Answer: In garden beds, there are several approaches you should use in combination. First off, never let one go to seed. (Pick off any blooms before they set seed and break apart.) Next, with perennial thistles, dig out the root and repeat as needed. In spring, rake the soil surface to disturb germinating seeds, then apply your summer mulch to smother any that may remain. You may need to apply a herbicide selectively to kill off any persistent perennial thistles. A spot spray or wipe on application of glyphosate for example, be sure to read and follow the instructions on the label.
The best defense against weeds in lawn grass is to grow as healthy a lawn as possible because the turf grass will crowd out most weeds. I would suggest you work with your local county extension to develop a lawn care plan that includes thistle control. If a chemical control is needed, they will have the most up to date recommendations on what is available for use in your state and can tell you when and how best to apply it for maximum results.
Good luck this summer!
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