Yard and Garden: Late Summer Lawn Care

AMES, Iowa — Many tasks can be done in Iowa during late summer and early fall that help lawns grow well. Most lawns in Iowa are comprised of Kentucky bluegrass and other cool-season grasses. Proper mowing, overseeding, aeration, weed control and fertilization late in the growing season can promote lush, […]

AMES, Iowa — Many tasks can be done in Iowa during late summer and early fall that help lawns grow well. Most lawns in Iowa are comprised of Kentucky bluegrass and other cool-season grasses.

Proper mowing, overseeding, aeration, weed control and fertilization late in the growing season can promote lush, green, good-looking, cool-season lawns. In this month’s “Yard and Garden,” Aaron Steil, consumer horticulture specialist with Iowa State University Extension and Outreach, offers late summer lawn care advice.

What are best mowing practices for the fall?

During the stressful, hot summer months, lawns are mowed at 3.0 to 3.5 inches tall. The cooler temperatures of fall promote better growth. In September, lower the mowing height to 2.5 to 3 inches. Continue to mow the lawn until the grass stops growing in late October/early November. When mowing the lawn, never remove more than one-third of the total leaf area at any one time. 

How do I overseed my thin and sparse lawn?

Mid-August through mid-September is the best time to renovate a thin lawn. Sowing grass seed in late summer has several advantages over spring seeding. Cool-season grass seeds germinate quickly in the warm soils of late summer. Once the grass germinates, the warm days and cool nights of fall promote rapid turf growth. Also, there will be less competition from weeds, as few weed seeds germinate in late summer and fall. 

To reduce the competition from the established turfgrass, mow the lawn at a height of 2.0 to 2.5 inches before seeding. Successful overseeding requires good seed-to-soil contact. Core aerators, vertical mowers and slit seeders can be used to ensure good seed-to-soil contact. 

After seeding, keep the upper 1 inch of soil moist with frequent, light applications of water. The seeds of most turfgrasses should germinate in two to three weeks if the seedbed is kept uniformly moist. Gradually reduce the frequency of watering, but water more deeply, when the grass seedlings reach a height of 1 to 2 inches. 

Why and how should I aerate my lawn?

Early September is the best time to aerate lawns in Iowa. Aeration relieves soil compaction, improves water and nutrient movement in the soil, and prevents thatch accumulation. 

Aerate lawns with a core aerator. Core aerators have hollow metal tubes or tines that remove plugs of soil. Avoid spike-type devices that simply punch holes (compacting the soil) in the ground. Core aerators can be rented from hardware or machine rental stores or the service can be done by a professional lawn care company.

Remove soil cores that are approximately three-fourths of an inch in diameter and 3 inches long. For best results, aerate lawns when the soil is moist. Avoid aeration when soils are dry or wet. The tubes or tines will not be able to penetrate deeply when the soil is dry and may get plugged with soil when the soil is wet. Lawns that are properly aerated should have 20 to 40 holes per square foot. Since most core aerators won’t remove the proper number of holes with a single pass, several passes are often necessary. After aeration, pull a drag mat or weighted piece of fencing material across the lawn to break up the soil cores on the soil surface. If the lawn is smaller, use a stiff rake to break up the soil cores.

When and how should I apply herbicides for weed control in my lawn?

In small areas, some weeds can be controlled by pulling and digging. This method is best accomplished after a soaking rain or deep watering. Unfortunately, pulling and digging is often ineffective on deep-rooted weeds. 

In many situations, herbicides are the only practical method of weed control. Perennial broadleaf weeds, like dandelion, clover, thistle and plantain, are best controlled with a herbicide application in late September to early November in Iowa. Effective broadleaf herbicides include 2,4-D, MCPP, MCPA, dicamba, triclopyr and others. The most effective broadleaf herbicide products contain a mixture of two or three herbicides as no single compound will control all broadleaf weeds. Broadleaf herbicides can be applied as liquids or granules. Do not apply herbicides to newly seeded lawns until the new grass has been mowed at least twice. Herbicides must be used according to label instructions on the package.

Perennial grassy weeds, like quackgrass and tall fescue, have few options for selective control. If hand digging or pulling is not practical, the most effective herbicide option is the nonselective herbicide glyphosate (Roundup). Nonselective herbicides kill virtually all plants (both desirable and weedy) to which the material is applied. Spot treat the weedy perennial grasses. More than one application may be necessary to kill difficult to control weeds. Always apply herbicides when winds are calm and temperatures are cool to prevent drift and damage to desirable plants.

While perennial weeds are best controlled in the fall, the control of annual broadleaf and grassy weeds like crabgrass and prostrate knotweed is best done in spring with a pre-emergent herbicide.

When should I apply fertilizer on my lawn?

Fertilizer applications can be made in mid-September and again in late October/early November. Mid-September fertilization promotes a moderate rate of shoot growth and helps to thicken the turf. An application of fertilizer in late October/early November (at the time of the last mowing) promotes root growth and early green-up next spring. With each application apply 1 pound of actual nitrogen per 1,000 square feet utilizing a slow-release fertilizer.

For low maintenance lawns, a single application in late October/early November is all that is needed. High maintenance lawns can utilize a third application at the same rate in spring (April or May).

Shareable photo: Fall lawn.

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