Hello Rick,

To effectively control and prevent the return of Canadian thistles, it's important to employ a combination of strategies, as they are notoriously difficult to eradicate. Here are some recommendations:

1. Mechanical Control: Mowing the thistles can be an effective strategy, but timing is crucial. Mow the thistles after they have bolted (when the flower stalk is typically hollow) but before they have formed flowers. This will help prevent the plants from producing seeds and regrowing.  Probably now, as you indicated that flowers are coming>

2. Herbicide Application: If you're already using herbicides, ensure that you apply them early in the growing season when the thistles are in the rosette stage. Herbicides containing active ingredients such as aminopyralid or clopyralid are somewhat effective against thistles. It's also important to wear proper personal protective equipment when applying herbicides, especially if the thistles are over shoulder height.

3. Sequential Applications: If you're using herbicides, consider applying them in a sequence. After cutting the thistles back two weeks after the initial application, allow them to regrow prior to the second application. This can help manage established populations more effectively.

4. Prevention: To prevent the thistles from coming back, consider implementing measures to prevent seed production. This can include mowing the area before the thistles have a chance to flower and produce seeds.

5. Cultural Practices: Avoid disturbing the soil to prevent the spread of thistles by wind-borne seeds or root fragments.

Remember, controlling thistles can be a multi-year process, and it's important to be persistent with your efforts. 

Here is some more information:https://extension.umn.edu/identify-invasive-species/canada-thistle