Thistle- If you reside somewhere within the whitetails’ range, thistles can be a problem in your food plots, especially in your perennials. They can be enduring and difficult to eliminate. However, with a little persistence and ingenuity they can be eliminated. Sometimes you may need to use more than one method to eradicate these wicked weeds.
Frequent and aggressive mowing is one of the best deterrents for thistles in perennials. You want to keep the thistles from going to seed. If you can keep this up for the entire year without letting them flower and seed, you should be in good shape for next year. In bad cases it can take several years to get the problem under control.
Spot spraying with glyphosate can also work. This; however, will leave you with bare spots, so you will also need to seed the areas vacated by the thistles and the residual clover that you also kill with over-spray. Otherwise more unwanted weeds will fill the bare spot. This is very labor intensive, but can work well.
A Grass Works Weed Wiper (http://www.weedproblems.com) is the best tool for the job. Because the thistles grow a little taller and faster than the perennial clovers and chicory, a Weed Wiper will take out the thistles with an herbicide application, but the applicator will travel above your desired crop leaving it unharmed.
Back before the Weed Wiper was invented, some managers used to take an old blanket soaked in a glyphosate solution and stretch it around the bucket of a tractor – then set the bucket at a height so it traveled above the clover, but low enough to touch the thistles (or whatever weed you wanted eliminated, so long as it was taller than your clovers).
Whatever you do, DON’T just “let the thistles be.” They will go to seed and keep spreading. Besides maintaining their foothold where they are, the wind will carry the seeds and they’ll take up residence wherever there is an open patch of dirt. KILL THEM DEAD!