If you want a weed-free lawn, it’s best to be proactive and prevent them from sprouting their ugly heads in the first place. 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. Learn more about when and how to apply pre-emergent herbicides so you can have a healthy, lush lawn.
- What is pre-emergent herbicide, and how does it work?
- When to apply pre-emergent herbicides
- How to apply pre-emergent herbicides
- How to determine which pre-emergent is best
- Tips for applying pre-emergent herbicides
- FAQ about applying pre-emergent herbicides
What is pre-emergent herbicide, and how does it work?
Pre-emergent herbicides inhibit seed cell division and stop weed growth in its tracks. They keep weeds from emerging through the soil by depriving the weed seeds of essential nutrients and creating a chemical barrier in the top layer of the soil, thus preventing root development.
Pre-emergent herbicides can be selective or non-selective.
- Selective herbicides target only specific weeds, so they’re safe to use in areas with other plants and grasses. If only one type of weed grows in your yard — or a few — look for a pre-emergent herbicide that can block that weed from growing.
- Non-selective herbicides kill everything they touch (weeds, plants, and grasses), so we recommend using a steady hand when applying a non-selective herbicide. Typically, homeowners choose non-selective herbicides when targeting a localized area like a driveway.
To successfully target weeds, you have to know what weeds thrive in your yard and what season they grow.
When to apply pre-emergent herbicides
To get ahead of the game in your fight against weeds, you have to know what weeds grow in your yard and which season they prefer. Ground temperature also has to be just right for a weed-free yard.
Check your weather app before applying your pre-emergent so that it doesn’t immediately wash away with a summer rain shower or blow away in the wind. Temperatures must be warm and relatively humid, but not too humid as dew can dilute your product and lower its effectiveness. Wait for the ground to dry thoroughly before applying the herbicide, especially if you’ve experienced a period of consistent rain.
Weeds pop up according to season; some thrive in the warmer months, while others make an appearance when temperatures drop significantly. Time your pre-emergent application according to what grows in your yard, and consider a post-emergent if these troublesome plants still manage to squeeze their way into your yard.
For winter and fall weeds:
Apply your pre-emergent from August to November to kill weeds germinating when temperatures drop. Consider a fall application even if your grass looks weed-free at the beginning of fall.
During this time, you might notice weeds like:
- Bluegrass (poa annua)
- Prickly lettuce
- Mouse-ear chickweed
- Hairy bittercress
- Downy brome
- Field pennycress
For late spring and summer weeds:
These types of weeds emerge in spring or summer, develop and produce seed during the summer, and die when the first frost hits. To prevent appearance, apply pre-emergent herbicide in early spring (around mid-March), after your grass or flowers have started growing but before weed germination has started.
The timing must be right to avoid killing all seeds in the ground (for example, if you’ve overseeded).
Typical late spring and summer annual grassy and broadleaf weeds include:
- Canada thistle
- Broadleaf plantain
- Creeping charlie
By ground temperature
Once you know the right time of year to apply pre-emergents, you must also pay attention to the ground temperature.
For summer weeds:
- Apply a pre-emergent herbicide once the ground temperature has been 55 degrees for four to five days straight.
For winter weeds:
- Apply a pre-emergent herbicide in the fall when ground temperatures reach 70 degrees and continue to drop.
To check ground temperature, stick a thermometer 2 inches below the soil.
How to apply pre-emergent herbicides
To get the most out of your pre-emergent herbicide, you have to pay as much attention to how you apply it as when you apply it. To spread your pre-emergent correctly, you need the right tools, the correct type of pre-emergent herbicide (granular or liquid, selective or non-selective), and a systematic approach to your application.
Granular herbicides involve little mixing or measuring and cost less than liquids, but require a spreader and some dexterity for an even application. They’re recommended for smaller yards. In contrast, liquid herbicides allow for a more precise and even application on a larger area, providing faster results and requiring less watering. The downside is that they need more prep time, which can be a headache if you’re unfamiliar with these chemicals.
How to apply granular herbicides
What you’ll need: A broadcast spreader (either manual push or hand-held), your granular herbicide of choice, chemical-resistant gloves, work clothes with long sleeves, protective eyewear, socks, and shoes.
Prep: Before you do anything, read the label carefully. It will include information on application safety, protective equipment, directions for use, application rates, and precautions.
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. Finally, figure out the throw distance by choosing a crank speed on your spreader. The faster the crank speed, the wider the throw distance of the granules, targeting wider growing beds (or areas).
Application: For best results, walk in opposite directions on both sides of the area you’re targeting and release the handle on your equipment. Remember that your walking pace will influence the amount of product applied to your yard, so try to keep a steady pace for an application that rivals the pros.
To prevent an uneven application, make sure you pass over the same area a few times without going over the required application rate. When you’re done, water vigorously with ½ to 1 inch of water to ensure correct absorption.
