Images: (top) Developing seedhead on a mature ragweed plant (Michigan State University) and (bottom) a young ragweed seedling (Virginia Tech).
Common ragweed is native to the continental US and Canada. Herbicide resistant populations of common ragweed are present in many US states throughout the Midwest, South, and Northeast. Some populations have multiple-resistance to herbicide groups 2, 9, and 14. Herbicide resistant populations require diversified management practices in the absense of these herbicide options. While it is not traditionally a weed of top concern in most states, the development of herbicide resistance makes it a higher concern, and it can dominate fields if small infestations are not controlled. Integrated weed management recommendations include:
- Plant into weed-free soil by controlling all ragweed seedlings prior to planting
- Till at night and/or pre-plant burndown herbicides. Tillage during the day stimulates seedling germination
- Till in the mid-late spring after ragweed seeds have germinated
- Scout often to catch plants before they reach 4-6 inches tall
- Scout 2 weeks after the first post-emergence herbicide application
- Apply herbicides to plants under 4-6 inches tall for optimal control
- Where herbicide-resistant populations are present, avoid using the herbicide groups they are resistant to, and diversify tank mixes with at least two other effective herbicide sites of action (check the resources below for herbicide recommendations).
- A rotation of small grains overseeded with clover suppresses ragweed. Ragweed also does not tolerate mowing well.
Herbicide resistant populations:
- Group 2 (ALS)
- Group 9 (glyphosate)
- Group 2 + 9
- Group 2 + 14 (PPO inhibitors)
- Group 2 + 9 + 14
Locations of resistant populations:
Alabama, Arkansas, Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Jersey, North Carolina, North Dakota, Ohio, Pennsylvania, Tennessee, South Dakota, Virginia, Wisconsin (Search by State to find which resistant populations are present in each state)