September brings cooler weather and with cooler weather brings cool season weeds. Pre-emergent should be applied before cool season grasses and weeds start to appear. A pre-emergent forms a barrier in the soil. That barrier prevents seeds from sprouting and growing. For the organic gardeners apply corn gluten meal at the rate of 20 lbs per. 1000 sq. ft. Corn gluten meal has a 9% nitrogen content which will help keep the lawn green. There are also several chemical pre-emergent on the market. Hi-Yield Weed and Grass Stopper w/ Dimension, Hi-Yield Crabgrass Control, XL 2G and more. All of these will work well if applied early before winter weeds and grasses start to sprout. Remember that any pre-emergent will stop all seeds from sprouting. So do not apply where over seeding on lawns spread or gardens will be planted.