Before applying pre-emergent and new grass seed, it is essential to keep in mind that the seeding must be completed no less than 45 days before the average first fall frost. Once some pre-emergents are applied, you only have to wait seven to 10 days before reseeding. The same is true of laying sod after pre-emergent.
Crabgrass post-emergent herbicides are different than pre-emergents because they are applied to crabgrass that is already visible. These post-emergents are made with glyphosate and are mixed at a ratio of 2/3 ounces for every gallon of water for crabgrass less than 6 inches high. This ratio increases if the crabgrass is taller. Seeding after crabgrass preventer is applied is similar to seeding after pre-emergents, and lawns can be reseeded one week after the glyphosate is applied. If you are unsure, read the instructions on the packaging since some manufacturers may have different directions.
When to Use Pre-emergents
Horizon Online explains that early fall and early spring are the best times of year to apply pre-emergents since this is right before the time when most weeds sprout. Knowing the type of weeds to target is essential since pre-emergents are designed for different species. The package will have directions for mixing the solution, so these must be followed. The pre-emergent spray should blanket the area rather than being used to spot-cover smaller sections. There should also be directions for watering the area after the pre-emergent is applied.
Rake over any bare spots and turn over the dirt. Topsoil can be added to any remaining bare spots, as this will enrich the soil. Add the grass seed to the spots and water daily until it germinates. Frequent watering will help the root system get established.
The waiting time for seeding after crabgrass preventer depends on what type of product is used. Crabgrass pre-emergents may have different ingredients that require longer waiting periods before reseeding can commence. If the grass is planted too soon afterward, the grass will not grow. Waiting times depend on the chemicals in the pre-emergent; for example, if it has bensulide, it may take up to four months before reseeding is recommended.
Pre-emergent herbicides need to be mixed correctly for the spray solution to be at the appropriate strength. Take the time to read the manufacturer’s recommendations and don’t forget to calibrate your sprayer!
The weed will only be killed when it begins to sprout from the seed and hits the herbicide barrier. It is possible for seeds to remain dormant and not be harmed by the pre-emergent herbicide application. This is why weed control is a constant process. There will always be seeds under the surface and a portion will germinate each season. Annual applications must be made to significantly reduce large infestations.
Principle #3: Pre-emergent herbicide must be watered in.
How Pre-Emergent Herbicide Works
To get a better idea of how pre-emergent works, let’s look at 3 key principles of pre-emergent weed control.
What weeds do you want to control?
Thorough coverage is key. Think of pre-emergents like a blanket – you need to cover an entire area through which the weed seeds cannot germinate. Spot spraying achieves nothing, as there is plenty of open space for weeds to come through. Manufacturer instructions will indicate how much product to use “per 1000 square feet” or “per acre”, which determines how much herbicide to use for each gallon of water. Note that it usually takes 1 to 2 gallons of spray solution to cover 1000 square feet.
Most well-known example: Annual Bluegrass (Poa Annua)
Other examples: Shotweed, Chickweed, Mustards.
Life Cycle: 1 year – Germinate in fall. Flower and produce seed quickly, then die in spring.
Pre-emergent timing: Late summer/early fall (rule of thumb is by September 15th)
Dithiopyr – Effective on about 45 grassy and broadleaf weeds, Dithiopyr is the active ingredient in Dimension and several other brands. It is one of the few pre-emergent lawn care products that has some effectiveness against weeds that have already sprouted. But it needs to be used at maximum strength for that to happen. Effective for about four months after application.
Herbicides that contain isoxaben, simazine, or oxyfluorfen, for example, kill some broadleaf weeds but are ineffective against others and against invasive grasses. Herbicides classified as a dinitroaniline, napropamide, metolachlor and dichlobenil will kill invasive grasses and some – but not all – broadleaf weeds.
Pay particular attention to the label of the pre-emergent product if you have a new lawn, or intend to reseed. If you pick a variety that kills grassy weeds, likely it will kill any new desirable grass seed as well. Most pre-emergence products lose effectiveness after about six to eight weeks, so wait at least that long before reseeding.
Could It Damage Your Lawn?
You can find localized soil temperature readings online or from your county’s Extension Service. For the most localized data of all, plunge a gauge into your own turf. Simple soil temperature gauges can be found online or at garden shops for $8-$15. A meat thermometer with a 3-inch probe will serve the same purpose.
“The timing is a little tricky,” says Dr. Rebecca Grubbs-Bowling, assistant professor and turfgrass specialist at Texas A&M University. She is the author of the newly published “A Homeowner’s Guide to Herbicide Selection for Warm-Season Turfgrass Lawns.”
Some herbicides can hurt your grass. Pre-emergent herbicides can damage new lawns and shouldn’t be used until the grass has settled in for a few months. Products containing dicamba can damage St. Augustine and carpetgrass if applied at the wrong time. Also, methylated seed oil, often used in treating crabgrass, should never be used on St. Augustine, carpetgrass, Bermuda, or centipede grass.
“Be realistic and reasonable,” Grubbs-Bowling says. “The best defense against weeds is a healthy and competitive yard.”