Pre-emergent herbicides block weed seeds from germinating before they emerge in your lawn. The key with pre-emergents is to get the timing right, choose the right kind for your yard, and apply the herbicide correctly.
Coupled with post-emergent herbicides, your yard should remain green, lush, and weed-free. But you need both — pre-emergents and post-emergents. Much as bookends keep your books in place, using both types of weed killers will support a healthy lawn.
In this article, we cover how pre-emergents work, when and how to apply pre-emergent herbicides, and some common pre-emergent application mistakes.
How pre-emergent herbicides work
Pre-emergent herbicides include chemicals like prodiamine or dithiopyr, which work in three ways to stop weeds like clover, crabgrass, thistle, and dandelions from emerging through the soil.
When to apply pre-emergent herbicides
The time of year you apply your pre-emergent herbicide is the most crucial factor to keep weeds from springing up in your yard. Two factors — season and ground temperature — determine when it’s the right time to apply your herbicide:
Pre-emergents are applied to prevent weeds from sprouting each season. Pre-emergents typically last 3-5 months, and a post-emergent can be used when you start seeing weeds pop up after that time.
To avoid winter and fall weeds like bluegrass and prickly lettuce, apply a pre-emergent herbicide from August to November.
To get ahead of early spring and summer weeds, apply pre-emergent herbicides around mid-March.
By ground temperature
Once you know the right time to apply pre-emergents, you need to pay attention to the ground temperature, too.
To prevent summer weeds, apply a pre-emergent herbicide once the ground temperature has been 55 degrees for at least 48 hours.
To prevent winter weeds, apply a pre-emergent herbicide in the fall when ground temperatures reach 70 degrees and continue to drop.
How to measure ground temperature: Stick a thermometer to 2 inches below the soil.
How to apply pre-emergent herbicides
How you apply your pre-emergent herbicide is just as important as when you apply your weed deterrent.
To apply your weed deterrent correctly, you need the right tools, you need the right type of pre-emergent herbicide (granular or liquid, selective or non-selective), and you need to spread or spray your weed killer systematically.
Oh, and check your weather app before applying your pre-emergent so that your herbicide doesn’t immediately wash away with a summer rain shower. You should also wait for the ground to dry thoroughly before applying the herbicide, especially if you’ve experienced a period of consistent rain.
How to apply granular pre-emergents
What you’ll need: A spreader, either manual push (similar to a wheelbarrow) or hand-held styles.
What to do next: Fill your spreader with granular pre-emergent herbicide according to the application rate on the label. Calibrate the spread to fit the area of your yard where you plan to apply your pre-emergent herbicide.
To apply your granular pre-emergent, go across your entire lawn and spread the granules as evenly as possible by releasing the handle on your equipment. Make sure you pass over the same area a few times without going over the required application rate to prevent an uneven application.
How to apply liquid pre-emergents
What you’ll need: A sprayer. Most liquid pre-emergent herbicides come with their own sprayer tool, but you also can purchase a portable pressure sprayer tool, which comes in various sizes and types.
What to do next: Mix your liquid pre-emergent with water according to the correct ratio found on the herbicide label. Apply your liquid herbicide to the area of your yard systematically, row by row, just as you mow your lawn.
Apply your liquid pre-emergent evenly.
Selective vs. non-selective herbicides
How you spread your pre-emergent also depends on if you chose a selective or non-selective herbicide. For instance, if you chose a selective pre-emergent specifically for crabgrass, you can apply the pre-emergent evenly over any area with that specific weed without worrying about harming other plants or grasses in that area.
If you choose a non-selective pre-emergent, you’ll need a steady hand and an eye for detail. Applying a non-selective pre-emergent puts you at risk of killing healthy, non-weedy vegetation, so you have to make sure to carefully spray the area in which you’re looking to prevent weeds.
Water your lawn after applying herbicide
Watering and pre-emergents go hand in hand. Almost all types of pre-emergent herbicides — organic, synthetic, liquid, or granular — require water to work.
You should apply water relatively soon after (within 3-5 days) to ensure the herbicide moves through the soil to the seeds.
Pre-emergents should not be applied on wet lawns. Wait until your grass is dry, then water your lawn after you have applied your herbicide.
Pro Tip: Granular pre-emergents tend to need more water than liquid types, so make sure to saturate the area thoroughly to ensure the herbicide breaks through to the seeds.
Common pre-emergent application mistakes
Uneven application: If you miss some areas or are inconsistent in applying your pre-emergent herbicide, you may see patches of weed pop up in your yard.
Applying too little or too much herbicide: If you apply too little herbicide, the product won’t work and you’ll soon have weeds sprouting in that area. Applying too much herbicide can harm the grass and other vegetation surrounding the weeds, and can cause issues later on when you go to reseed that area.
Don’t combine pre-emergents with other lawn care
Applying your pre-emergent is a job that should be done on its own. If you have recently aerated, overseeded, or fertilized your lawn, it may hinder the herbicide’s performance.
You should also wait at least a year to place new sod over locations where you used pre-emergent herbicides.
Kinds of weeds pre-emergent herbicides block
Pre-emergent herbicides prevent weeds depending on the season, so knowing what weed thrives in your yard is essential to the timing of your weed prevention.
For example, seeds like spurge, which appears in the summer, and fescue, which shows up in the winter, grow extremely fast.
Typical summer annual and late spring grassy and broadleaf weeds include:
In the winter, you might notice weeds like:
- Bluegrass (poa annua)
- Prickly lettuce
FAQ about pre-emergent herbicides
Applying pre-emergent herbicides on grass is OK, with a few exceptions — if your turf has been damaged by drought, pests, or is struggling to grow. Also, if you recently overseeded your yard, you should wait at least 60 days to apply pre-emergent herbicides so the chemicals don’t keep your grass seeds from growing.
Certain pre-emergents, such as organic corn gluten, are safe for pets, but many synthetic herbicides are not. If you apply synthetic pre-emergent herbicides, block off the area of grass and wait at least 24 hours for the herbicides to be absorbed into the ground before letting your pets or children on the grass.
It may seem like a quick fix to use your sprinklers to apply your liquid pre-emergent herbicide, but it can be dangerous. Placing these chemicals in your irrigation system can ruin the equipment but harm the ecosystem. The best way to apply a liquid weed preventer is through a sprayer.
You should apply a pre-emergent herbicide two or three times a year — along with a post-emergent herbicide — to keep your yard weed-free.
Next step: Post-emergent herbicides
Weeds are some of the most durable plants, and it can be tough to prevent them entirely in your yard. Pre-emergent herbicides are used as a barricade for the weeds, but post-emergent herbicides work on weeds that have already grown.
Post-emergent herbicides don’t have to be applied in a specific season like pre-emergents. Apply post-emergent herbicides when you see dandelions, crabgrass, and other weeds dotting your yard.
In fighting weeds in your yard, think of pre-emergents and post-emergent herbicides as a one-two punch. With the double impact of pre- and post-emergent herbicides, you can effectively knock out most weeds that would mar your gorgeous green lawn.
Applying pre-emergent herbicides and post-emergents can be an easy job for homeowners, but if you don’t have the time or are concerned that you won’t apply the herbicide correctly, a Lawn Love pro near you can take this lawn care job off your hands.