How to apply liquid herbicides
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 in various sizes and types. You’ll also require your granular herbicide of choice, chemical-resistant gloves, work clothes with long sleeves, protective eyewear, socks, and shoes.
Prep: Mix your liquid pre-emergent with water according to the correct ratio found on the herbicide label.
Application: Apply your liquid herbicide to the area of your yard systematically, row by row, just as you mow your lawn. This will ensure an even application of herbicide. When you’re done, wait 12 to 24 hours for the liquid to be absorbed and your grass to dry, and resume your normal watering schedule. You don’t want the product to be washed away after all your hard work.
How to determine which pre-emergent is best
Pre-emergents target various weed types, and how you choose the right one should depend on your needs. We’ve rounded up the most common ingredients found in pre-emergents and listed the weeds they target – or don’t:
- Prodiamine – Prevents both grassy weeds and broadleaf weeds. These include chickweed, witchgrass, clover, thistle, crabgrass, creeping bentgrass, and dandelion. Prodiamine struggles to control nutsedge. It’s safe to use on established lawns.
- Dithiopyr – Prevents chickweed, oxalis, clovers, dandelion, bittercress, crabgrass, goosegrass, and poa annua. Dithiopyr has a tough time controlling spurge and broadleaf weeds.
- Benefin (aka Benfluralin) – Prevents crabgrass, carpetweed, ryegrass, chickweed, annual bluegrass, sandbur, pigweed, foxtails, goosegrass, and knotweed.
- Trifluralin – Prevents summer grass, chickweed, redroot, annual bluegrass, and red-dead nettle. Trifluralin isn’t your best choice to prevent thistle, clover, daisies, and wild turnips.
- Oxadiazon – Prevents crabgrass, clover, dandelion, thistle, and knotweed. There are better options to prevent chickweed, spurge, and pearlwort.
Tips for applying pre-emergent herbicides
Pre-emergent application must be done a certain way to ensure weed-control success. If you’re a lawn care beginner, bear these tips in mind as you go along:
- Keep your application even: 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. To prevent this, calibrate your spreader or sprayer according to the instructions, and walk across your yard at a steady pace. You can mark your lawn to know where to walk and where you’ve already applied the product.
- Apply the right amount of herbicide: If you apply too little herbicide, the product won’t work, and weeds will soon sprout 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.
- Avoid combining pre-emergents with other lawn care products: Applying your pre-emergent is a job that should be done on its own. Mixing it with other products may hinder its performance, especially if you have recently aerated, overseeded, or fertilized your lawn.
- Choose the right product for your needs: Pre-emergents are formulated for specific categories of grass and weeds. Selecting your herbicide should go hand in hand with the type of grass you have and the weed you’re trying to prevent.
- Wait before laying new sod: Experts recommend waiting approximately one year after applying any type of herbicide before laying down sod.
- Apply pre-emergent to dry grass: Wait until your grass is dry to apply your pre-emergent, then water your lawn to ensure absorption (this applies to granular pre-emergent).
- Ensure proper soil conditions: This means a soil pH window of 6.0 to 7.0 and not too much organic matter, as this can interfere with some herbicides and reduce effectiveness. It can also push the pH outside the recommended window.
FAQ about applying pre-emergent herbicides
Are pre-emergent herbicides safe for turfgrass?
Applying pre-emergent herbicides on grass is OK, with a few exceptions — if your turf has been damaged by drought or pests, or is struggling to grow. Also, if you’ve 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.
Are pre-emergent herbicides safe for pets?
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.
How often should I apply pre-emergents?
You should apply a pre-emergent herbicide two or three times a year.
How long after applying pre-emergent can I overseed?
Experts agree that you should wait between 8 and 10 weeks to overseed your lawn after applying pre-emergent. This gives the chemicals in the herbicide enough time to act and dissipate, preventing any harm to the grass seeds you spread. Bear in mind that the entire process depends on the type of grass you have, so adapt your overseeding and pre-emergent application accordingly.
What are post-emergent herbicides?
Weeds are some of the most durable plants, and preventing them entirely in your yard can be tough. Pre-emergent herbicides are used as a barricade for the weeds, but post-emergent herbicides work on already-grown weeds.
Post-emergents 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.
Think of pre-emergents and post-emergent herbicides as a one-two punch that can effectively knock out most weeds that would mar your gorgeous green lawn.
Keep your yard weed-free with a pro’s help
Applying pre-emergent herbicides is a fairly straightforward process that most homeowners can take up, but that doesn’t mean difficulties can’t arise. If you don’t have the time or are concerned that you won’t apply the herbicide correctly, an expert near you can take this job off your hands.
While you’re busy with pesky weed removal, don’t forget about other lawn care tasks necessary for a healthy, breathtaking lawn year-round. A local lawn care professional can aerate, dethatch, fertilize, and mow your lawn to your satisfaction – so you don’t have to.