Control of annual grass weeds, like crabgrass, in the lawn is difficult. Pre-emergent herbicides prevent the seeds from germinating. Timing is important. Pre-emergent herbicides are used when the soil temperature is between 59 F and 65 F - or as the forsythia flowers begin to fade - which is just about now in the Lehigh Valley. 
Chemical pre-emergents provide a chemical barrier in the soil that prevents seed germination. There is a long list of products both with and without fertilizer. Organic corn gluten meal inhibits root development during seed germination, at least partially by desiccating the soil and reducing water uptake. Since pre-emergent applications do not discriminate among seeds, over-seeding the lawn can be done in fall. Whether you use chemical or organic pre-emergent treatment read the label carefully and follow directions. 
Information on “Chemical Weed Control” from Penn State Center for Turf Grass Science Corn meal gluten information from Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor,Puyallup Research and Extension Center, Washington State University.
Chemical pre-emergents provide a chemical barrier in the soil that prevents seed germination. There is a long list of products both with and without fertilizer. Organic corn gluten meal inhibits root development during seed germination, at least partially by desiccating the soil and reducing water uptake. Since pre-emergent applications do not discriminate among seeds, over-seeding the lawn can be done in fall. Whether you use chemical or organic pre-emergent treatment read the label carefully and follow directions. Information on “Chemical Weed Control” from Penn State Center for Turf Grass Science Corn meal gluten information from Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor,Puyallup Research and Extension Center, Washington State University. .
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