How to Get Control of Giant Ragweed

Giant Ragweed (2)
Giant ragweed is taking the Corn Belt by storm. The Weed Science Society of America (WSSA) recently published an update on the progress of the weed’s distribution and herbicide resistance across the U.S.

The weed is most abundant near the upper Mississippi River and north of the Ohio River in Indiana and Western Ohio. Farmers outside of these locations aren’t in the clear quite yet though. Fields that employ minimum tillage, continuous soybeans or use the same herbicide mode of action year after year increase giant ragweed populations.

Difficulty Managing Ragweed


Data courtesy of Weed Science Society of America

In 60% of the counties studied, giant ragweed was resistant to ALS (group 2), glyphosate (group 9) or both. Farmers should be on the lookout for the summer annual now and have a plan in place for control. Weed scientists recommend spraying the weed before it is 4” tall and using multiple, effective herbicide modes of action.

Left unchecked, giant ragweed can cause significant yield loss. University of Illinois research indicates one plant per square meter in soybeans can reduce up to 52% of yield and two per square meter in corn reduces yield by up to 37%.

ALS Inhibitor and Glyphosate Resistance

ALSLegend GlyphosateLegend
Data courtesy of Weed Science Society of America

Look for these characteristics while scouting giant ragweed and be prepared to act quickly:

  • Cotyledons leaves are round, thick and large with purple hypocotyl.
  • Hairy stems.
  • True leaves are opposite, three- or five-lobed, hairy, with toothed edges and can be 4” to 8” wide by 6” long.
  • Small, green flowers, but crop experts urge growers not to let giant ragweed get to the flowering stage. 
The best time to kill a weed is before it emerges. But if the weed has already emerged, be sure to catch it before it’s over 4”--some herbicides won’t be effective on taller giant ragweed plants.


Are you seeing giant ragweed in your fields? How are you managing it? Let us know in the comments. 

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