Chemical weed preventers, also called preemergent herbicides, are usually granules or liquids, but both require water to work. As the preventer soaks into the ground, it leaves a residual film in the top 1-inch of soil. Because most seeds germinate at or just below the soil’s surface, these preemergent herbicides remain active against any germination processes for up to four months, depending on the chemicals involved. Organic weed preventers work in a similar way. With many weeds being members of the grass family, all seeds, including desired lawn species, fail to germinate and sprout after you’ve used a weed preventer.
Cool-season grasses are usually seeded, as opposed to warm-season grasses that usually need to be grown from sod or plugs. Because cool-season grass seeds germinate best in fall, apply your chemical preventer in spring to actively kill off weeds in spring and summer. In general, temperatures between 65 and 70 degrees Fahrenheit are good for weed preventer application. Hot days often cause the chemicals to vaporize into the atmosphere, reducing their effectiveness. By the time fall seeding weather arrives, the chemicals are no longer active and the grass seeds will be able to sprout.
Time It Right
Avoid the need for weed preventers by keeping your lawn healthy. Once established, only water your turf once a week during the growing season. Up to 1 inch of water during this watering session allows roots to search deeply for moisture to create strong grass. Shallow grass roots die in stressful conditions, like drought, and allow weeds to grow in thinned spots. Allow your turf to grow to a healthy height as well, typically between 1 and 3 inches, depending on the species. Long grass blades mean the grass can produce enough energy to stay healthy and compete with weeds. In short, healthy and well-maintained grass has less problems with weed growth.
Spreading seed is an inexpensive way to grow a lush lawn, but exposed soil between germination and establishment makes it vulnerable to weeds. Although chemical weed preventers have different mixtures and instructions, you should not apply them while seeding or immediately afterward. You must allow one to four months between applying this type of chemical and spreading seed.
Even if you time your weed preventer and seeding periods correctly, you need to do the job right to get an even lawn with no bare patches. Apply seeds uniformly across your yard using a drop spreader on a mild fall day. Spread up to 1-inch of organic mulch over the seeds to conserve moisture and encourage germination. Water the seeds at least twice a day for short, 10-minute sessions. You do not want to wash away the seeds, but they need consistent moisture to grow. Hand pull any weeds that appear while the grass seedlings develop. Do not apply any chemicals for weed control.
Let’s start with the herbicides. Weed and feed products designed for early-spring application usually contain pre-emergent herbicides. They work by preventing weed seeds from sprouting, and they can be an effective way to control crabgrass and some broadleaf weeds.
But here’s the problem: Early spring probably isn’t the best time for you to fertilize your grass or apply herbicides unless you have a history of weed problems.
And unless your weeds are running rampant, try spot spraying them in the fall instead of putting down herbicide over your entire lawn. That’s just one small step you can take for sustainability.
Trouble is, this assumes that you’ve got weed seeds in your soil ready to sprout. If you’ve been using pre-emergent herbicides regularly or otherwise doing a good job of controlling weeds and keeping them from going to seed, you may have exhausted the supply of weed seeds in the soil. If that’s the case, applying pre-emergent herbicides is like clapping your hands to keep the lions away.
Editors note: Rossi is a turf specialist and associate professor in the Department of Horticulture, Cornell University. This is the first in a series where Rossi debunks common lawn myths. His advice targets cool-season grass growing regions in the Northeast, but may be applicable in regions with similar growing conditions.
For perennial weeds such as Plantain, Dandelion, Knotweed and Clover, apply a post-emergent herbicide in fall to send the killing chemicals directly to roots. This treatment will help reduce the numbers of these weeds in spring.
Use only on established lawns of the following turf types: Bermudagrass, centipedegrass, all varieties of St. Augustinegrass (including floratam) and zoysia.
Fall is also an excellent time to control perennial weeds, because that’s the time of year when plants begin a process of winter food storage, shifting internal foodstuffs from leaves to roots. If you treat perennial weeds with herbicide in fall, the chemical moves from leaves to roots, essentially killing the weed at the root.
Apply anytime – spring through fall at temperatures between 50 degrees F and 90 degrees F.
The best treatment for Annual Bluegrass and Crabgrass is a Crabgrass preventer, a pre-emergent herbicide that combats grassy weeds and can be applied in spring or fall. Unfortunately, these products aren’t traditionally sold year-round in all regions. If you know you’ll be dealing with these weeds in fall, it’s best to purchase a Crabgrass preventer in spring and store it in a cool, dry place until autumn’s cooler days arrive.
Do not use on Bahiagrass, Carpetgrass, Centipedegrass, St. Augustine including Floratam or Dichondra.
3.2 fluid ounces (6+1/2 tablespoons) of product mixed with 1 gallon of water.