That is horse mint, and it is sometimes called lemon mint. Strip a few of the leaves off and smell it. it has a nice mint smell. Tec is correct on the coloring. I live in West Texas and my horse mint is a dark purple. The bees love it. In our area, it blooms in late spring, but in a dry year it very well may bloom in the summer dearth if a rain comes in July. I wish I had a hundred acres of it. Oh, and it blooms for several weeks.