Crabgrass--that ugly, sprawling coarse grass that seems to pop up all over the lawn--is an opportunist. It thrives when lawn grasses are stressed. It is a summer annual grass that begins germinating as soon as the soil temperature reaches 55 degrees in spring and continues to germinate until the soil temperature breaks 80; usually sometime during mid-summer. The best way to control this pest naturally is to keep the rest of the lawn healthy and free from stress--and to spend a little time rooting out survivors.
Get out early and aerate the lawn in spring; it brings air and light directly to lawn grass roots and stimulates growth. Rake the lawn vigorously to remove dead grass that can contribute to thatch, another environment in which crabgrass flourishes. Fertilize spring lawns with light (30 percent or less than their yearly requirement) feedings of slow-release nitrogen only.
Try some of the organic alternatives to chemical pre-emergent weed killers. The Northwest Coalition for Alternatives to pesticides suggests corn gluten as a pre-emergent herbicide. Its developer, Dr. Nick Christian at the University of Iowa, recommends application at a rate of 20 pounds to 1,000 square feet of lawn.
By the time the soil temperature hits 60 and you are putting in garden annuals, you'll have some young survivors; try sprinkling baking soda on plants to dry them up or spraying young plants with vinegar, a mild acid. If one works for you, you may be able to save some digging effort later.
Mow high in spring; from 2 1/2 to 3 inches for most grasses. This seemingly counter-intuitive act keeps lawn grass tall enough to shade crabgrass seedlings which don't grow upward but rather spread flat out along the ground. Because you're keeping the grass relatively long during a growing spurt, you'll need to mow frequently, which will stimulate more growth.
Root out survivors while the ground is still soft and moist. Start digging out crowns in early summer before they begin to produce seed. Water deeply and infrequently; crabgrass roots are grow close to the surface and will grab surface sprinkling. Deeper watering gets to lawn grass roots and encourages deeper, stronger growth.
Pamper your lawn grass come fall. Purdue University and the University of Illinois Extensions recommend applying 60 to 100 percent of the nitrogen that your lawn needs in late summer and fall. Keep watering the lawn until just before the first freeze to give grass roots water to store for a strong spring start.