Common and Giant Ragweed Identification and Management
February 1, 2025
Common ragweed (Ambrosia artemisiifolia) and giant ragweed (Ambrosia trifida) are two weed species that appear in many fields every year. Both have become more prevalent in some areas and are getting harder to control because of herbicide resistance. At maturity, the plants can be robust with common ragweed reaching heights of 3 to 6 feet and giant ragweed reaching up to 17 feet.1,2 Proper identification is critical for their management.
Regional names for common ragweed include hay fever weed, bitterweed, wild tansy, annual ragweed, and blackweed. Common ragweed has a shallow, fibrous root system with fern like leaves and green to light pinkish red stems that are hairy (Figure 1). Seeds can begin germinating in May, with most seedlings emerging by early June. The first true leaves have one or two deep clefts in each margin that form lobes. Plants emerging in May can produce 30,000 to 60,000 seeds (Figure 2).3 Later emerging ragweed plants produce less seed because flowering is related to day length. Seeds are distributed by animals, birds, water and even equipment. Seeds can survive for 30 plus years in the soil. Soybean yield loss has been reported to be 10 percent when there are four ragweed plants per 30 feet of row.1
Regional names for giant ragweed include buffalo weed, kinghead, crown weed, wild hemp, horse weed, bitterweed, tall ambrosia, and tall ragweed.4 Giant ragweed is a little different from common in plant structure. It has large, simple, opposite leaves that have 3 to 5 deep lobes with teeth like edges Figure 3). Unlike common ragweed giant can emerge as early as March and continue emerging into July. Blooming can occur from July through October. Depending on the crop, this species of ragweed can get 1 to 5 feet taller than the crop. Giant ragweed can produce large crown-shaped seeds with points and ridges along the tops of the seeds (Figure 4). Observations have shown that a single plant can produce around 5,100 seeds in soybean fields and up to 3,500 seeds/sq yard when competing with corn.2
Management
- With associated risks, tillage can be used for management. By tilling the soil, germination can be sped up because seed to soil contact is increased. However, tillage can put the seed deeper in the soil layer and take longer to germinate.
- Practicing no-till can help keep ragweed seed on top of the ground making it harder for the seed to germinate and exposing them to insect seed predators.
- Herbicides - a good burndown program with residual additives is a good start. A rigorous post emergence program with multiple modes of action along with more residual chemistries can help take out ragweed plants before they head out and go to seed. A two-pass program should help increase the amount of control.
Continued field scouting and properly identifying weed species that are present are key steps in managing ragweed species and other weed species.
Channel Agronomist
Michael Mullins
Sources
1Jordan, T., Nice, G., Smeda, R., Sprague, C., Loux, M. and Johnson, B. 2007. Biology and management of common ragweed. The Glyphosate, Weeds, and Crop Series. GWC-14. Purdue Extension Knowledge to Go. Purdue University. https://ag.purdue.edu/btny/purdueweedscience/wp-content/uploads/2021/01/gwc-14.pdf
2Johnson, B., Loux, M., Nordby, D., Sprague, C., Nice, G., Weshoven, A., and Stachler, J. 2007. Biology and management of giant ragweed. The Glyphosate, Weeds, and Crop Series. GWC-12. Purdue Extension Knowledge to Go. Purdue University. https://www.extension.purdue.edu/extmedia/GWC/GWC-12.pdf
3Jordan, J. 2023. Weed of the month – common ragweed. Clemson Cooperative Extension. Clemson University. https://hgic.clemson.edu/weed-of-the-month-common-ragweed/
4Mohler, C.L., Teasdale, J.R., and DiTommaso, A. 2021. Giant ragweed. Manage Weeds On Your Farm. Sustainable Agriculture Research and Education (SARE). https://www.sare.org/publications/manage-weeds-on-your-farm/giant-ragweed/
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