Crabgrass is an invasive plant species. While the plant itself is an annual that dies off at the first hard frost, the seeds stay dormant until the following spring thaw. Because of this method of propagation, crabgrass must be treated during the fall to prevent it from reemerging in the spring. Once treatment and control have been achieved, effective maintenance is key to preventing the return of crabgrass.
Crabgrass is killed by exposing the roots to air. This is the standard method to control and destroy crabgrass, because it dries out the roots. Digging out crabgrass roots must be timed during the fall, before the plant goes to seed. If the grass goes to seed, pulling out the original plant will not stop the seeds from producing new plants.
Pre-emergent herbicides are chemicals used to treat lawns for crabgrass in the fall, before it grows. Pre-emergents stop the seeds from germinating and can be specifically designed for crabgrass. Pre-emergents can be used over a wide area because they do not chemically alter any plants for which they are not designed. Postemergent herbicides are chemicals used to treat lawns after crabgrass has begun to bloom or has grown to maturity. Postemergent herbicides are a spot treatment because they are not plant-specific and can affect any plant with which they come into contact.
Various preventative measures can stop the onset of crabgrass invasion into a lawn. Keeping the grass higher than 2 inches allows the grass to shade out the seeds left by crabgrass. Crabgrass needs full sun to grow; cutting grass too shallow allows the crabgrass to compete and then crowd out lawns. Using the highest setting on the lawnmower helps prevent crabgrass from gaining a foothold.
Crabgrass has shallow roots. Because of this, frequent short bursts of water are beneficial to crabgrass, because these plants can grab the moisture before it reaches the deeper root systems of grass. Watering less frequently but for longer periods of time allows moisture to reach deep into the sod so the lawn can make the most of the moisture. One inch of water is the recommended rate for watering lawns.
For lawns that have been adversely affected by crabgrass, there may be many bare spots once the weed is removed. Reseeding should be done in the late autumn to allow the seed to germinate over winter and sprout in spring. Reseeding the area can restore the lawn, but the potential for crabgrass to return is good if the grass was originally prone to succumbing to crabgrass. To prevent this, use grass seed that is less susceptible to crabgrass in place of the original grass. Zoysia grass is one type of grass that can withstand crabgrass incursion.