A post-emergent herbicide is a chemical treatment designed to eliminate weeds already visible and actively growing in the lawn, such as crabgrass. This differs fundamentally from a pre-emergent product, which creates a chemical barrier in the soil to stop weed seeds from germinating. Once crabgrass has emerged, only a post-emergent application can effectively control the plant. The success of this treatment relies heavily on precise timing, targeting the weed when it is most vulnerable.
Identifying the Critical Treatment Window
The effectiveness of any post-emergent crabgrass killer is highly dependent on the weed’s stage of growth, which defines the critical treatment window. Crabgrass is an annual warm-season grass, meaning it germinates in late spring, grows throughout the summer, and dies with the first hard frost of fall. The most successful applications occur when the crabgrass is young and actively growing, typically in late spring through early summer.
The ideal time to treat crabgrass is when it is in the two- to four-leaf stage, before it has developed more than one or two tillers, which are the side shoots that branch out from the main stem. Young weeds have thinner cuticles and more active internal growth, allowing for maximum absorption and translocation of the herbicide into the root system. Treating crabgrass at this juvenile stage often requires a lower herbicide rate and results in a higher kill rate.
As the plant matures and develops three or more tillers, its cell structure thickens, making it far more resistant to chemical control. The latest acceptable time for application is before the plant begins to form seed heads, which usually happens by mid-summer. Once a crabgrass plant produces seed heads, the systemic herbicide is less likely to move efficiently throughout the plant to achieve a complete kill.
Applying post-emergent treatments to mature, seeding crabgrass is often a futile effort that wastes product and time because the plant is focused on seed production rather than vegetative growth. Even if the visible plant is killed, the thousands of seeds it has already dropped will ensure a new infestation the following spring.
Optimal Weather and Time of Day for Application
The micro-timing of the application, focusing on daily weather conditions, is just as important as the macro-timing of the growth stage. Post-emergent herbicides are systemic, meaning they must be absorbed through the leaf tissue and transported to the roots, a process that relies on the plant’s metabolic activity. This process is most efficient when the ambient air temperature is consistently between 65°F and 85°F.
Applying the killer when temperatures are too cool (below 60°F) significantly slows the plant’s metabolism, reducing the rate at which the herbicide is absorbed. Conversely, application during extreme heat (above 90°F) can cause the herbicide to volatilize, reducing its effectiveness and potentially causing damage to the surrounding desirable turfgrass. High heat also stresses the desirable turfgrass, making it more susceptible to herbicide injury.
It is necessary to apply the herbicide when the air is calm, with wind speeds below 10 miles per hour, to prevent the spray from drifting onto non-target plants. The leaf surface must also remain dry for a sufficient period after application, so check the forecast to ensure no rain is expected for at least 24 hours. Rain shortly after application will wash the product off the leaves before it can be fully absorbed, rendering the treatment ineffective.
The best time of day for application is typically early to mid-morning, after any overnight dew has evaporated from the leaves. Applying to a dry leaf surface maximizes adherence and absorption before the intense heat of the mid-afternoon begins. This timing provides the chemical with several hours of optimal absorption during the plant’s most active period.
The Necessity of Follow-Up Treatments
A single application of post-emergent crabgrass killer is rarely sufficient for achieving complete eradication, especially for a dense or mature infestation. The need for a follow-up treatment is based on the plant’s ability to survive the initial exposure and the requirement to treat any crabgrass that emerged after the first application. Many products specifically recommend a sequential treatment plan to ensure thorough control.
The typical waiting period between the first and second application is 10 to 14 days. This is necessary to allow the initial dose of herbicide to fully circulate and exert its effects on the target plants. Applying the product too soon can overstress the desirable turfgrass without providing any added benefit to the weed control. This two-week window allows time to assess the efficacy of the first treatment and identify any surviving or newly emerged crabgrass plants.
If the crabgrass was treated when it was larger, with multiple tillers, the initial application serves primarily to stunt its growth and make the second treatment more effective. The repeat application ensures that any parts of the plant that did not receive a lethal dose are targeted again. The treatment cycle should continue until all visible crabgrass is completely dead, typically indicated by a change in color to brown or purple.