FREQUENTLY ASKED QUESTIONS
WHAT ARE PREEMERGENCE HERBICIDES?
A preemergence herbicide is an herbicide that is designed to control weeds by interfering with seedling germination and emergence. They are commonly referred to in the lawn care industry as “weed preventers“. They form a protective barrier on your lawn during critical seasons when weeds are most actively germinating. Postemergence herbicides will instead work to control established weeds that have already germinated and emerged. Some herbicides have both pre- and postemergence activity.
HOW DO THEY WORK?
Herbicides are classified by their site of action which refers to the location within the plant where the herbicide interferes with development. Different preemergence herbicides may have different sites of action or manners in which they work. However, many of the preemergence herbicides found in lawn care products for homeowners are classified as inhibitors of microtubule assembly. In simpler terms, these products inhibit cell division, resulting in seedlings that are stunted, deformed, and unable to emerge as healthy plants.
WHAT ARE THE POTENTIAL BENEFITS OF USING A PREEMERGENCE HERBICIDE?
- Protection during critical seasons when turfgrass may be less able to compete with weeds (spring and fall).
- Preemergence herbicides are generally the most effective chemical option for controlling challenging annual weeds like crabgrass (Digitaria spp.) and annual bluegrass (Poa annua ).
- In some cases, preemergence herbicides can reduce the number of postemergence herbicide applications required to maintain a healthy lawn.
- When used as directed by the label, many preemergence products can be safer to use around established plants in the landscape compared with select postemergence products.
WHAT OTHER FACTORS SHOULD I CONSIDER?
- ALWAYS FOLLOW THE LABEL. Consider all parts of the label including recommended personal protective equipment (PPE), as well as appropriate application rates and timing.
- Preemergence herbicides can injure newly-established or overseeded turfgrass lawns. Follow label recommendations and consult your AgriLife County Extension Agent when in doubt.
- Over-application of select preemergence herbicides can injure established turfgrass. Never exceed the maximum annual rate specified by the product label.
- The performance of preemergence herbicides can be significantly affected by timing, precipitation, environmental conditions, and the specific weeds you are targeting. For the most effective program, work with your AgriLife County Extension office regarding their recommendations for your area.
Two premergence herbicide applications per year are generally recommended for best results: one in the spring and one in the fall. Recommendations for preemergence herbicide timing are generally based on the soil temperatures at which common turfgrass weeds such as crabgrass and annual bluegrass germinate. For this reason, we typically recommend that you apply preemergence products based on the soil temperatures outlined above. Application timing can fluctuate by several weeks based on geographic region and annual weather conditions. When in doubt, contact your local AgriLife County Extension office.
MONITORING SOIL TEMPERATURE
You can monitor soil temperature yourself using a soil thermometer or even a household meat thermometer. Measure the soil temperature for the uppermost 1″ of soil, where most weed seeds will be concentrated. Be mindful that some seeds may be on the soil surface, which will warm more quickly leading to earlier germination. Soil temperatures are also generally monitored at different depths for many areas throughout the state of Texas. Contact your local AgriLife County Extension office for recommendations on the most reliable sources for soil temperature data in your county.
- Check out aggieturf.tamu.edu/turfgrass-weeds/ for assistance with weed identification.
- Download A Homeowner’s Guide to Herbicide Selection for Warm-Season Turfgrass Lawns from aggieturf.tamu.edu or the AgriLife Bookstore for a more thorough overview of turfgrass weed management.
- When searching for additional lawn care products, don’t forget your local producer’s co-op and/or feed store.
Dr. Becky Grubbs, Extension Turfgrass Specialist