Crabgrass: Fighting Lawn Cares Enemy

For all the fear "crabgrass" strikes in homeowners, it is a relatively simple plant to control. The secret to fighting crabgrass involves learning its modus operandi and applying control strategies before and after it strikes.  Read on to find out how to win the crabgrass battle.

The Enemy's Profile

Crabgrass is a summer annual plant, which means it reproduces from seed each year. Two main crabgrass types likely appear in homeowners' yards - large crabgrass (Digitaria sanquinalis) and smooth crabgrass (Digitaria ischaemum). The plant's light apple green color and soft blades easily identify itself to homeowners. Tiny hairs also appear on the blades. Crabgrass is sometimes confused with either quackgrass (Agropyran reptans) or tall fescue (Festuca arundinacea), particularly in the spring, as both of these are also unattractive and stand out in the lawn. Because crabgrass does not germinate until mid-to-late spring, it is typically a very small plant during the growing season, and should not be confused with larger weeds like quackgrass or tall fescue.

Once it germinates, crabgrass can dominate lawns very quickly. Crabgrass germinates in the spring when soil temperatures consistently reach 55°-58°F then persists throughout the summer until the first frost. The plant germinates throughout the entire growing season, from late spring through summer. In fact, if the appropriate temperature and amount of light are available, crabgrass will germinate after every rainfall or irrigation. Prior to the first frost, crabgrass goes to seed, thereby ensuring its continued existence the following year. The length of the crabgrass season depends on when soils warm to proper germination temperatures and when the first frost occurs.

Attack Strategies

Crabgrass can be controlled both before and after it appears in a lawn through pre- and post emergent methods. There are also many basic lawn maintenance techniques that can reduce the likelihood of a crabgrass invasion.

Pre-emergent Control

The best and easiest strategy for controlling weeds is to attack them in a juvenile state. For crabgrass, this means preemergent herbicide control. (The term "preemergent" can be slightly misleading - the crabgrass plant does germinate, and the emergent leaf of the small, young plant absorbs the herbicide and quickly dies.) The preemergent herbicide lasts four to six months, preventing crabgrass from establishing throughout the season. Following are tips for preemergent herbicide timing:

• Apply the herbicide on the lawn before crabgrass appears. The ideal window is between four to six weeks long, when the first signs of spring appear and when the soil temperatures reach 55°-58°F. 
• The forsythia bush is a good indicator for application timing. Apply the preemergent after its bright, yellow flowers bloom, but before the blooms drop.
• If the window has passed and you haven't applied the preemergent, you may have missed the first germination of crabgrass. Go ahead and make the application, but watch for crabgrass that may still appear and be prepared to control it on a post emergent basis. This will likely only occur once in early summer and then the preemergent will control the crabgrass from then on.
• Some herbicides have both pre- and post emergent capabilities, such as Dimension®. This herbicide is a good choice if you are late in the application window.

One issue with preemergent herbicides is that most have what is known as non-selective control of grass seedlings and young grass. This means that if you are establishing a yard or repairing bare spots, the preemergent herbicide can prevent the grass seedlings from properly germinating. There is one selective herbicide available to control crabgrass when establishing a lawn, called siduron (Tupersan®). It can be applied with turfgrass seed and will only prevent the crabgrass from establishing. It is not necessary to use siduron on established turf, as it is expensive compared to other preemergent herbicides.

Post-emergent Control

If you miss the preemergent window and crabgrass appears in your lawn, you will need to rely on a post emergent strategy. It's more difficult than preemergent control because the post emergent herbicides available to the homeowner are limited. Look for products containing MSMA or DSMA (Ortho® Weed-B-Gon® Crabgrass Killer is a good choice). They usually require multiple applications, seven to 10 days apart. Although these herbicides discolor grass, certain turfgrass species are more tolerant than other species. For example, bermudagrass lawns are a good choice when applying MSMA.

If you are willing to have a professional, licensed applicator assist with post emergent control, there are two other herbicides that effectively control crabgrass: fenoxaprop (Acclaim®)and quinclorac (Drive®).  Drive® is very effective with cool season grasses and young grasses, making it a good choice in the springtime for newly established yards.

Good Lawn Care Practices

Basic lawn maintenance, as simple as it seems, can also help alleviate crabgrass invasion. Proper mowing, fertilizing and watering techniques promote a dense, healthy lawn that will prevent the crabgrass from receiving the sunlight necessary for germination. Mowing practices that achieve proper frequency and height are at the top of the list. Scalping the grass is the worst thing a homeowner can do when lawns are susceptible to crabgrass germination. Proper fertilizing is also important, as a lack of nitrogen will lead to low turf density and crabgrass invasion. In addition, too much or too little water can stress a lawn, resulting in open areas for weeds such as crabgrass to take hold.

Prepared for battle

Equipped with a plan of attack to control crabgrass, homeowners can enjoy a lawn free of this pesky weed. Just remember to stop the enemy in the spring to avoid the more difficult work of controlling crabgrass after it appears. And as always, keep up with proper lawn care practices to avoid this problem and enjoy your lawn this spring.