Crabgrass is an annual grassy weed that plagues lawns across the country, distinguishing itself from desirable turfgrasses by its coarse texture and rapid, sprawling growth habit. As an annual, the plant completes its entire life cycle—from germination to seed production—within a single growing season before dying off with the first hard frost. Effective control relies heavily on chemical intervention through herbicides. These herbicides are divided into two main categories: those that prevent seeds from sprouting and those that kill the plant once it is visible.
Understanding the Life Cycle and Timing
Timing the application of a herbicide is the most important factor for successful crabgrass control. The weed’s seeds lie dormant over winter and begin to germinate in the spring when conditions are ideal. Germination is determined by the soil temperature, which must reach a consistent 55 to 60 degrees Fahrenheit at a depth of one inch for several consecutive days.
The goal of management is to prevent the plant from reaching maturity, which is when it produces seeds for the following year. A single mature crabgrass plant can produce as many as 150,000 seeds, making prevention a much more manageable task than eradication.
Pre-Emergent Active Ingredients
Pre-emergent herbicides are the most effective tool because they target the weed before it appears above ground. These products create a chemical barrier in the top layer of the soil that kills the seedling shortly after it sprouts. The herbicide is absorbed by the developing root or shoot, preventing the cell division necessary for growth.
The most common active ingredients for homeowners are Prodiamine, Pendimethalin, and Dithiopyr. Prodiamine and Pendimethalin halt cell division in the young plant. Dithiopyr is unique because it offers a small window of early post-emergence control, killing crabgrass that has just sprouted and has not yet developed tillers.
Pre-emergent products must be applied before the soil temperature reaches the 55-degree mark for germination. Once applied, the herbicide must be activated by water, either from rain or irrigation, to form the necessary barrier in the soil. These herbicides will also prevent the germination of desirable grass seed, which is a limitation to consider if overseeding is planned.
Post-Emergent Active Ingredients
When crabgrass plants have emerged and are visible, a post-emergent herbicide is required to kill the established weed. These products are sprayed directly onto the foliage, where the active ingredient is absorbed and translocated throughout the weed’s system. Quinclorac is widely considered the standard for post-emergent crabgrass control due to its effectiveness.
Quinclorac is a selective herbicide, meaning it targets the crabgrass while leaving most desirable turfgrass species unharmed. The chemical works by causing the weed’s leaves and stems to curl, twist, and change color before the plant dies. For residential users, Quinclorac-based products are a primary option.
Alternatives such as Fenoxaprop-ethyl and Mesotrione are also effective. However, the effectiveness of any post-emergent treatment significantly decreases as the crabgrass matures, particularly once it has developed multiple tillers and is forming seed heads. For mature plants, repeat applications spaced two to three weeks apart are often necessary to achieve control.
Safe and Effective Application Methods
Safe and effective application begins with carefully reading and following the product label directions. The label specifies the correct application rate, the target weeds, the type of turfgrass the product is safe for, and the necessary personal protective equipment. Proper sprayer calibration is necessary for liquid applications to ensure the correct amount of active ingredient is distributed evenly across the area.
Herbicide formulations come in both granular and liquid forms, requiring different application methods. Granular products are applied using a broadcast spreader, and pre-emergent granules must be watered in immediately to activate the chemical barrier. In contrast, post-emergent liquids should remain on the leaf surface, ideally keeping the foliage dry for several hours to allow for maximum absorption into the plant.
Environmental conditions influence application success, with temperature being a primary concern. Post-emergent herbicides can damage the lawn if applied in extreme heat, such as when temperatures exceed 85 degrees Fahrenheit. Avoiding application before heavy rain is also important, as this can lead to product runoff into non-target areas and reduce efficacy.