The herbicide 2,4-dichlorophenoxyacetic acid, better known as 2,4-D was invented in 1945 and has gained a reputation as one of the most powerful tools in the constant battle against weeds in gardens, crop fields and pastures. The herbicide is safe for use around humans and edible plants when used properly, but it cannot control all weeds, and crabgrass is resistant to the herbicide.
The herbicide 2,4-D is designed to kill broad-leaf weeds, which include common lawn pests like dandelions and clovers. However, because the herbicide must kill these common weed without harming the lawn or landscape, it has no effect on grass species. While crabgrass can take over a lawn and cause serious troubles for a landscaper, it is still a type of grass and is therefore unaffected by the chemical powers of 2,4-D.
While it may look like other types of grass at first glance, crabgrass gets its name from its distinctive spread, which starts at a central stem or root and sprouts blades that resemble a crowd. The grass is coarse and generally takes on a lighter green color than many other grasses. The grass sprouts in a matter of weeks and spreads by dropping its seeds each fall, ensuring that it will invade the same area next year.
Crabgrass thrives in hot, dry weather, which is generally the time that other species of grass are struggling for survival. The quick spread and fast growth of crabgrass allows it to quickly choke out other grasses, absorbing all of the water and nutrients from the soil and leaving other species to struggle and die.
While 2,4-D doesn't work on crabgrass, there are a number of herbicides that will. If you know you have a crabgrass problem, apply a pre-emergent herbicide such as benefin or bensulide in early spring, before seeds begin to appear and sprout. If the crabgrass has already taken hold or it is too late for a pre-emergent chemical treatment, use MSMA or MAMA. All of these herbicides are available for homeowner use at garden centers and nurseries; follow all instructions and warnings carefully.
Crabgrass Lawn Treatment
Keep your grass mowed higher than the general recommendation for your species; if you are unsure, keep it higher than 3 inches. Higher grass will shade the weeds and keep the soil cool. Remember that crabgrass grows in hot, dry areas. Water your lawn to a depth of 4 to 6 inches but only once every week and a half to two weeks. If you water shallowly and often, you are allowing the weed seeds constant access to moisture. Fertilize only in the early spring and late fall, when the grass can take advantage of the food but the weeds cannot.