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.