Crabgrass, botanically known as Digitaria, is a common warm-season annual weed that plagues many lawns. It is easily recognized by its coarse, light-green blades and its habit of growing in dense, unsightly clumps close to the ground. As an annual, it completes its life cycle within one growing season, but it produces thousands of seeds that ensure its return the following year. Successfully managing this persistent weed requires stopping it before it germinates and safely eliminating the plants that have already emerged. This article details methods for controlling crabgrass while maintaining the health of your turfgrass.
The Foundation: Stopping Crabgrass Before It Starts
The most effective long-term defense against crabgrass is preventing its seeds from successfully germinating. Pre-emergent herbicides work by creating a chemical barrier in the top layer of the soil where the weed seeds reside. When the crabgrass seed begins to sprout, it absorbs the herbicide and its growth is inhibited before it can emerge above the soil line. This preventative approach avoids the need for harsher post-emergent treatments later in the summer season.
Timing the application is the most important factor for pre-emergent success. Crabgrass seeds begin to germinate reliably when the average soil temperature, measured at 2 inches deep, consistently reaches approximately 55°F for several consecutive days. Applying the product too early risks the chemical barrier breaking down, while applying it too late means the seeds have already sprouted and the product will be ineffective. Monitoring local soil temperatures, rather than relying solely on calendar dates, provides the most accurate guide for application timing.
Many homeowners rely on a single application of a pre-emergent herbicide, which is sufficient if the product offers season-long control. However, in regions with long, hot summers, a split application approach can be more effective. This method involves applying a reduced rate in the early spring, followed by a second, lighter application six to ten weeks later to renew the protective barrier for the mid-summer heat.
Precise application techniques are necessary to ensure the safety of your lawn. Carefully calibrate your spreader to avoid overlapping passes, which can cause an area to receive a double dose of the herbicide and potentially harm the desirable turfgrass. It is important to apply the product when rain is expected or to water it in soon after application, as this action moves the chemical into the soil to form the necessary barrier.
Targeted Removal of Actively Growing Crabgrass
Despite preventative measures, some crabgrass plants will inevitably emerge, requiring a targeted, post-emergent approach. Post-emergent herbicides are absorbed by the leaves of the actively growing weed, moving systemically through the plant to cause its demise. This strategy is necessary once the plants are visible and the pre-emergent chemical barrier is no longer effective.
The most effective chemical for controlling established crabgrass is quinclorac, which is highly selective and generally safe for most common turfgrass species. Quinclorac works by interfering with the weed’s cell division and growth processes, causing the leaves and stems to curl and the plant to eventually die. For best performance, it is often recommended to combine quinclorac with a methylated seed oil (MSO) or nonionic surfactant to help the herbicide stick to and penetrate the waxy crabgrass leaves.
Targeting crabgrass when it is young, specifically before it has produced more than four to five tillers, significantly increases treatment efficacy. Once the weed matures and begins to develop seed heads, it becomes far more resilient to chemical controls. Treating the weed early also prevents it from dropping thousands of seeds back into the soil, which reduces population pressure for the subsequent year.
Protecting turfgrass during post-emergent application requires careful attention to environmental conditions and dilution rates. Never apply these herbicides when the lawn is under stress from high temperatures (generally above 85°F) or severe drought, as this increases the risk of turf burn or discoloration. Always follow the product’s label instructions precisely regarding dilution, since applying too concentrated a solution can injure or kill the surrounding grass.
Protecting Your Turf Through Proper Lawn Care
Long-term crabgrass control requires adopting sound cultural practices that favor the turfgrass, in addition to chemical treatments. A dense, healthy lawn naturally crowds out weeds by competing for sunlight, water, and nutrients. When turfgrass is maintained properly, it creates an environment hostile to the germination and survival of crabgrass seeds.
Mowing height is perhaps the simplest and most effective cultural practice for suppressing crabgrass. Taller grass blades, ideally maintained at a height between 3 and 4 inches, create a dense canopy that shades the soil surface. This shading prevents the sunlight necessary for crabgrass seed germination from reaching the soil, effectively acting as a natural pre-emergent barrier. Short mowing, conversely, exposes the soil and invites weed growth, stressing the desirable grass.
Modifying your watering schedule also plays a significant role in discouraging crabgrass. Crabgrass is a shallow-rooted weed that thrives in moist, frequently watered topsoil. By contrast, desirable turfgrass benefits from deep, infrequent watering that encourages the roots to grow deeper into the soil profile. Watering deeply, until the top 6 to 8 inches of soil are moist, and then allowing the surface to dry out, deprives the crabgrass seedlings of the consistent moisture they need to survive.
Applying fertilizer at the appropriate times supports the growth of the desirable grass species, allowing them to thicken and out-compete the weeds. Cool-season grasses benefit most from fall fertilization, while warm-season grasses thrive with summer feeding. Ensuring that the turf receives the nutrients it needs to maintain density is a proactive step that minimizes open areas where crabgrass can become established.