Crabgrass, an annual weed common to North American lawns, is an iconic scourge of homeowners as weather grows warm. Three common varieties exist: smooth (Digitaria ischaemum), southern (Digitaria ciliaris) and large or hairy (Digitaria sanguinalis). All are pale green with five flat blades and thrive in moist, fertile conditions in warm weather, especially in full-sun.
Overwintering as seeds, germination begins when spring temperatures consistently remain over 55 degrees. Crabgrass propagates through generous seed production. A single plant will produce as many as 150,000 seeds, which spread easily by wind. Because these invasive weeds sit very low to the ground, lawn mower blades do not reach the plant, but will agitate the seed heads, scattering seeds and accelerating propagation. Unmanaged, crabgrass can quickly overtake a lawn or appear along walkways or in pavement cracks during spring and summer months. Crabgrass will not survive the first frost, but the seeds remain to begin the cycle again when warm weather returns.
If crabgrass has been a problem in previous years, seeds lie in wait. The application of a pre-emergent herbicide a few weeks before the last frost date will keep seeds from germinating, solving a crabgrass problem before it begins. Once crabgrass appears, pre-emergent herbicides may still be applied, but other strategies must also be employed to prevent the spread of seeds from developed plants. Pre-emergent herbicides can affect the development of newly seeded lawns. Do not treat newly seeded areas and wait three to four weeks before seeding land on which a pre-emergent herbicide has been applied.
Hand-pulling weeds early will limit spread, but other cultural consideration will help prevent growth. Crabgrass thrives in sunlight. A lawn with bare patches or is cut too short will allow this low-profile weed easy growing conditions. A healthy lawn free of bare patches and cut taller will outcompete these low-profile weeds for sunlight, reducing its spread. In most cases, grass should be cut no shorter than 3 inches. Overseeding in the fall will help produce a thicker turf in the following season, limiting crabgrass germination. Seeding should be limited to cool weather months. Seeding or fertilization of cool weather grasses should be avoided during warm months in which crabgrass grows easily.
Proper watering techniques can also inhibit crabgrass growth. If irrigation is necessary in hot weather conditions, frequent watering encourages crabgrass germination. Instead, soak turf deeply and infrequently, allowing the soil to dry completely between watering.
If cultural controls were established too late to prevent an oppressive crabgrass presence, herbicides may be employed though early summer, but effectiveness may be limited leaving a brown, patchy lawn in its wake.
Although it can be frustrating when crabgrass has overtaken an otherwise healthy lawn, herbicides are of little value as a late-season strategy. The first frost is soon to come, killing the crabgrass and leaving just the seeds behind. Plan a pre-emergent treatment before next season and overseed as needed. With a cultural control strategy in place and vigilant weeding when warm weather returns, next year will bring a healthy, crabgrass-free lawn.
Originally: How to Combat Crabgrass