The chemical solution for controlling crabgrass without damaging desirable turfgrass involves the precise application of selective herbicides. Crabgrass is a warm-season annual weed that completes its life cycle within a single season and reproduces by seed. Eliminating this grassy invader is challenging because it shares many biological characteristics with perennial turfgrass, such as Kentucky Bluegrass or Tall Fescue. Selective control is achieved by using specific chemical classes and critical timing, exploiting biological differences between the weed and the lawn.
Preventing Crabgrass with Pre-Emergent Herbicides
The most effective strategy against crabgrass involves preventing its emergence before it sprouts from the soil. Pre-emergent herbicides accomplish this by creating a chemical barrier on the soil surface. These chemicals inhibit cell division in the root or shoot of the germinating seedling, effectively killing the weed before it can be seen.
Timing is the single most important factor for the success of a pre-emergent application. The chemical must be in place before crabgrass seeds begin to germinate. Germination typically occurs when the soil temperature at a four-inch depth consistently reaches 55 degrees Fahrenheit for several consecutive days. Applying the herbicide too early risks the chemical breaking down, while applying it too late misses the opportunity to establish the necessary barrier.
Common active ingredients for pre-emergent control include Dithiopyr, Pendimethalin, and Prodiamine. Dithiopyr offers a unique advantage because it provides a short window of early post-emergence control, targeting very young crabgrass seedlings. For full-season protection, especially in areas with long, warm summers, a split application is often recommended. This involves applying a portion of the product in the spring and another portion 8 to 10 weeks later to maintain the chemical barrier.
Eliminating Existing Crabgrass with Post-Emergent Treatments
When crabgrass has already sprouted and is actively growing, post-emergent herbicides are required. These systemic products are absorbed by the plant’s foliage and translocated internally to disrupt growth processes. The primary chemical for this purpose is Quinclorac, which is effective against both newly emerged and more mature crabgrass plants.
Quinclorac is safe for use on many common turf varieties, including Tall Fescue, Kentucky Bluegrass, Bermuda, and Zoysia. It provides superior control for crabgrass even after the weed has developed four or more tillers. For the largest and most established weeds, a follow-up application two to three weeks after the initial treatment is often necessary to ensure complete eradication.
Another effective post-emergent option is Fenoxaprop-p-ethyl, often used for grass-on-grass control in cool-season turf. This chemical works well on crabgrass at nearly all growth stages, though it should not be mixed with certain broadleaf herbicides. The application of these systemic chemicals often requires adding a non-ionic surfactant or an oil concentrate. This additive helps the herbicide stick to the waxy leaf surface of the crabgrass for better absorption.
How Selective Herbicides Target Crabgrass
Selective herbicides kill crabgrass while sparing turfgrass due to differential metabolism. This selectivity is not a simple rejection of the chemical by the turf, but rather a difference in the speed at which the two plants can detoxify the chemical. Desirable turfgrass species possess specific enzyme systems, such as cytochrome P450 monooxygenases and glutathione S-transferases, which rapidly metabolize the absorbed herbicide.
These enzymes break down the toxic herbicide molecule into inactive, non-toxic compounds before they can accumulate to lethal concentrations within the plant. Crabgrass, being a different species, either lacks these specific detoxification enzymes or produces them at a much lower rate. This difference means the herbicide remains in its active, toxic form within the weed, interfering with crucial metabolic pathways and leading to plant death.
Quinclorac, for instance, acts as a synthetic auxin, mimicking and then overloading the plant’s natural growth hormones. The perennial turfgrass can quickly process and neutralize this chemical signal, while the crabgrass cannot, resulting in uncontrolled, destructive growth. Therefore, selectivity is based on the turfgrass’s superior ability to chemically defend itself.
Ensuring Safe Application and Protecting Turfgrass
To prevent damage to the lawn, it is imperative to follow the product label directions with precision, as they specify which turf species are tolerant to the chemical. Some common turfgrasses, notably St. Augustine and Centipede grass, are sensitive to many herbicides and require specific, lower application rates. Applying any chemical at a rate higher than the label specifies significantly increases the risk of damaging the desirable turf.
Avoid applying post-emergent herbicides when temperatures exceed 85 degrees Fahrenheit or when the turfgrass is under drought stress. High heat and drought weaken the turf, making it less able to metabolize the chemical quickly and increasing the likelihood of injury. Weeds under drought stress also develop a thicker waxy leaf cuticle, which reduces herbicide absorption and lowers effectiveness.
For granular pre-emergent products, proper calibration of the spreader ensures an even distribution of the protective barrier. After application, granular products require irrigation or rainfall to activate the chemical and move it into the soil. Conversely, most post-emergent liquid applications must remain on the leaf surface for a specified period, often 24 hours. This allows for maximum absorption before the lawn is watered or mowed.