How to Prevent Crabgrass
The best prevention for crabgrass is to provide such great growing conditions that the turf crowds it out.
Crabgrass is an annual strain of wild grass (a new plant will grow from seed each year). Seeds travel in the wind and are transported by birds and animals. Once in the ground, the seeds can remain 
dormant for up to 15 years, germinating when conditions are right (the right amount of soil contact, heat and water).
Cutting this grass short during the summer will inhibit flowering and seeding and should help reduce next year’s crop. In the fall, re-seed or patch your lawn. Light infestations can be hand weeded. After a good rain, gather the whole set of leaves in your hand and pull each root out slowly.
If you have crabgrass, mow your lawn frequently to the maximum height recommended for your grass species. The frequent cutting will prevent it from flowering and seeding, and your lawn grass will have an easier time crowding out all other weeds.
Crabgrass Control: Organically
“But do I really need chemicals for crabgrass control?” you ask. The answer is, No. While the best way to kill crabgrass is with pre-emergent herbicides, the best way to control it is by having healthy green grass. Here are some ways to promote lawn health — at the expense of crabgrass:
• Fertilize (compost is fine) more heavily in autumn than spring. By autumn, frosts will have already killed any crabgrass.
• Don’t let bare spots remain uncovered for long, else opportunistic crabgrass will take root. In the fall, fill in those bare spots by over-seeding.
• When irrigating the lawn, water more deeply and less frequently. Crabgrass is a notoriously shallow weed.
• One of the most important tips on mowing lawns that I can offer as it relates to weed control is to “mow “high.” This means leaving the lawn grass at a height of 2 1/2″-3″. Doing so will allow the lawn grass to “protect its own turf” better, depriving crabgrass seeds of the light they need to germinate.
Final Notes on Control
A great organic “weed and feed” product is corn gluten, a safe and effective natural alternative to synthetic pre-emergent herbicides. Synthetic herbicides often contain chemicals that are harmful to people, pets, lawn critters you want to keep (like earthworms), and other parts of the ecosystem. An organic pre-emergent herbicide, corn gluten will suppress germination, while fertilizing your lawn.
Don’t forget good old weeding as a method of control. Hand-pulling small patches before it goes to seed makes eminent sense. To facilitate weeding, water the lawn first (weeds are more easily extricated from wet soil). The grass grows from a central root, spreading tendrils like a crab. (Hence its name). Do it early since this weed grows very fast, and will soon become unmanageable, until the first frost kills it.
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