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weed and seed fall

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.

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.

Preventer Science

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.

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 By the time fall seeding weather arrives, the chemicals are no longer active and the grass seeds will be able to sprout.

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.

2. Turfgrass Water Conservation Alliance; “Simple Tips,” October 2014.

Cool-season grasses, such as Kentucky bluegrass, perennial ryegrass and fescues, flourish in northern regions with warm summers and cold winters. Cool, moderate, early fall temperatures send these grasses into their most active growth season, which is the ideal time for fertilizing and strengthening.

A high-nitrogen lawn fertilizer with added potassium, such as Pennington UltraGreen Lawn Fertilizer 30-0-4, stimulates dense, healthy growth and supports winter and summer hardiness. The added boost of natural organisms from Pennington’s Myco Advantage technology increases the surface area of grass roots, improving your lawn’s ability to absorb water and nutrients, and enter winter prepared.

Fertilizing Warm-Season Grasses

1. Michael Goatley Jr., Shawn Askew and David McCall, “Fall Lawn Care,” Virginia Cooperative Extension.

Like lawn grasses, common lawn weeds are either warm- or cool-season plants that follow the same seasonal growth peaks that lawn grasses do. Annual weeds, such as warm-season crabgrass, complete their life cycle in one year – but leave plenty of seeds behind for years to come. Perennial weeds, including cool-season plantains, live many years. Untreated, they come back from the roots year after year and spread extra seed, too.

Pennington and Myco Advantage are registered trademarks of Pennington Seed, Inc. Amdro and UltraGreen are registered trademarks of Central Garden & Pet Company.

Feed warm-season grasses their last feeding of the season six to eight weeks before the average fall frost date in your region. They’re still actively growing, but they’ll soon need to harden off and prepare for winter. Bermuda grass has higher nitrogen needs than some other warm-season grasses, so it can be fertilized up to four to five weeks before frost. 1 But don’t feed warm-season lawns any later. Fertilizing too late in the growing cycle can stimulate late-season growth that delays dormancy and makes lawns more susceptible to winter injury.

Dandelion, clover, spurge, and numerous other broadleaf weeds are stimulated as well. If you should have just a few weeds here and there, pull them by hand. However, if you are overrun, fall is an excellent time to control those broadleaf weeds. Perennial broadleaf weeds are busy sucking up much-needed nutrients to store for overwintering. An application of Gordon’s Trimec or Speedzone can be applied 28 days after grass seed germination. A fall application of Gordon’s Trimec with Uncle’s Stikit, spreader sticker or a granular application of Loveland Weed and Feed will be quickly absorbed by the broadleaf weeds and is sure to kill to the roots.

Each year during the fall over seeding season, we get a lot of folks that come in to the Grass Pad and say, “I followed your fall lawn renovation program; I used your weed-free grass seed. Why do I have weeds in my yard?” Our response is “Well, it means you really did it right!”

When properly preparing your lawn for fall overseeding, you created a pathway for grass seed to reach the soil. If grass seed didn’t reach the soil, it would not grow. Raking, verticutting, dethatching, or core aerating are all methods to ensure seed to soil contact.

Controlling Broadleaf Weeds after Seeding

Disturbing the soil (verticutting, aerating, dethatching or raking) exposes soil and any pre-existing weed seeds that lay dormant in the soil. These weed seeds could be from last year or several years ago, suspended under the soil, too deep to germinate waiting for their opportunity to spring into life.

Grassy weed seeds appreciate the same fertilizer and additional watering from your fall renovation program and flourish. Don’t panic – annual grassy weeds will be slow to germinate as soil temperatures cool in fall. Mother Nature will take care of these annual weeds at first frost. Using Prevent, crabgrass preventer in mid-April will eliminate their return.