Last year my son's lawn and area underneath an evergreen tree had lots of thistles growing in it. I dug some out and sprayed the others. The yard looked better late summer/fall. The thistles are back this year. I'm wondering what is the recommended way of removing them? Spraying or pulling them out? If spraying is more effective, is there a chemical I should be looking for when I buy the product? What is recommended for preventing them in the future? Thanks, Rick