Best Time to Kill Thistles such as Non-Native Canada Thistle? Spring and Fall!
Spring and fall are excellent times to attack non-native thistles such as Canada thistle (Cirsium arvense). Here’s a photo of a very young Canada thistle rosette taken in early spring in a small prairie area that we are restoring.
I used our Large Foam Herbicide Dispenser with the mesh brush attachment to wipe foam herbicide on the rosettes. One nice thing about early spring is that the plants are well spaced and you can easily isolate the target weed. Thus, you can avoid harming desirable native plants.
As shown below, you can get a perfectly targeted application of the foam weed killer by wiping the foam directly onto the leaves of the thistle.
In terms of herbicide rates, most labels for glyphosate suggest rates that are pretty high for wiping. In some cases, the recommended wiping solution may have 20% active ingredient. In my opinion, this is too high. (That high rate may be suggested because conventional wiping applications with sponges or fabric wipers result in a lot of drippage.) Foam herbicide reduces the drippage to a minimum. Using foam herbicide, I have had success with rates that are about 4% to 8% active ingredient.
In terms of timing, in the spring make sure you treat the Canada thistle in the rosette stage before it starts to bolt. Once it is bolting, it will be hard to control with herbicide. In the fall, wait until you have had a frost or the weather has become pretty cold in your area. As long as the thistle leaves remain green, you can treat them.