Crabgrass is a summer annual weed that germinates each spring, grows aggressively during the warm months, produces thousands of seeds, and then dies with the first frost of autumn. Precise timing is the single most important factor for effective control. Because a single crabgrass plant can produce over 150,000 seeds, proactively stopping the germination process is significantly easier than reacting to a widespread infestation later in the season.
The Critical Role of Soil Temperature for Prevention
The most effective method for controlling crabgrass is prevention, which requires applying a pre-emergent herbicide before the seeds begin to sprout. This timing is directly tied to soil temperature, not the calendar date. Crabgrass seeds typically start to germinate when the soil temperature at a one-inch depth reaches 55 degrees Fahrenheit for three to four consecutive days.
For a pre-emergent product to be successful, it must be applied just prior to this 55-degree trigger and before the temperature climbs to the 70-degree range, where the majority of seeds will sprout. This application window can be visually estimated by natural cues, such as when the Forsythia shrub is in full bloom. Using a soil thermometer provides a more accurate measure than relying on air temperature or an ornamental plant’s bloom cycle.
In regions with extended growing seasons, a split application technique is often employed to maintain the protective barrier throughout the summer. This involves applying the full recommended rate in two halves, with the second application occurring approximately six to eight weeks after the first. This strategy helps to counteract the natural breakdown of the herbicide barrier in the soil. Active ingredients commonly used in pre-emergent products include prodiamine and dithiopyr, which create a chemical layer that kills the seedling as it attempts to emerge.
Treating Crabgrass After Germination
When pre-emergent control is missed or fails, post-emergent herbicides become necessary to manage visible crabgrass plants. The effectiveness of these products decreases drastically as the plant matures. Post-emergent control is most successful when the crabgrass is small and actively growing, ideally before it develops more than two or three tillers.
Once the plant develops three or more tillers, it may require multiple applications of the post-emergent herbicide for complete control. The active ingredient quinclorac is widely used for post-emergent control and is effective on crabgrass plants up to the four-tiller stage or even larger. Other active ingredients like fenoxaprop are also effective on later stages, controlling crabgrass up to the five-tiller stage.
Spraying crabgrass becomes pointless once the plant has reached its reproductive stage and set seed, typically in late summer or early fall. The plant will die with the first significant frost, making application at this stage a wasted effort. Focusing on control before the plant produces seeds is paramount, as this prevents a greater infestation the following year.
Application Factors for Optimal Results
Herbicide application, whether pre-emergent or post-emergent, must be paired with the correct conditions for maximum effectiveness. For granular pre-emergent products, water is necessary to move the active ingredient from the granule into the soil to form the protective barrier. While some products need immediate watering, most require irrigation or rainfall within seven days of application for activation.
Conversely, post-emergent liquid applications are best applied when the foliage is dry and rain is not expected, allowing the herbicide to absorb fully into the leaf tissue. Avoid application during extreme heat (temperatures exceeding 85 to 90 degrees Fahrenheit), as this can stress the turf and increase the risk of damage.
Mowing at a higher setting, such as three inches, helps the turf shade the soil. Shaded soil remains cooler, delaying the onset of crabgrass germination.