Crabgrass, a warm-season annual weed, is a common nuisance for lawn owners. It survives the winter as dormant seeds, sprouting and maturing during the heat of summer, competing with turf for water and nutrients. Since this weed relies entirely on seeds for the next year, timing is the most important factor for effective control and prevention. Control methods are divided into two categories: preventative applications designed to stop seeds from sprouting, and treatments that target the weed after it has already emerged.
Timing Pre-Emergent Crabgrass Preventer
The goal of a pre-emergent herbicide is to establish a chemical barrier on the soil surface before crabgrass seeds begin to germinate. This protective layer disrupts the cellular division in the sprouting seedling, effectively halting its growth before it ever breaks ground. To achieve this, the application must occur just before the soil warms to the temperature that triggers germination.
Crabgrass seeds start to germinate when the soil temperature at a two-inch depth consistently reaches 55 degrees Fahrenheit for several consecutive days. The ideal application window is when the soil temperature is approaching 55 degrees Fahrenheit, typically in early to mid-spring.
Visual cues can provide a reliable signal for timing the application. A traditional indicator is the blooming of local forsythia bushes, as their bright yellow flowers often coincide with the soil warming to the target range. Applying the preventer just as forsythia blooms, or when lilac buds are beginning to swell, helps ensure the barrier is established before germination begins. Once applied, the product needs to be watered into the soil to activate the barrier, making application before a gentle spring rain ideal.
When to Apply Post-Emergent Treatments
If the preventative window is missed, post-emergent herbicides are necessary to treat actively growing crabgrass. These products are applied directly to the weed foliage and are most effective when the plant is young, before it develops a thick cuticle or extensive root system. The best time to apply a post-emergent killer is when the crabgrass is in the two- to four-leaf stage, before it begins to tiller.
For maximum effectiveness and to prevent damage to surrounding turf, applications should be made when daytime temperatures are between 65 and 85 degrees Fahrenheit. Applying when temperatures are above 85 degrees Fahrenheit can stress the lawn and risk injury to the desirable grass. The best time of day is typically in the morning after the dew has dried, allowing the herbicide to be absorbed by the leaves.
Apply on a calm day with no rain forecast for at least 24 hours to ensure the product remains on the foliage and does not drift. Since crabgrass continues to germinate throughout the summer, a second application is often required. This follow-up treatment is typically scheduled seven to ten days after the first, targeting any missed plants or new seedlings.
Late Summer and Fall Control
Although the first hard frost of the fall will naturally kill crabgrass, late-season control efforts are important to reduce the volume of seeds dropped into the soil for the next year. A single mature plant is capable of producing tens of thousands of seeds, which survive the winter and contribute to the following spring’s infestation.
Spot treating any late-maturing plants with a post-emergent product can significantly lower the seed count for the next season. Fall is also the optimal time to focus on cultural practices that thicken the lawn, which is the best long-term defense against weeds. Aeration and overseeding create a dense turf canopy that shades the soil, keeping it cooler and blocking sunlight. This thicker, healthier lawn naturally crowds out future weed growth, preparing the area for the next spring’s preventative measures.