A crabgrass patch looks like a coarse, light green starfish pressed flat against the soil while your turf stands upright around the edges. Crabgrass in lawns is a summer annual weed that comes up from seed each spring and finishes a whole life cycle in one season. The plant takes the open ground your turf gave up and holds that ground all summer. In October you get bare soil back.
Seed needs warm soil, steady moisture and high light intensity to germinate, which is why bare dirt and thin turf sprout crabgrass first while shaded, dense lawns seldom do. The first thing above ground is a single blade. You walk right past that blade because you read it as young turf, and by the time the plant stands out, a fibrous root system has locked it in place.
Next the plant flattens out. The lower stems branch and run across the ground instead of climbing, so the patch widens sideways into your turf week by week. Those stems have swollen nodes that root wherever they press into soil. Each rooted node becomes a new crown, so one seedling ends up a small colony. In an open spot with no competition, the stems can push up to 2 feet (60 cm) tall.
The timing is what hurts you. Crabgrass peaks in July and August, the same weeks that slow cool-season turf down, so the weed takes water and root space at the moment your lawn has none to spare. Your grass thins a little more. The patch takes a little more ground.
Seed is the real damage. Seedheads run from July through September, and one plant can produce thousands of seeds in a season. Why crabgrass is bad comes down to that number. Pulling a few plants in August does nothing about the seed already dropped into your soil, and that seed is what greets you next spring.
Frost kills the plants off in fall. Each one turns reddish-brown, collapses and dies, and your lawn goes into winter with crabgrass bare patches where the colonies were. Bare soil warms faster in spring and takes far more light than turf does, which is the exact condition the buried seed needs. The same thin spots fail on you again the next year.
Look at where the patches sit and you can name the cause. A hot strip along the driveway edge. The lane where your mower scalps a high spot every pass. Turf gives up in those places first, and that is where seed finds the bright, open soil every seedling needs.
Michigan State University Extension treats crabgrass in lawns as a sign, not a random invader. Open, weak turf raises soil temperature and cuts the competition, so the weed lands where your grass already gave up ground. A heavy infestation is telling you to raise the mowing height and water deeper and less often. Then get seed into the thin areas this fall.
Mow at the top of the range for your grass type and water deeply and less often. Turf dense enough to shade the ground blocks the light crabgrass seed needs for germination.
Buy the pre-emergent if you want, and the spring application does help. But turf thick enough to shade the ground is what keeps crabgrass in lawns from starting, and that part costs you nothing beyond mowing height and a better watering habit.
Read the full article: Crabgrass: The Timing That Decides Every Season
