Crabgrass can't grow in a healthy lawn, but anyone who's started with perfect sod and clean, deep topsoil knows crabgrass is the first weed to attempt to invade the turf. Perhaps it's impossible to eradicate crab grass and its weedy relatives, but control is possible. Required are a basic understanding of how crabgrass grows, the right timing and dogged persistence.
The most common varieties of digitaria, or crabgrass, to plague home lawns are the smooth and hairy species. Crabgrass is a grassy weed with long leaves called blades that are lighter in color and wider than most lawn grasses and grow from large clumps. Digitaria species are annual grasses, growing each spring from seed that settles each fall in the top half-inch of the soil.
Because crab grass is an annual grass, it must establish a new root system from seed each spring. To do so, the digitaria seed begins germinating earlier than perennial turf grasses begin to green up. Ohio State University's Extension website explains that seeds begin to germinate in spring when nighttime soil temperatures warm to a minimum of 52 to 54 degrees for at least five nights in a row and enough moisture is available.
Any successful crabgrass management program must make the most of on this early spring period of germination. Overseed and top-dress existing lawns in fall rather than spring to avoid competition; pre-emergent herbicides affect any grass seed, not just crabgrass. Turf grasses that germinate in fall and establish a root system will survive the treatment.
Pre-emergent treatments disrupt the germination process and must be applied in very early spring as the ground begins to warm. Purdue and University of Illinois joint cooperative extension service (PUI) lists benefin, oxidiazon, pendimethalin or a compound containing one of these as effective herbicides. OSU suggests bensulide, DCPA and siduron can give good to excellent results. Experiments at Iowa State University have established that several applications of corn gluten may provide an organic control alternative. Water areas treated with pre-emergent herbicides immediately to carry the herbicide to the seed.
OSU suggests that repeated applications of pre-emergents may keep new generations of crabgrass from emerging in early summer. Monosodium methyl arsonate (MSMA), disodium methyl arsonate (DSMA), fenoxaprop and quinclorac are used as contact herbicides on young plants, but post-emergent treatments are less effective and should be used before daytime temperatures rise above 85 degrees, according to PUI. After plants have become mature in midsummer, mowing to avoid allowing plants to set seed and hand-digging are recommended as the best ways to control crab grass.