|Crabgrass and other annual grassy weeds are common problems in home lawns that can be treated through both chemical and nonchemical methods.
Proper lawn care practices to encourage a dense stand of vigorous grass are the best way to prevent weeds from invading. For example, mowing height can have a big impact; lawns mowed higher (over 2 inches) tend to have less problems with annual grasses such as crabgrass. Close-mowed lawns tend to open up, allowing weeds like crabgrass to invade.
Light, frequent watering also favors crabgrass. Crabgrass often invades areas seeded in late spring because of bare soil, frequent watering, and onset of hot weather, ideal for its growth.
Herbicides (weed killers) are also available to manage annual weeds. Preemergence
herbicides prevent annual grassy weeds such as crabgrass from emerging. Timing of
application is very important, as the weed killer should be applied before the crabgrass emerges from the soil. Crabgrass will germinate when soil
temperatures are greater than 55 to 60° F for 7-10 consecutive days, and continues until soils reach 95° F. Other annual grasses germinate as soils get warmer than 60°.
Late March to early April is the suggested time for applying a preemergence crabgrass herbicide. If the spring is very warm, consider late March. In cold, “late” springs, these materials could be put down well into April. Using forsythia blooming as a guide is not dependable. Many
preemergence crabgrass herbicides are available in combination with lawn fertilizer at your local Co-op’s garden supply center, so the crabgrass prevention and spring fertilization can be done at the same time. Preemergence herbicides include
benefin, benefin/trifluralin, bensulide, dithiopyr, oxzdiazon,
pendmethalin, prodiamine, and siduron.
Some herbicides may be reapplied for extended control; refer to the label for timing and rates. Core aerifying or dethatching should be done based on label instructions. One of the management problems
associated with preemergence herbicides is seeding or over-seeding practices. With the exception of a herbicide siduron (Tupersan), preemergence annual grass weed killers will also damage germinating desirable grass seed. Oftentimes siduron is combined with starter fertilizer.
If crabgrass plants are appearing in lawns in mid to late summer, remember that they are annual plants and die as temperatures drop in fall. Postemergence crabgrass
herbicides need to be applied when crabgrass plants are very small; typically crabgrass is noticed too late for these to be effective. The suggested strategy to avoid crabgrass next season would be to improve the lawn through cultural practices and consider a preemergence herbicide in spring.