When Should You Apply Pre-Emergent To Your Lawn?

Pre-emergent herbicides are truly the unsung heroes of weed prevention because they form an invisible barrier in the soil that stops weed seeds from sprouting. Gardeners and landscapers alike use them to keep unwanted plants at bay without disturbing their beloved greenery. 

"Pre-emergents prevent seeds from germinating–hence the ‘pre’ indicating before, and the ‘emergent’–as in when seeds start to emerge," American Meadows’ Tabar Gifford tells Southern Living. "The benefits of using a pre-emergent is that you can stop weed seeds from germinating in areas where you have existing desirable gardens, grass or live plants."

Gifford cautions, "It is important to note that pre-emergents are non-selective and can inhibit the germination of both weed and desirable plant seeds. Therefore, it should not be used in areas where you plan to sow seeds."

So when should you apply pre-emergent? Gifford says before weed seeds germinate, typically in early spring for summer weeds and late summer for winter weeds. 

"Proper application involves spreading the meal evenly over the soil and ensuring it remains dry for a few days to activate its pre-emergent properties," she says. "Moisture shortly after application can reduce its effectiveness."

Gifford tells Southern Living the pre-emergent she is most familiar with is  Corn Gluten Meal (CGM), because it’s what she used to plant her grass plugs.

CGM is a natural byproduct of corn processing that is used as an organic pre-emergent herbicide. 

"Its high protein content can inhibit seed germination by desiccating emerging new roots," Gifford explains. "However, its effectiveness varies as it takes some time to build up in the soil and be effective. You typically need several applications, sometimes over a year to two to be effective."

While this may seem like a lot of work, this is also one of the most “gentle” options as a natural byproduct of corn. 

"Other pre-emergents can be organic, or chemical so it’s important to know what you’re using and read all of the instructions and precautions carefully before you begin," she advises, adding that CGM is non-selective and can inhibit the germination of both weed and desirable plant seeds, so it should not be used in areas where you plan to sow seeds.

Additionally, due to its nitrogen content, CGM acts as a fertilizer, which can inadvertently promote the growth of existing weeds. 

"Other natural pre-emergent options include products containing ingredients like citric acid, botanical oils, and vinegar," Gifford says. "These substances can suppress weed growth, but their effectiveness varies, and they may require multiple applications."

When Should You Apply Pre-Emergent?

When it comes to applying pre-emergent to your yard, timing is everything. 

"For summer annual weeds like crabgrass, apply pre-emergents in early spring before the top two inches of soil reach temperatures between 50 to 55 degrees Fahrenheit for five consecutive days," Gifford says. "For winter annual weeds, a fall application is your best bet. Always monitor soil temperatures and be aware of your local climate to get it just right."

How To Apply Pre-Emergent

When you’re ready to get started Gifford advises:

Preparation: Mow your lawn to the recommended height and clear away any debris.

Application: Use a spreader for granular formulations or a sprayer for liquid ones, following the manufacturer's instructions to the letter.

Post-Application: Water the treated area with about 0.5 inches of water to activate the herbicide, unless the product specifies otherwise.

What Happens If You Apply Pre-Emergent Too Early

Gifford emphasizes how important it is to correctly time the application of pre-emergent.

If pre-emergent is applied too early, the herbicide may break down before the weed seeds start to germinate, rendering your efforts futile.

What Happens If You Apply Pre-Emergent Too Late

If pre-emergent is applied too late and weeds have sprouted, Gifford says "pre-emergents are about as useful as a screen door on a submarine. You'll need post-emergent herbicides or some good old-fashioned elbow grease."

Related: The Best Time To Fertilize Your Lawn, According To An Expert

Tips For Applying Pre-Emergent

Gifford stresses that it is important to remember that while pre-emergent herbicides are a powerful tool in weed management, "they're not a silver bullet." Combining them with other practices like mulching and manual removal will help you maintain a healthy, weed-free garden.

Other tips Gifford offers:

Soil Preparation: If aeration is on your to-do list, do it before applying pre-emergents to avoid disrupting the herbicide barrier.

Safety: Suit up with gloves, long sleeves, and eye protection. Keep kids and pets away from treated areas until everything is dry and settled.

Product Selection: Choose a herbicide that's labeled for the specific weeds you're targeting and is safe for your type of grass or plants.