Weed of the Week: Horseweed (marestail)

Horseweed lifecycle photos
Photos: Horseweed seedling (Penn State), young horseweed rosette (Virginia Tech), large horseweed mid-summer (Virginia Tech), horseweed seeds (Penn State)

Have you seen horseweed beginning to pop up with the recent spring weather? At this time of year, you could be seeing either spring-emerging horseweed or fall-germinated seedlings that are re-emerging for the spring. Now is the time to begin plans to till or use an herbicide burndown so that the field is free of horseweed prior to putting crop seed in the ground.

Horseweed is a winter annual that emerges in both the spring and the fall, growing to between 4-6 feet tall by August. The seedlings that overwintered have begun to re-emerge in some parts of the country, and spring seedlings emerge from late March to early June. Over 80% of fall-emerged seedlings can survive into the spring and re-emerge with more developed taproots that can make them more challenging to control. Horseweed is a particular problem in no-till fields because it prefers less disturbed habitats. Because of this, tillage is very effective at knocking back horseweed. Glyphosate-resistant populations of horseweed continues to spread in major growing regions of the US. There are also populations resistant to ALS (group 2), ALS+glyphosate, and group 22 herbicides.

Here are 8 important IWM tips for successful management of horseweed:
  1. Control before 5” tall – plants above this height can survive most herbicide applications
  2. Control fall-emerged seedlings in the fall to prevent spring re-emergence
  3. Use multiple effective herbicide modes of action
  4. Shade it out with cover crops, narrow row spacing, and quick canopy closure – horseweed isn’t very competitive under shade
  5. Rotate in corn, small grain – soybean management options are limited for horseweed. Rotating in corn & small grains increases options for mowing, herbicide rotation, and breaks the lifecycle of horseweed.
  6. Start clean – control all horseweed before crop planting
  7. Spray residuals for late-spring emerging plants
  8. Don’t depend on POST applications – options are very limited, especially in soybean.