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.
Post contains content updated on August 20, 2018:
I remind people every year that late summer and fall are great times to apply herbicides to kill perennial invasive plants. I have discussed the timing of herbicide applications in several previous posts. So, get out and enjoy that cooler weather (it will come) and, for those of us farther north, beautiful fall colors!
In terms of priority, I would put perennial weeds into three groups for purposes of application timing: First are the perennials that need to be treated before a frost. Plants such as bindweed, crown vetch, and Japanese knotweed, die back after a frost. Therefore they need to be treated in late summer or early fall before a killing frost.
Second are the plants such as perennial thistles. Canada thistle is a prime target. Canada thistle can be treated a little later in the fall because it is more frost tolerant. In fact, this frost tolerance can be used to your advantage. If nearby desirable plants have died back or lost their leaves, you can apply herbicide to the thistle with less potential for damage to the desirable plants. Just make sure you apply the herbicide to foliage that is still green.
Third are the woody perennial weeds. These can be treated with cut stem or frill treatments from the fall into late winter (just do it before warm temperatures start pushing plant sugars up to the branches for leaf out). The application should be into the vascular system of the tree or shrub, e.g., cut stump, frill, or injection. The Directions for the Green Shoots Foam Herbicide System show how to do these applications.
You can also do foliar applications to woody perennials in the late summer and fall. Timing can be a little tricky: if you have had a drought, the leaves of the weed tree may be sparser and may not absorb the herbicide as well.