To be effective, a pre-emergent weed killer should be applied based on soil temperature. When considering the use of pre-emergents, refer to the following guidelines:
- Spring Pre-Emergents should be applied when the soil temperature has been at 55℉ (12℃) for 2–3 days.
- Fall Pre-Emergents should be applied when soil temperatures come down to 70℉ (21℃) for 2–3 days.
- To Determine Soil Temperature search for soil temperatures by your state. Or you can use this soil temperature map to view data for your region at no cost.
Pre-emergent weed killers are a powerful lawn care tool because they kill weeds before they’re visible. If applied at the right time, a good pre-emergent will form a barrier that keeps your lawn weed-free for months.
If the timing of pre-emergent application is wrong, however, all the money and effort spent spreading the product will be for nothing. If you apply pre-emergent too early or too late, new weeds will come roaring to life. Timing and applying pre-emergent weed killer application is critical to success and keeping a healthy lawn.
What is Pre-Emergent Weed Killer and How Does it Work?
Pre-emergent (or pre-emergence) weed killer is one of the most misunderstood weed control methods. Here are the things you need to know before you get started using pre-emergents for weed control:
- Pre-emergents kill weed seeds as they sprout. This process happens below the soil surface. As the seed germinates and sprouts upward, it encounters the pre-emergent present in the soil. This kills the sprout.
- Pre-emergents do not kill mature plants or dormant root systems. If the seed has grown enough to break through the soil surface, pre-emergents won’t harm it.
- Pre-emergents do not kill dormant seeds. Only sprouting seeds are affected by pre-emergent weed killers.
- Pre-emergents work like a barrier. Once they’ve been applied and watered into the soil, they will kill sprouting weeds for 6–8 weeks
- Some varieties of pre-emergents are selective, meaning they only kill weeds. Many varieties will stop all plants from sprouting. If you have seeded your lawn with new grass, wait 6 weeks before applying pre-emergent. If you have already applied pre-emergents, wait 3 months before seeding your lawn.
What Month Should You Put Down Pre-Emergent Weed Killer?
The best month for applying pre-emergent weed killer varies by region. In northern areas of the US and throughout most of Canada, temperatures may not be ideal for spring pre-emergent application until as late as May. Warm coastal regions in Florida and California may require spring pre-emergent weed killer as early as January.
For your pre-emergent herbicide to be most effective, time your application based on soil temperature, not the date on the calendar. This will ensure you stop weeds before they sprout.
When to Apply Spring Pre-Emergents
Spring pre-emergent weed killers are designed to stop invasive grasses, such as crabgrass and foxtail, as well as spring weeds like dandelions. To get the timing right, wait until soil temperatures have been at 55℉ (12℃) for 2–3 days. Check soil temperature by viewing a local soil temperature map online or through the use of a soil thermometer.
The reason temperature is so important is because warming soil triggers weed seeds to germinate. Crabgrass and foxtail—two of the most common weeds in North America—germinate soon after soil temperatures reach 55℉, with other weeds following closely behind. By applying pre-emergent once soils consistently reach this temperature, you will stop the widest range of weeds.
When to Apply Fall Pre-Emergents
If you live in the warm southern states, or along the west coast, fall and winter are aggressive growing seasons for winter weeds like annual bluegrass and deadnettle. If you live in one of these regions and don’t want to spend your winter battling weeds, a fall application of pre-emergent weed killer can be a lifesaver.
Once soil temperatures drop down to 70℉ (21℃) for 2–3 days at the end of summer, apply a fall pre-emergent formulation and water it into your lawn. This will keep those winter weeds from sprouting.
Can You Apply Pre-Emergent Too Early?
If pre-emergent herbicide is applied too early, it may have reduced effectiveness or no effect at all. Typically, a pre-emergent weed killer remains in the soil 6–8 weeks after application. This may seem like a large window with a margin for error, but it’s important to keep the following in mind:
- Pre-emergents applied too early may lose effectiveness completely before weeds sprout.
- If you’re a few weeks early with your pre-emergent application, the weed killer may lose effectiveness just as later-season weed seeds begin sprouting. If this happens, you’ll see many more weeds as spring moves into summer.
- Very heavy rainfall (or a late spring snowfall and subsequent thaw) might somewhat reduce the active lifespan of your pre-emergent. The better you time your pre-emergent application, the more value you’ll get out of it.
