Put Canada thistle in the crosshairs

We have Canada thistle patches. Residual herbicides won’t touch them. What can we do early postemergence?
The Indiana certified crop advisers panel answering this question includes Gene Flaningam, crops consultant, Vincennes; Carl Joern, field agronomist with Pioneer, northeast Indiana; and Greg Kneubuhler, agronomist, G&K Concepts Inc., Harlan.
Flaningam: Canada thistle is a creeping perennial that reproduces from seeds and a belowground root system. Knowing characteristics of your weed helps determine what chemicals to apply. Look at something systemic such as glyphosate, XtendiMax on Xtendflex soybeans, or Enlist on Enlist soybeans. All these products translocate into the weed for better control.
Joern: The greatest success in managing Canada thistle is when weeds are in the late-bud to early-flower stage. Unfortunately, this isn’t in alignment with your ideal timeline. Pull the trigger on a “late post.” It will target the underground structure of the plant. Glyphosate is the most efficacious option for this extremely competitive weed.
Kneubuhler: Canada thistle is tough to eradicate due to fibrous roots. They can extend 10 to 15 feet wide and deep. It’s a process over time to eliminate them. I highly recommend an upfront residual with dicamba and/or 2,4-D to slow them. In crop, the best is a high rate of glyphosate. Canada thistle is more effectively controlled down to the root when it’s allowed to bud or flower. An early post won’t time that correctly. Basagran can be effective as an early post. Full control can take years. Fall applications of dicamba or 2,4-D can start the process.
Editor’s note: Steve Gauck, a CCA and research agronomy manager with Beck’s, Greensburg, Ind., determined that when Canada thistle patches aren’t sprayed until mid- to late post, soybean plants are affected. They grow taller with fewer nodes. Often, they’re lighter green and play catchup even after thistles die.