Giant ragweed is a summer annual known for inducing allergies across the Midwest. Beyond its ability to cause misery in humans, however, the weed can do significant damage to ROI. According to Michigan State University Extension, one giant ragweed plant per 10 square feet can reduce corn yield by 55% and soybean yield by 52%.1
- Common name: Giant ragweed
- Scientific name: Ambrosia trifida
- Cotyledons: Round to oblong in shape with a purple stem
- Leaf shape: First pair of true leaves are unlobed and lance-shaped with toothed margins. Subsequent leaves are 3-lobed (occasionally 5-lobed) and lance-shaped with toothed margins.
- Reproduction: Monoecious (with male and female characteristics on one plant)
- Flowers: Long, slim clusters of small, green flowers
- Common ragweed tends to emerge early in the Midwest growing season, around March. However, some populations can emerge as late as the end of July.
- One giant ragweed plant can produce about 5,500 seeds.
- The seeds are larger than those of other weeds, allowing the plants to emerge from deeper planting depths. Uneven emergence is one factor that makes common ragweed difficult to control.
- Giant ragweed grows very quickly and can reach up to 16 feet high. This is another factor that makes it difficult to control. The size of the plant makes it tough competition for crops for sunlight and nutrients.
- Michigan State University Extension has found one giant ragweed plant per 10 square feet can reduce corn yield by 55% and soybean yield by 52%.1
- According to WeedScience.org, 13 states have reported herbicide-resistant ragweed in various crops. The weed has shown resistance to glyphosate and ALS-inhibitor (Group 2) active ingredients.
A weed control program with multiple modes of action can help control giant ragweed. A strong program should consist of a burndown or tillage, followed by preemergence and postemergence applications that include residual activity. When choosing solutions, keep in mind giant ragweed is resistant to glyphosate (Group 9) and Group 2 active ingredients.
- Crop rotation can help keep giant ragweed at bay. Michigan State University recommends including either forage or small grains in rotation. The weed can’t tolerate mowing, so forage is a good choice and small grains can suppress giant ragweed.1
- Tillage is tricky in the case of giant ragweed, because the practice can not only control emerged seedlings but also stimulate germination.1
1Giant Ragweed: https://www.canr.msu.edu/weeds/extension/giant-ragweed
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