If you’ve kept up your pastures by liming, fertilizing and renovating when necessary, you may not have weeds to worry about. However, if you’ve got a field where the forage stand is getting weak or soil fertility isn’t up to par, weeds may be showing up.

Perennial weeds are often the toughest and most troublesome weeds in a pasture. In this part of the country, Canada thistle is an opportunistic perennial weed that is tough to control once it’s established.

If you’ve ever see its root system, you understand why. Merrill Ross, a retired Purdue University Extension weed specialist, used to display a tall, narrow wooden box with a Plexiglas front at field days. The box was about 4 feet tall and filled with soil. Once Canada thistles planted at the top of the box matured, the entire area below the soil line was full of roots. If you destroy top growth but don’t harm the roots, new shoots will appear and form healthy plants again.

Strike soon
The key to controlling Canada thistle is to apply the right herbicides that will translocate into the roots at the right time. If the plant moves herbicide into the roots, you have a better chance of destroying the roots and attaining real control of thistles. If herbicide doesn’t move into the roots, then at best, only top growth will be killed.

Herbicide options for this fall in many pasture situations include 2,4-D, Banvel and Crossbow, or generic versions of Crossbow. The active ingredient in Crossbow, triclopyr, is marketed under various product names. You won’t be able to apply these herbicides if lactating dairy animals are grazing in the pasture, or if there are legumes present. If it’s a grass pasture and lactating dairy cows aren’t present, then these options should be available to you. Read herbicide labels carefully before applying. 

You may also want to check out Milestone, a newer herbicide for broadleaf weed control in grass pastures from Dow AgroSciences. The active ingredient in Milestone is aminopyralid. It’s actually labeled to take out legumes, so it’s not an option if you want to keep legumes in your forage mix.

Right time
Fall is an attractive time to go after Canada thistle and other perennials with similar underground rooting capability because the plants are typically actively growing. Their goal is to transport food into the roots to serve as reserves during the winter. If you apply a herbicide that translocates to the roots, then it will be moved down the stem into the roots along with the potential food reserves.

Once you get herbicide into the roots, you have a better chance of actually killing the plant. Dead roots can’t send out new shoots in the spring.

You’re more likely to be successful if the thistles were mowed earlier. Herbicide applications are usually more effective before seed heads appear.

Wiping out every thistle in one application is likely unrealistic. But if you hit the patch hard this fall and then follow up next spring, you can make progress.

Parker raises hay, cattle and timber near Morgantown.