- Soil disruptions, such as digging or uprooting weeds/trees may compromise the effectiveness of your pre-emergent in the digging area. If you apply too early, there’s a bigger chance digging animals or your routine lawn maintenance will damage the pre-emergent barrier before it has a chance to work.
- A correctly timed spring pre-emergent will stop early weeds just as they start to sprout and linger long enough to deter later-sprouting species that require warmer temperatures.
Is April Too Late for Pre-Emergent Herbicides?
Depending on the climate in your region, or the current year’s weather patterns, April may be too early or too late to apply a pre-emergent herbicide. In warm regions, such as Florida and California, some species of weeds and invasive grass may begin sprouting in January or February. In the northeast and midwest, cool temperatures may keep weeds at bay until May.
Because pre-emergent herbicide only kills weed seeds as they begin to sprout, plan to apply spring pre-emergents once soil temperatures have come up to 55℉ for 2–3 days. Rather than look at the calendar, track soil temperatures in your region to determine the best time to apply your pre-emergent controls.
How Long Does Pre-Emergent Last?
If properly applied and watered into the soil, pre-emergent herbicide lasts 6-8 weeks. That means that for up to 2 months, any weeds that try to sprout in your lawn will be killed as they germinate.
To ensure maximum effectiveness of your pre-emergent, spread your weed killer at concentrations recommended by the manufacturer, using a sprayer (for liquid products) or a drop spreader (for granular products).
For longest-lasting results, after application, water the pre-emergent into your soil with 0.5 inches of water. That’s about half an hour of watering with a sprinkler. If you don’t have a watering system, plan to spread your pre-emergent 24–48 hours before rainfall and let nature do the watering. By watering pre-emergent into the soil, you ensure it forms a barrier beneath the surface to stop those sprouting seeds.
How Often Can You Apply Pre-Emergent Weed Killer?
You can apply a pre-emergent weed killer up to twice a year. If timed correctly, one application in spring and one application in fall should control the vast majority of weeds year-round.
If you live in a region with aggressive late-growing summer weeds (such as chickweed), then you can apply a second round of pre-emergent 8 weeks after your spring application. This application schedule should not cause any harm to your yard.
Will Heavy Rain Wash Away Pre-Emergent?
One of the positives of pre-emergent is that it resists breakdown by water quite well. Pre-emergents are designed to bond with the soil and remain for up to 8 weeks.
That said, all herbicides are soluble if inundated with enough water. The chances of your pre-emergent herbicide being washed away by heavy rain increase if the ground it is applied to is sloped or if there is very little grass and/or other vegetation to hold the soil in place.
While heavy rain will somewhat reduce the effectiveness of pre-emergent herbicide, it is generally resistant to natural precipitation patterns. You may see a small drop off in effectiveness if rainfall is heavy, but it takes a lot to wash away pre-emergent herbicide.
Can You Apply Pre-Emergent to Wet Grass?
It is not recommended to apply pre-emergent herbicides to wet grass. Doing so may cause the herbicide to stick to grass blades rather than reach the soil.
For maximum effectiveness, apply pre-emergent to dry grass shortly before rain or planned watering. Watering after application will help draw the pre-emergent down into the soil, where it must be to function.
When Can You Mow After Applying Pre-Emergent?
Wait until the pre-emergent herbicide has been watered into the soil before mowing any areas where the product has been spread. If you mow before pre-emergent has been watered in, the undissolved herbicide will be removed or spread unevenly around the lawn by the mowing process. This will reduce or negate the effectiveness of your pre-emergent weed killer.
The best practice is to apply a pre-emergent herbicide, water it into the soil (or wait for rainfall), allow the grass to dry, and then resume mowing as normal.
When to Put Down Pre-Emergent Herbicide
For maximum pre-emergent herbicide effectiveness, apply spring pre-emergents once soil temperatures have remained at 55℉ (12℃) for 2–3 days. When using fall pre-emergents, apply them when soil temperatures have dropped to 70℉ (21℃) for 2–3 days. The reason for this is that pre-emergents kill weed seeds as they sprout, and seasonal seeds dormant in the soil are triggered to sprout when soil temperatures reach a certain point.
By consulting a soil temperature map rather than the calendar for timing your pre-emergent application, you will achieve the best results and keep your lawn weed-free all year.