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Saturday, May 18, 2002

By Sandy Feather

Q. We built a house three years ago. We have concentrated on getting the house finished and have planted trees and shrubs. However, we have not paid much attention to our lawn. It has grown in well, but there are quite a few weeds. Are there steps we can take to improve the quality of our lawn without using lots of pesticides? I am willing to use herbicides to get the weeds under control but would prefer to minimize our pesticide applications as much as possible.

A. Proper cultural management of your lawn will go a long way toward improving its health and appearance without using a lot of pesticides. You may need to make a few herbicide applications to get the weeds under control. Once you have accomplished that, you can control the occasional weed by hand-pulling or spot-treating with a liquid herbicide.

	Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

Start controlling broadleaf weeds such as dandelions and plantain now. You can use a granular weed-and-feed product if you have not fertilized your lawn yet. Be sure to apply weed and feed to turf that is moist from dew or rain so that the granules stick to the leaves of the weeds. That gives the product a chance to be absorbed by the weeds.

If you have already applied fertilizer, choose a liquid broadleaf weed killer instead. Herbicides labeled for home lawn use generally contain a combination of 2,4-D, MCPP and dicamba. Tough-to-kill weeds such as clover and ground ivy will require additional applications. You often get the best control of tough weeds by making herbicide applications in late summer (mid-August to early September). You still may need two or three applications to get sufficient control.

You will have to use a liquid herbicide rather than weed-and-feed for repeated applications, because that much fertilizer would burn your lawn. Check the label of the herbicide you are using for suggested application intervals. Even though some of these weeds seem indestructible, keep after them and repeat your applications at the shortest recommended intervals until you are satisfied.

Mowing practices greatly determine the quality of turfgrass. If you allow your lawn to grow long, then cut it short, you are not doing it any favor. Most species of turfgrass should be cut at a height of 2 1/2 to 3 inches. There is a direct relationship between the height of cut and the depth and extent of the root system. The shorter you cut it, the less root system it will have.

Since summer heat and drought are the most stressful for the cool-season grasses we grow, leaving the grass longer to encourage an extensive root system will make your lawn more drought-tolerant. Leaving the grass longer shades the soil, moderating soil temperatures and helping to conserve soil moisture. Longer grass also shades out germinating weed seeds and crowds out weeds that try to become established.

Do not remove more than one-third of a blade of grass in one mowing.

Be sure to sharpen your mower blade regularly. A sharp mower blade makes a clean cut that the grass recovers from easily. Dull mower blades shred the grass, making jagged wounds that are harder to heal. They can serve as a point of entry for insect and disease problems. How often you should sharpen your mower blade depends on the size of your lawn and the number of obstacles it's likely to encounter. Once a month for very large (or obstacle-filled) lawns or every other month should be sufficient. Very small lawns can get by with once a year.

Deep, infrequent watering during hot, dry weather is important to maintain a healthy lawn. Deep watering encourages the turf to develop a deeper, more extensive root system. Conversely, frequent, shallow watering encourages a shallow root system, which means a lawn that is under drought stress when the top couple of inches of soil dry out.

Use a sprinkler or an irrigation system to apply 1 to 2 inches of water weekly to your lawn if we are not receiving any rain. This is best applied in one long, deep soaking session, rather than watering your lawn a little bit every day. Our clay soils can absorb only about 1/2 inch of water an hour, so it should take two hours of watering to apply an inch of water.

To measure how long you have to run your sprinkler or irrigation system, take a flat-bottomed container such as a coffee can and mark off 1/2-inch increments. Place the can or cans where it will be hit by the water, and time how long it takes to gather 1/2 inch of water. Then run your sprinkler twice as long. You may need to apply water even more slowly to steep slopes to avoid wasting water to runoff. It is best to water in the morning. If you water during the heat of the day, too much water is lost to evaporation. If you water at night, the grass stays wet too long and may get disease problems.

A soil test will help you design a fertilization program that provides what your lawn needs for optimum health and growth. Inadequate or excessive fertilization can limit turf growth. A soil test will tell you what type of fertilization program is required to provide your lawn with the nutrients it needs to perform well. It will also tell you what you have to do to get your soil pH (acidity or alkalinity) into the optimum range of 6.5 to 7.0 that most turfgrasses prefer. Soil test kits are available from your local Penn State Cooperative Extension office for a nominal fee.

Typically, late spring (mid- to late May), late summer (late August to mid-September) and late fall (mid-November) are the best times to apply fertilizer. Liming is best done in fall. Limestone moves through the soil very slowly and takes time to effect the desired change in pH. If your soil test reveals that your lawn needs 100 pounds or more of limestone per thousand square feet, break it into two applications, fall and spring.

You should check your lawn for thatch. Thatch is nothing more than a layer of organic matter between the soil surface and the crowns of the grass plants. Dig up a small square of turf so you can look at the soil profile. The thatch layer is easily visible. Thatch is created when growing turf-grass sloughs off dead stems and roots. A thin layer of thatch -- 1/2 inch or less -- is desirable. It acts as a mulch, moderating soil temperature and maintaining soil moisture.

More than that creates problems, though. A thick layer of thatch can keep water from reaching the soil, so your lawn is constantly drought-stressed. And that creates more thatch. Thatch is a breeding ground for insect and disease problems. Even worse, if your lawn does develop a problem with white grubs (soil-dwelling insects that feed on turf roots), a thick thatch layer can make it very difficult to get an insecticide down to where the grubs are feeding.

The causes of thatch include the variety of grasses in your lawn. Bluegrass and creeping red fescue are the worst thatch formers of the cool-season grasses. If your soil pH is lower than 6.5, the microbes that live in the soil and break down thatch can't function. If you overfertilize your lawn, or if you water shallowly and infrequently, you encourage thatch buildup. Allowing your grass to grow too long, then cutting it too short and leaving the grass tips on the lawn will also contribute to thatch buildup.

A moderate layer of thatch -- between 1/2 and 1 inch -- can be removed by dethatching. Dethatching is stressful and should be done only in fall. You can rent dethatchers, or hire a lawn service. Run the dethatcher in one direction, then go over your lawn in the perpendicular direction. A good dethatching should make you want to cry when you look at your lawn. Overseed with varieties of turfgrass that match your lawn to help it recover.

If you have more than 1 inch of thatch, consider a total renovation -- removing your existing lawn and starting again from scratch. The knives of most dethatchers will not go deep enough to get through the thatch and down to the soil.

If you do not have a thatch problem, but the soil is compacted, rent a core aerator. They are available from many tool rental shops, or you can hire a lawn service to do the aerating for you. Core aerators pull 3- to 4-inch plugs of soil out of the ground and leave small holes behind. This helps aerate the soil (yes, roots need air!) and alleviate soil compaction. Fall is ideal for core aeration, but it can be done in the spring as well. If your lawn does not have a lot of activity on it, core aeration every three years or so will keep the soil aerated sufficiently. If all the neighborhood kids play in your yard, consider yearly core aeration in late fall to keep soil compaction to a minimum.

It is good to leave the cores on the soil surface, where they will break down during winter's freeze-thaw cycles. If the cores are creating too much of a muddy mess (when children and/or pets play on the lawn), you can rake them up and put them on the compost pile.