The only organic methods of controlling this weed are to improve the growing conditions of your grass. Is your soil well aerated or could it use a good coring at some point? Make sure that your maintenance practices encourage lush, thick turf but be sure not to over water or over fertilize. Your grass should be kept as high as possible for the variety of turf that you are growing...there's a direction correlation between the length of the grass blade and the vigor and length of the root system. Have a weed pulling session occasionally.