Pre-emergent herbicides are a form of chemical weed control applied to the soil before weed seedlings emerge. They function by creating a chemical barrier near the soil surface that inhibits the growth of newly germinated seeds. The active ingredients interfere with the plant’s cell division, preventing the seedling from establishing a root system and shoots. Because these herbicides do not affect established plants or seeds that have not yet germinated, successful application hinges entirely on precise timing synchronized with the target weed’s life cycle.
Understanding Soil Temperature
Relying solely on a calendar date for pre-emergent application is unreliable because soil conditions, not arbitrary months, dictate weed germination. Proper timing requires measuring soil temperature, as this directly triggers the weed seed growth cycle. The chemical barrier must be in place before the soil warms enough to signal the seeds to sprout.
For warm-season annual weeds, like crabgrass, germination begins when the soil temperature consistently reaches 55°F at a depth of 2 to 4 inches for several consecutive days. Applying the herbicide too early risks the product breaking down due to microbial activity. Applying it too late means the weeds have already sprouted, rendering the pre-emergent ineffective.
Spring Application Guidelines for Oklahoma
The primary spring application targets crabgrass and other summer annual weeds. Because Oklahoma is geographically diverse, timing varies significantly from south to north, often spanning over a month. Monitoring local soil temperature is the most accurate method, which can be done using a soil thermometer or by checking online agricultural extension resources.
Regional Timing
Residents in Southern Oklahoma (e.g., Lawton and Ardmore) apply between late February and early March, as their soil reaches the 55°F germination point sooner. Central Oklahoma (e.g., Oklahoma City and Tulsa) typically shifts the application window to the middle of March.
The latest application is reserved for Northern Oklahoma and the Panhandle region, where soils remain cooler longer, pushing the window into late March or early April. Regardless of location, the goal is to apply the herbicide when the soil is consistently between 50°F and 55°F, just before the main wave of germination begins. A second application is advised six to eight weeks later to sustain control throughout the long summer growing season, as the initial barrier degrades over time, lasting only 8 to 12 weeks.
Fall Application Guidelines for Oklahoma
The fall application is a separate, second line of defense against cool-season annual weeds, such as henbit, chickweed, and annual bluegrass (Poa annua). These weeds germinate in autumn and remain active through the winter before setting seed in the spring. This application is essential for maintaining a clean lawn during the dormant season.
The critical temperature for fall timing is a drop in soil temperature below 70°F, signaling the beginning of the germination window for these cool-season weeds. Applying the pre-emergent as temperatures fall through the 70s and toward the low 60s ensures the barrier is in place before the seeds sprout. For most of Oklahoma, this window typically occurs from mid-September through October.
This fall treatment prevents winter weeds from becoming established, which reduces the amount of competition for your lawn when it begins to green up in the spring. Properly timed fall and spring applications establish season-long weed control.