Crabgrass, the bane of many homeowners, is an annual, smooth-leaved grass that forms a crab-like rosette that can grow up to 6 inches high. The grass typically emerges in the spring when soil temperatures reach 60 degrees Fahrenheit, produces seeds and dies with the first frost. But crabgrass is a tough adversary, making it almost impossible to eradicate. But you can take steps to minimize it, including applying mulch to flower beds and keeping your lawn lush and healthy.
Crabgrass (Digitaria ischaemum) is native to Eurasia but has spread to all parts of the United States. It has smooth, 1/4-inch-wide leaves, and when it's young it is recognizable by its light green foliage. Mature crabgrass typically has dark green foliage that forms a flat, spreading crown. Left unmowed, crabgrass can grow 6 inches tall.
While completely eradicating crabgrass is almost impossible, homeowners should take measures to minimize its growth. Left unchecked, crabgrass quickly invades lawns and flower gardens, crowding out other plants. In a lawn infested with this visitor, the grass is unsightly and uncomfortable on which to walk.
Maintaining a healthy, vigorous lawn will help minimize crabgrass growth, according to the University of California, Davis. Mow the lawn at the height recommended for your particular species, which is usually between 2 1/2 to 3 inches. Irrigate it deeply but infrequently to encourage grass to develop a strong root system, allowing the soil to dry out between watering. Fertilize the grass in September and November, applying 1 1/2 lbs. of nitrogen fertilizer per 1,000 square feet of lawn at each application. Fertilizing in the spring encourages a flush of weak growth, providing optimal conditions for crabgrass infestation, according to Purdue University.
Pre-emergent herbicides should be applied in early spring before crabgrass germinates. These are the safest type to use and are considered the most effective chemical treatments. Apply them as early as March 1 according to package directions. Select pre-emergents, including bensulide and pendimethalin, specifically labeled to treat crabgrass. Pre-emergents will prevent turf grass seed from germinating, as well, so avoid their use if you are trying to grow a healthy lawn.
Treat germinated crabgrass with a post-emergent herbicide, such as fluazifop or quinclorac. These products require careful handling to avoid killing wanted vegetation and are not as effective as pre-emergents. Water the lawn thoroughly before applying them, which is best done on a cloudy, windless day.
Once crab grass has become well-established in mid-to-late July, herbicides are largely ineffective. At this point you're only alternatives are to dig the crabgrass out by hand or keep mowing it and wait for it to die. Crabgrass seeds are viable for three years, according to the University of California, Davis, so preventing the grass from going to seed through vigilant mowing is the most important aspect of managing mature crabgrass. It typically dies with the first frost and won't return the following year unless it has produced seed.