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Lawns with dead spots, spraying newly seeded grass, and quackgrass in raspberries

"Fielding Questions" columnist Don Kinzler also answers questions about spraying newly seeded grass and dealing with quackgrass in raspberries.

Lawn May 21, 2022.jpg
A reader wonders what they can do about dead patches in lawns after last year's drought.
Contributed / Special to The Forum
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Q: It seems the drought last summer caused a very patchy and sparse lawn with areas of dead grass. Should I seed the larger areas and top-dress the sparse areas? When should fertilizer be applied and how often? Any other suggestions would be appreciated. — Valerie U.

A: My email inbox is filled with photos just like yours, showing lawns with dead patches and thin spots. The key to getting lawns back to health is patience and doing all the small recommended steps.

Here’s a summary of coaxing lawns into recovery: Rake any dead patches and thin spots so grass seed will contact some soil. You can also top-dress the areas with a half inch of topsoil. Select a grass seed with 50% Kentucky bluegrass cultivars for sunny areas or 40-50% fescue cultivars for shaded spots.

Fertilize at seeding time with a lawn fertilizer labeled as a starter-type fertilizer which won’t burn tender new seedlings the way standard-strength lawn fertilizer can. Apply starter fertilizer to the entire lawn now in May, and in September apply a standard-strength lawn fertilizer. The September fertilizing is a very important step for all lawns.

Areas that are reseeded need to be kept continually moist at the surface for grass seed to sprout. A light covering of lawn patch products helps reduce evaporation.

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Raise the mowing height to 3 or 3.5 inches. Research consistently shows that lower mowing heights create a less-healthy lawn. Lawns mowed shorter develop a shallow root system, weeds are more prevalent, moisture evaporates quicker and heat stress is common. A higher height, mowed crisply, creates a thick, dense turf.

Q: I sprayed our lawn for weeds, and now I’d like to reseed some of the bare areas. How long do I have to wait before seeding, now that I’ve sprayed? — John H.

A: Most common lawn herbicide labels say to wait about four weeks before seeding grass. Lawn herbicides and their residue can cause damage to tender grass seedlings that aren’t yet tough enough to withstand chemicals.

Applying weed killers and seeding grass aren’t often compatible until enough time has passed between the two operations. When seeding bare spots or overseeding a thin lawn, a better approach to weed control is hand-digging or careful spot-spraying of weeds instead of applying herbicides to the entire lawn.

Q: What products are available for use in a mature raspberry patch to remove quackgrass? — Sandy C.

A: Luckily, there are grass-killing herbicides that will selectively remove quackgrass from non-grass plants like raspberries, peonies, asparagus and others. Two such products are Ortho Grass-B-Gon and Bonide Grass Beater.

These herbicides can be applied right over the tops of actively growing perennials or raspberry plants. They can even be used on perennial flowers like daylily and iris, which are both listed on the product labels, among many other types of plants.

It’s important to follow all directions on the label. The quackgrass must be actively growing and 4 to 6 inches high. I’ve found these products to be effective, but they are slow-acting, so results might not be evident for seven to 14 days.

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Quackgrass is tricky. There are usually latent buds in the underground rhizomes that wait until the rest of the quackgrass plant appears dead, and then they begin growth. If a second flush of quackgrass appears, apply the herbicide again. Persistent weeds can be conquered if our persistence outweighs that of the weeds.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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