This leafy weed spreads very quickly throughout the lawn. The leaves start out green but then turn yellow. They form a dense mat and never rise above a few inches. It seems to die out in the winter, then starts back up at the end of spring. The root is just a long single stalk, so it's easy to pull up, but it keeps coming back. I don't want to try any herbicides until I know what this darn thing is. Please help!