Pre-emergent only works before weed seeds sprout. Timing is the whole job.
In North Texas, that means two windows a year: late winter and early fall.
How pre-emergent works
A pre-emergent herbicide sits in the top layer of soil. It controls weeds that grow from seed there for several weeks.
It does not kill weeds that are already up. Texas A&M AgriLife Extension says most pre-emergents must be applied before the target weed emerges, or they won’t control it.
Go by soil temperature, not the date
The calendar is only a guide. Weather moves the window from year to year.
Soil temperature is the better signal. Use a soil thermometer or a local soil-temperature report. Measure about 2 inches down, not the air.
Late winter: crabgrass and summer weeds
Crabgrass is the first summer weed to sprout. AgriLife says it typically starts around early March in North and Central Texas.
The soil signal is about 55°F for several days in a row. AgriLife articles say at least three days. Its St. Augustinegrass calendar says four to five.
Soil warms at a different time each year, so the window shifts. Watch the soil, not the date.
The follow-up application
One application doesn’t last all summer. Weed seeds keep sprouting as the season goes on.
A second application later in spring keeps the barrier in place. The label sets how long to wait. Don’t guess.
Early fall: henbit, chickweed and annual bluegrass
Winter weeds sprout when it cools off. The fall target is about 70°F soil for four to five days in a row.
AgriLife’s Bermudagrass guide gives mid-August to mid-September. Its calendar runs September into October. Pick your date by the soil, because late-summer heat changes it.
A spring application won’t cover winter weeds. Treat again in the fall. It matters most on lawns that had henbit or annual bluegrass last winter.
Does your grass type change the timing?
Not much. AgriLife’s St. Augustinegrass and Bermudagrass calendars use the same soil-temperature triggers. Zoysia is warm-season too, so the same windows apply.
What changes is the product. Each one is labeled for certain grasses. Using the wrong one can hurt the lawn.
Water it in
Pre-emergent has to reach the soil. Rain or irrigation does that. AgriLife says it must be watered in before the weed emerges.
The label tells you how much water to use. If it doesn’t get watered in, it doesn’t work well.
What pre-emergent can’t do
- It can’t kill weeds that are already growing.
- It can’t control perennial weeds like nutsedge. See our nutsedge guide.
- It won’t catch every grassbur, since they sprout over a long season. See our grassbur guide.
Once crabgrass is up, post-emergent options are fewer and results can vary.
Common timing mistakes
- Waiting until you see crabgrass. It’s too late for a pre-emergent by then.
- Going by air temperature instead of soil temperature.
- Skipping the fall round because you did one in spring.
- Putting it down and not watering it in.
- Applying it right before seeding or sodding.
Missed the window?
You still have options, but they’re weaker. AgriLife says post-emergent control of crabgrass can be inconsistent. It can also discolor some grasses for a short time. Follow the label for your grass type.
The better fix is to be ready for next year’s window.
New sod or seed
Pre-emergent can stop grass seed from sprouting too. AgriLife says to check the label before putting it down around sprigging, sodding or seeding.
Planning to overseed with ryegrass in the fall? AgriLife says skip the fall pre-emergent.
Fertilizer carriers
Some pre-emergents come on a fertilizer carrier. In early spring, pick one with little to no nitrogen. Bermudagrass shouldn’t get nitrogen before it’s growing.
Follow the label
Follow the product label for every application. Keep kids and pets off until it has dried, as the label directs.
Want it handled?
Soil temperatures, watering in, and the right product for your grass are easy to get wrong. Green Ops’ weed control and fertilization program is handled by a licensed applicator who picks a product labeled for your grass type. Ask for a free quote in Mansfield, Tarrant and Johnson counties.