Crabgrass, an aggressive summer annual weed, can quickly dominate a lawn, particularly in areas where the desired turfgrass is thin or stressed. This weed is a warm-season grass that germinates when soil temperatures rise, often forming dense, low-growing mats that compete with established cool-season lawns like fescue and bluegrass for water and nutrients. Successfully eliminating crabgrass without harming the surrounding turf requires a two-pronged strategy: preventing new growth and selectively treating existing plants. The solution lies in understanding the crabgrass life cycle and employing specific herbicides and cultural practices to target the weed while sparing the lawn.
Preventing Future Growth
Preventing crabgrass from appearing is the most effective long-term control method, relying on the use of pre-emergent herbicides. These products work by creating a chemical barrier in the top layer of the soil that interrupts the germination process of the weed seeds. When a crabgrass seed begins to sprout, the emerging root and shoot absorb the herbicide, which prevents cell division and halts the seedling’s establishment. The selectivity of pre-emergent products comes from their mode of action, which targets only the actively germinating seed, not the established, desirable turfgrass plants. Common active ingredients used in these formulations include dithiopyr and pendimethalin; dithiopyr is particularly useful because it also offers a slight measure of early post-emergent control for very young crabgrass seedlings. This preventive approach is dependent on timing, as it is ineffective once the crabgrass plant has become established above the soil line.
Eliminating Existing Weeds
When crabgrass has already sprouted and is visible in the lawn, a different approach using post-emergent herbicides is necessary to eliminate the mature plants. Selective post-emergent products are formulated to target the crabgrass directly without causing significant damage to the surrounding turfgrass, such as Kentucky bluegrass or perennial ryegrass. Quinclorac is the primary and most effective active ingredient used for controlling established crabgrass, offering a high degree of selectivity. Quinclorac acts as a synthetic auxin, mimicking natural plant hormones that regulate growth. When the crabgrass plant absorbs this chemical through its leaves and roots, the herbicide moves systemically throughout the plant, causing abnormal, uncontrolled growth patterns that ultimately lead to plant death. This mechanism disrupts the weed’s ability to function normally, often resulting in visible yellowing and curling of the leaves within a couple of weeks. Established crabgrass often requires two applications of a post-emergent product, spaced approximately two to three weeks apart, to fully control the mature plants and any subsequent flushes of seedlings.
The Critical Role of Timing and Application
Successful selective control of crabgrass is highly dependent on precise application timing and technique for both pre-emergent and post-emergent treatments. Pre-emergent herbicides must be applied before the seeds begin to germinate, which typically occurs when the soil temperature at a one-inch depth consistently reaches 55 degrees Fahrenheit for several consecutive days; the blooming of forsythia shrubs is a common natural indicator for this period. Applying the pre-emergent product too late, after germination has begun, will greatly diminish its effectiveness because the chemical barrier is no longer effective on established seedlings. After application, the herbicide must be activated by watering or rainfall to move the product into the germination zone in the soil; if not watered in, it can sit on the soil surface, where it may degrade or be washed away by heavy rain. For post-emergent treatments, the timing of application is equally important, particularly concerning temperature, as applying products like quinclorac when air temperatures are above 85 to 90 degrees Fahrenheit can cause temporary discoloration or stress to the desirable turfgrass.
Strengthening the Turf
A dense, healthy lawn is the most effective long-term defense against crabgrass, significantly reducing the need for chemical intervention. Crabgrass seeds require ample sunlight and warm soil temperatures to germinate, conditions created by thin or short turf. Adjusting cultural practices to favor the growth of the desirable lawn species can naturally suppress the weed. Mowing the lawn at a higher setting (typically 2.5 to 3.5 inches) helps shade the soil surface, keeping it cooler and inhibiting the germination of crabgrass seeds; taller grass also allows the turf to outcompete the weed for light and space. Proper watering requires deep, infrequent irrigation rather than light, frequent sprinkling, as shallow watering promotes a weak, shallow root system in the turfgrass and keeps the soil surface moist, which helps crabgrass thrive. Finally, avoiding heavy fertilization during the peak heat of summer prevents giving the warm-season crabgrass an advantage over the cool-season lawn.