodes…

The above photo shows its rough stems with very short, stiff hairs…

A few big ants were not too happy to see me…

The inflorescence (flower stems) is quite interesting and somewhat complicated to explain.

The staminate (male flowers) grow in spike-like racemes and kind of droop downward. The pistillate (female) flowers grow in small auxiliary clusters and include one floret (petal) and a stout, conical beak… That’s as far as I go without good close-ups… Plants flower from July through September.

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A large colony of Ambrosia trifida along the edge of the south hayfield on 7-17-21.

The leaves grow in an opposite manner along the stems but some of the upper leaves are often alternate. The leaves have long petioles (leaf stems) that are sometimes narrowly winged. The leaves are broadly to nearly circular in outline, 3 or 5 lobed with teeth, and can grow to 12” long x 5” wide. Dry conditions can cause the leaves to turn brown and fall off. Smaller leaves toward the base of the inflorescence (flower stem) are more lanceolate and usually have pubescent (hairy) undersides. 

Another photo showing the rough stems and where the stems branch out at the nodes. This photo appears to show another set of leaves growing rather than branches… Hmmm…

Flowers give way to round to oval fruit that contains large, tough coated seeds. The seeds are dispersed by burs and are viable for many years.

The Giant Ragweed is no joke. They can form huge colonies of very large plants in ideal conditions. They are probably the number one species when it comes to causing hay fever. When flowering and plants are disturbed, they can send out clouds of pollen. I avoid mowing when these plants are blooming. It seems like the older I get, the more susceptible to ragweed pollen I become… A few years ago I mowed off the ragweed on the pond bank. The pollen flew and I have had issues ever since during ragweed season…

Ambrosia trifida behind the chicken house on 9-24-21… The roots of the Chinese Elms are above the ground making it impossible to mow in this area…

Same patch of Ambrosia trifida from a different view…

One of the big problems with farmers and those using spray to control this species is that many populations have become herbicide-resistant…

I live on a small farm in Windsor, Missouri where I enjoy gardening, collecting plants, and identifying wildflowers. The farm is in Pettis County but Henry County is across the street, and Benton and Johnson aren’t far away. I have grown over 500 different plants and identified over 250 species of wildflowers (most have pages listed on the right side of the page). I am not an expert, botanist, or horticulturalist. I just like growing, photographing, and writing about my experience. I rely on several websites for ID and a few horticulturalists I contact if I cannot figure them out. Wildflowers can be somewhat variable from location to location, so sometimes it gets a bit confusing. If you see I have made an error, please let me know so I can correct what I have written.

I hope you found this page useful and be sure to check the links below for more information. They were written by experts and provide much more information. Some sites may not be up-to-date but they are always a work in progress. If you can, I would appreciate it if you would click on the “Like” below and leave a comment. It helps us bloggers stay motivated. You can also send an email to me at thebelmontrooster@yahoo.com. I would enjoy hearing from you especially if you notice something is a bit whacky.

FOR FURTHER READING:
PLANTS OF THE WORLD ONLINE (GENUS/SPECIES)
INTERNATIONAL PLANT NAMES INDEX (GENUS/SPECIES)
TROPICOS (GENUS/SPECIES)
FLORA OF MISSOURI (GENUS/SPECIES)
FLORA OF NORTH AMERICA (GENUS/SPECIES)
WORLD FLORA ONLINE (GENUS/SPECIES)
WIKIPEDIA (GENUS/SPECIES)
USDA PLANTS DATABASE
DAVE’S GARDEN
MISSOURI PLANTS
MSU-MIDWEST WEEDS AND WILDFLOWERS
iNATURALIST
WILDFLOWER SEARCH
ILLINOIS WILDFLOWERS
MINNESOTA WILDFLOWERS
KANSAS WILDFLOWERS AND GRASSES
IOWA PLANTS
PFAF(PLANTS FOR A FUTURE)
GO BOTANY
LADY BIRD JOHNSON WILDFLOWER CENTER
FRIENDS OF THE WILDFLOWER GARDEN
INVASIVE PLANT ATLAS
TREE OF LIFE WEB PROJECT
U.S WILDFLOWERS

NOTE: The data (figures, maps, accepted names, etc.) may not match on these websites. It depends on when and how they make updates and when their sources make updates. Some websites have hundreds and even many thousands of species to keep up with. Accepted scientific names change periodically and it can be hard to keep with as well. Some of the links may use a name that is a synonym on other sites. In my opinion, Plants of the World Online by Kew is one of the most reliable and up-to-date plant databases and they make updates on a regular basis. I make updates “at least” once a year and when I write new pages or add new photos, but I do get behind. We are all a work in progress. 🙂