Problems in the landscape can be minor nuisances or major trouble. Determine what you have and decide what you can do about it. Pests can be weeds, water, viruses, fungi, bacteria, insects, birds, mammals, weeds -- anything that harms us, our homes or our plants. Here are some things to watch out for.

Insects and Diseases

Monitor for insect populations or diseases. Use pesticides only if a problem is present. When pests run out of control, resort to more drastic pesticides. Understand their impact before using them. Oils and soaps kill spiders and other beneficial insects and their eggs. Follow instructions for all materials.

Unhealthy plants are displaying signs of decline now. New growth weakens, yellows and dies. Get the problem diagnosed so you can take appropriate action in the future. Call your county Cooperative Extension Service or contact garden centers about a diagnosis. For trees, call a certified or consulting arborist.

Fungi are hard to reverse. Yews, dogwoods and many members of the heath family, including rhododendron, azalea, mountain laurel, leucothoe and Japanese pieris, are especially susceptible to phytophthora, one of several soil-borne fungus diseases, encouraged by excessive moisture and low temperatures. Effective chemical controls are limited. Installing plants so 25 percent of the root ball is above existing ground level, using soil from the hole to cover, will keep roots well drained. Plant with generous amounts of compost and leaf mold dug in over a wide area.

The hemlock woolly adelgid is a difficult insect to treat. If you see a white waxy substance lining the stem at the base of hemlock needles, look for adelgids. With a magnifying glass, you can see minute, flat, black crawlers slowly heading toward the new growth. They feed in spring and fall. Here are two ways to control them:

* Spray hemlocks with horticultural oil and insecticidal soap, once in May and again in June. Several materials are Sunspray Ultra-Fine Oil, Volck Oil Spray and Safer Insecticidal Soap. Insecticidal soap will wash off much of the waxy white substance from adelgids. Do a third spray with horticultural oil in September, before they stop crawling. Completely cover needles.

* Merit and Marathon (imidacloprid) are synthetic systemic pesticides that can be applied over the root system of the hemlock. It is absorbed by the roots of the tree and insects succumb as they feed. Apply during feeding times.

Mosquitoes

Adult mosquitoes hibernate and become active with the first warm weather. They feed most actively from dusk to dawn. Many different species occur throughout the world and can transmit diseases, such as encephalitis, malaria, yellow fever and West Nile virus.

Their closest habitat to you are your rain gutters. Keep those clean. Eggs are laid in shallow accumulations of water that can be fresh, stagnant or salty. Empty all standing water. Change the water in your bird bath every other day. Dump water that collects under outdoor plant containers, stray hubcaps by the garage or trash can lids. Keep water moving in your pond or other water feature. Use Bt (Baccillus thuringiensis) cubes, which contain bacteria that will kill the larvae when floated on water. One product on the market is Mosquito Dunks. A new control for larvae is Bite Free Granular Larvicide, which is safe for fish and other wildlife. Check www.farnam.com for more information. And, fish in your pond love mosquito larvae.

Repel mosquitoes from you with Skin-So-Soft Bug Guard Plus or plant-based repellents with soybean oil, such as Bite Blocker, which have been shown to provide protection. Studies suggest that these products do not last as long as those containing DEET, which repels the mosquito, making you unattractive for feeding.

More information about biting insects is available on the Centers for Disease Control and Prevention Web site (www.cdc.gov/ncidod/dvbid).

Drainage

When all downspouts are working and a grade slopes gently down and away from your home to the edge of the property, your basement will stay dry and the yard will usually remain well drained. Downspouts should be directed away from the wall. For aesthetics, bury a four-inch diameter or larger flexible corrugated or plastic PVC pipe to carry water eight to 12 feet from your wall. Pipes must slope downhill and have an open end for water to exit.

To assess a wet-basement problem, walk around the outside of your house. Look for low or wet areas where water can flow toward the structure. You will usually find that the problem is caused by a low spot at the wall or possibly from a downspout or gutter that doesn't carry water away. Add soil along the walls to create a downhill slope away from the house. Never place soil against siding or wood. Grade soil from below siding or wood.

A common error is making walks or patios perfectly level. A level surface will hold water and promote growth of fungus and algae. Paved surfaces should drop 1 to 2 inches over 10 feet.

Another kind of drainage is called percolation. Check drainage before planting by digging several holes 12 inches deep by 18 inches wide. Fill with water and time the drainage. Holes should drain (percolate) within minutes. Most plants will not survive if the hole doesn't drain. Improve poor percolation by adding compost to the top 10 to 12 inches over as wide an area as possible.

Erosion

Erosion control is always important. If you have a sloping property, controlling storm-water runoff is the first consideration that should be made to preserve the site. A deluge of water coursing down a slope can have tremendous cutting action. Depending on the steepness and amount of vegetation, a torrential rain can move tons of topsoil off your property.

The first step before planting is to slow down the water and spread it out. If you have hiked hilly woodland trails maintained by the National Park Service, you have seen a method it uses. In steep areas, diagonal channels are cut off to the sides to catch water on its downhill flow and direct it to the left or right traversing the bank. That slows and disperses the water, breaking up straight downhill cutting action. The diagonal channels are often created or retained with rocks or logs staked in place. Slow the water running down your yard with wider areas called swales. The swale should be level enough to support vegetation, and a mower if necessary.

Once you sculpt the land to divert the water, you need it to stay in that configuration, which takes tough, low-growing vegetation that will root firmly. Ground covers slow the water and allow it to percolate into the soil as it flows down the hill. Sod makes an excellent waterway for channeling storm water and holding the soil if you have full sun.

Crab Grass

Crab grass is an annual grass that germinates in spring and ruins your lawn by invading all summer. It has reddish-brown spiked seed heads in summer. Treat next March before it starts to grow.

A safe, all-natural pre-emergent control of crab grass is a corn gluten powder. It's sold under several names, including WeedBan, Weed Prevention Plus, Corn Weed Blocker and WOW Plus. You can find them at your local garden center. There are also effective synthetic pre-emergent herbicides on the market, such as Halts, Preen and Gallery. Pre-emergent herbicides are effective if used in fall to reduce fall and winter germinating weeds. Never use pre-emergent material when you are seeding your lawn. The seed won't germinate.

Broadleaf Lawn Weeds

Pull them, or control with a selective spray, such as Weed-B-Gon. Get them before they flower and go to seed. Keep weed seed from germinating with an all-natural, corn-based weed control such as those suggested for crab grass.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.

This erosion ditch occurred because ground cover was not planted on it.