If you want to control crabgrass and other spring weeds organically, try a product called corn gluten. A by-product of corn-feed processing, the granulated gluten prevents seeds from germinating for a five-week period. Crabgrass begins to germinate over a several-week period once soil temperatures exceed 55 degrees.
Lay the corn gluten and water it. You can apply it later this month to prevent subsequent germination of weed seeds. The natural product will prevent other weed seeds from growing but will not kill weeds that have already grown, such as dandelion, clover or henbit.
Corn gluten has the added benefit of adding a nitrogen feed to the lawn; this will thicken the grass and reduce further weed encroachment. Keep corn gluten away from newly seeded lawns or garden beds where annuals seeds have been scattered. Corn gluten brands are available at independent garden centers locally.