Crabgrass is one of the most prevalent grassy weeds found in Pennsylvania lawns. Crabgrass thrives in full sunlight and high temperatures and can easily out compete common cool-season grasses under these conditions. Crabgrass is in a group of plants known as summer annuals. These plants have a life of less than one year. Summer annuals germinate in the spring, grow through the summer and die with the first hard frost. These plants produce a tremendous amount of seed in the mid- to late summer when the day length starts to shorten. These seeds ensure next year’s crop of weeds but can also remain dormant in the soil for several years before germinating. It is likely that if you have crabgrass in your lawn, it will be there next year, too. “One year’s seeding equals seven years weeding,” as weed scientists like to say.
Weed management in turfgrass stands can be accomplished in various ways. Traditional methods include cultural management and mechanical and chemical controls. The primary and most effective weed control tactic in a lawn is proper mowing. In fact, it has been estimated that regular mowing eliminates some 80 percent of weedy species. Other cultural practices, such as judicious fertilization, can further reduce weed competition by increasing turfgrass vigor. Open and weak turfgrass areas promote crabgrass infestations because of higher soil temperatures, which enhance germination and decrease competition. The best defense against weed invasion is a dense, healthy turfgrass stand. This is particularly effective for annual weeds such as crabgrass that establish from seed every year. A thick turf canopy can effectively shade the soil and reduce the number of seedlings that are able to establish.
Knowing when crabgrass is likely to be present is helpful in proper identification and control. Homeowners who complain of crabgrass infestations in April and May are usually identifying tall fescue, nimblewill or quackgrass. Crabgrass germination typically begins in early May when soil temperatures reach 62 degrees Fahrenheit at a depth of 1 to 2 inches, or about two weeks after the forsythia blooms begin to drop. Heat accumulation or growing degree-days are a great way to estimate soil temperatures from readily available weather station data.
To be effective, preemergence herbicides must be in place before germination occurs. Preemergence treatments are preferred because they are generally more effective for crabgrass control and less injurious to the turfgrass than postemergence treatments. In general, preemergence herbicides should be applied when soil temperatures reach 50 to 55 degrees Fahrenheit, or when forsythia is in full bloom. This will allow the preemergence herbicide to form a barrier before the crabgrass seedlings emerge. Preemergence herbicides work by inhibiting the growth of young seedlings. These products do not eliminate established plants and must be applied before germination begins. In general, preemergence applications are preferred in lawns with a history of crabgrass pressure because of the difficulties associated with postemergence control. Preemergence herbicides and proper turfgrass management are the best combination for long-term crabgrass suppression.