If crabgrass has become established in your lawn, proper lawn maintenance alone may not be enough. A pre-emergent herbicide is the next line of defense. Pre-emergent herbicides work by killing the crabgrass seedlings as they germinate. When applying the pre-emergent herbicide, always follow the manufacturer's directions. In general:
- Timing is essential when using pre-emergent herbicides. Application times depend a great deal on weather patterns, which vary from region to region. If your area has experienced a warmer than usual winter, you'll probably need to apply the herbicide earlier than usual.
- Apply the herbicide when the ground temperature rises above 60 degrees. Since it's difficult for most of us to monitor the soil temperature, there's an easier way. When you notice shrubs blooming and trees budding, it's time to apply the herbicide. Warm nights and periods of rainfall encourage crabgrass germination. If your weather fits this pattern, get the herbicide in place right away.
- For newly seeded lawns, wait until you have mowed your lawn three times before applying the herbicide to avoid killing the new grass seedlings.
- Apply the herbicide uniformly across your lawn. If you miss a spot, crabgrass can get established and then spread to the rest of your lawn.
- Do not de-thatch or aerate the lawn after applying the herbicide. Doing so may break the chemical barrier of the herbicide.
- Wait two to four months to re-seed the lawn after using a pre-emergent herbicide.
- Use a pre-emergent herbicide during late winter or during early spring of the next year to prevent any crabgrass seeds left behind from developing at the next opportunity.
- Do not use a pre-emergent herbicide if crabgrass is already in the lawn or if you have just installed sod.
If the crabgrass seeds have already sprouted and crabgrass has appeared in your grass, the pre-emergent herbicide will do no good. However, you still have an option. Post-emergent herbicide products control crabgrass after it has already germinated.
Post-emergent herbicides work by killing the crabgrass plants. Apply these herbicides only to the crabgrass that is visible. Read and follow the manufacturer's directions on the product carefully. The amount of post-emergent herbicide that you can safely apply to your lawn depends on the type of grass you have. Here are some tips:
- Check the weather forecast before using a post-emergent herbicide. Apply the herbicide on a calm, sunny day. Rainfall shortly after application will wash the product away before the crabgrass has a chance to absorb it.
- For best results, apply the herbicide in the morning after the dew has dried. If you wait until late afternoon, dew or a shower may prevent maximum absorption.
- Post-emergent herbicides work best when temperatures are 60 - 90°F. These higher temperatures cause the plants to absorb the herbicide quickly; if the temperatures are too cool or weather conditions are too cloudy, the product is likely to be ineffective.
- Make sure the soil is moist before applying the herbicide. If not, you should water the area fairly extensively the day before treatment. If conditions are extremely dry, you may want to water again two days after the application. The waiting period will give the crabgrass time to absorb the herbicide.
- If you notice the lawn browning suddenly, you may have applied too much herbicide. In this case, water the area extensively as soon as possible to dilute the herbicide and keep it from further damaging your lawn.
- After treating the area with the herbicide, keep an eye out for newly germinated crabgrass plants. Any plants that may have germinated since the initial application will require a follow-up spot treatment.
- If the crabgrass plants are fairly well established, you'll need to apply the herbicide twice. Treat the affected areas again four to seven days after the first application. Make sure the soil is moist before the second application.
- Follow the manufacturer's instructions about when it's safe to re-seed grass. Seed new grass in the area as soon as possible to establish a healthy lawn crabgrass can't break through.
- If you use a post-emergent herbicide during the summer, care for your lawn according to the lawn maintenance tips above.
- If the majority of your lawn is crabgrass, it may be best not to remove it during the summer. Wait and renovate the lawn in the fall.
When using lawn treatments or lawn care products, always follow package directions regarding proper clothing, protective equipment, application procedures and safety precautions. When using herbicides, also make sure you understand which weeds the chemical treats and which desirable plants it may also kill.