Fixing a problem lawn

Eliminating weeds from the soil before planting the right grass for your area is the best way to solve most weed problems. How do you get rid of weeds without planting a new lawn? You'll need to know which weeds you have, since different weed types require different herbicides.

Two major weed types

Lawn weeds are either annual or perennial grass weeds or broadleaf weeds. Grass weeds have parallel leaf veins, while broadleaf weeds have branched veins.

Herbicides

These include post-emergence herbicides, which kill growing weeds, pre-emergence herbicides, which kill germinating weed seeds.

Post-emergence herbicides include four types. Nonselective types kill whatever plants they contact, while selective versions kill some plants but not others--most commonly broadleaf weeds but not grasses. Contact herbicides kill only the parts of a plant they contact, while systemic types circulate throughout the plant.

Herbicides are often combined with fertilizers, though these can be hard to apply at the right time for weed control or fertilization. You'll also find herbicides in spray bottles or as concentrates that you dilute and apply.

Guide to common lawn weeds details which herbicide works for which type of weed. Some herbicides can also harm more than just weeds, however.

Hint

Begin with the least-toxic weed remedies and use synthetic herbicides only as a last resort. Many can harm you and the environment, and can damage nearby plants through leaves or roots if applied incorrectly. Natural alternatives such as corn-gluten meal can prevent crabgrass and other weed seeds from germinating without threatening anything else.

For more tips, see the University of California's integrated Pest Management Web site (www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74113.html).

While some insects can become numerous enough to do serious damage, they're blamed for more lawn issues than they actually cause. A simply drench test will confirm whether insects are the problem.

Begin by adding 8 ounces of liquid dish soap to 2 gallons of water. Then sprinkle the mixture over 1 square yard of lawn, covering both healthy and damaged turf. Hidden insects will move to the surface within minutes, letting you identify which you have and how many.

You'll find an array of biological controls, botanicals, and synthetic insecticides. See Guide to common lawn pests for which controls work best for which insects. Some insecticides can harm more than just the insects they target, however.

Hint

Many common insecticides also harm or kill non-targeted insects, birds, or animals, and can harm the environment. Instead, begin by altering your environment to discourage the pest. Then use the least-toxic insecticides only if needed. Examples include Bacillus thuringiensis (BT) for caterpillars, such as sod webworm and predatory nematodes for grubs. Consider more toxic remedies only as a last resort.

For more tips, see the University of California's integrated Pest Management Web site (www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74113.html).

Most diseases are caused by fungus pathogens present in all lawns. Fungus will grow and expand at the lawn's expense when conditions favor the fungus or weaken the grass.

Trying to grow the wrong grass for your area is one way to encourage fungus growth (See our grass guide and zone map). Two other ways: improper fertilization (too little, too much, or ill-timed) and too much water.

Fungicides are readily available. But catching the disease while it's active is difficult as is choosing the right fungicide, since each controls a slightly different set of diseases. As with most problems, the answer lies in proper care. See Guide to common lawn problems for non-chemical solutions.

Sick lawns can often be revived without starting over. Specific areas of dead or weedy lawn can be patched. You can also overseed for lawns that are thin or weed-infested, or simply to add a more appropriate grass to the lawn you have.

Patching a bare spot

This is idea for small, damaged areas, since you're replanting only the problem area.

1. Dig up and remove the damaged section, plus 6 inches of surrounding, healthy lawn, cutting 2 inches deep.
2. Smooth and level the soil. Add a small amount of soil amendment and starter fertilizer, firm the soil, water, and level again if needed.
3. Replant with seed, sod, or sprigs. For seed, cover lightly, press into place, and keep moist until germination. For sod, cut a section to fit, press it into place to firm roots against soil, and water frequently until rooted. For sprigs or plugs, plant them a few inches apart, cover lightly and press into place, and water until rooted.

Overseeding thin or winter-dormant lawns

This involves sowing seed over your entire lawn to correct thin areas or add grass that can better tolerate shade or resist disease. Overseeding is also used in the South and West to cover a dormant warm-season grass during the winter months.

1. Mow at the lowest possible setting and remove all clippings. If weeds are excessive, spray a post-emergence, nonselective, and systemic herbicide such as glyphosate. Spray when weeds are growing actively and then wait one week for the herbicide to kill the plants before mowing.
2. Dethatch. If your mower can't cut to the soil level, use a dethatcher. The blades of this rental machine cut down through thatch and stems and slightly into the soil. Rake up and remove all loosened debris.
3. Use an aerator (which you can also rent) to pull out 2- to 3-inch-long cores of soil, opening and cultivating the soil to prepare it for seeding.
4. Spread seed with a drop, broadcast, or handheld spreader. On bare areas, broadcast the seed two to three times the normal rate. Cover the seed to a depth of 1/16 to 1/8 inch by raking it in and lightly rolling or firming the soil.
5. Cover the area. Spread a thin, 1/4-inch layer of mulch over the seeds with a peat spreader to protect the seeds and keep the seedbed moist until germination.