"Winter takes its toll . . . on humans and houseplants," observes John Peterman, owner of the National Diagnostic Houseplant Laboratory in Lexington, Ky. If your plants have made it this far, he says, "Don't let them die now."

This time of year, county extension horticulturists are flooded with calls -- as many as 100 a day -- from local residents. While the queries concern any of the 450 different types of houseplants found in this country, the owners grapple with only three emotions: guilt, frustration and a sense of failure.

"That's the American mentality toward plants," says Bob Stewart, a Prince George's County extension agent. "If someone gets a pot of geraniums for Mother's Day from their daughter in California, they'll go over backwards trying to keep that plant alive. Whereas in Europe, they'll enjoy the plant for the three or four weeks that it looks nice, then they'll toss it in the trashcan."

Surprisingly, although some people think of their houseplants "as part of their family," according to one horticulturist, most owners don't learn about proper care until their parlor palm or Chinese evergreen droops or shrivels.

The problems, says Stewart, usually are easy to diagnose and generally are caused by three interrelated cultural conditions: insufficient light, overwatering and improper potting soil.

"When you put a houseplant into your house," says Howard County extension agent Ray Bosmans, "there is no way you can match the light requirements it had when it was growing in the greenhouse." Compounding this problem is the fact that consumers greatly overestimate light levels in their home, says Stewart, and they often select plants that cannot flourish in that environment.

To increase your chance of success, Bosmans advises, "Borrow from the professionals' knowledge. Walk around in the shopping malls; look at the type of plants professionals have chosen because they grow well indoors."

Some plants that tolerate moderate light: Chinese evergreen, dracaena, heartleaf philodendron, cactus, ficus, grape ivy, parlor palm, snake plant, wax begonia, spider plant, peace lily.

Overwatering is as detrimental to the plant as insufficient light. Peterman tells his clients: "Use room-temperature tap water. Saturate the plant's root ball until 15 to 20 percent of the water runs out the drainage holes in the pot. Wait 45 minutes. Pour off the excess water from the saucer. Do not water that plant again until the soil becomes dry." If your plant is getting adequate light, that should be in about a week, but, Bosmans cautions, "Don't water the plant until it really needs it."

"If the potting soil mix is satisfactory," says Stewart, "it becomes difficult to overwater." Back yard soil is not suitable for houseplants. Nor is commercial potting soil used alone -- because it is finely decomposed peat moss, which holds a lot of water and excludes air, it inhibits root growth. Packaged potting soil is an excellent base, Stewart says, but must be mixed with other substances of fairly large size that will not compact or break. Perlite, vermiculite, gravel, even chopped up styrofoam, all work well. They provide the essential pore space which enables oxygen to enter the soil between irrigations.

After the three most common problems, says Stewart, "the insects come in." Because of higher indoor temperatures, insects are especially active now. But plants vary in their susceptibility to the four major predators -- aphids, mealybugs, red spider mites and scales -- all sucking insects.

"You have to learn how to live with them and keep them under control," says WGTS "Plant Talk" hostess Terry Pogue. "Almost every plant is going to have an insect or two on them, but the idea is to keep the population very low so the plant thrives." She maintains it would be "foolish to try to completely eradicate them."

People forget a plant uses photosynthesis to make its food, says Pogue. Instead, "they think fertilizing is wonderful and do it every week. Then the soluble salts from the fertilizer and the water, which are toxic to the roots, build up in the soil. Those who are overly attentive to their plants get into trouble."

Owners in that category who treat an ailing houseplant by repotting it every time they learn of a new soil mixture, she adds, never give the plant a chance to become established.

And keeping a home at 70 degrees or warmer, she says, will create problems for tropical plants, which thrive at 65 degrees. The higher temperature dries the air. The result: brown, brittle leaf tips and edges.

The normal evaporation of water from plant leaves exacerbates the harmful effect of low household humidity. During the winter months, group your plants to humidify the air immediately surrounding them, says Rita Pelczar, development coordinator of Fairfax County's Green Spring Farm Park program. Or, place a few inches of pebbles in a tray. Add water, but don't cover the stones completely. Then, set your plants on the pebbles so the water can evaporate around the leaves.

Some people claim they keep their plants healthy by talking or singing to them. No scientific evidence supports -- or refutes -- that belief. So when questioned by local residents about this method of plant care, Pelczar replies, "Do it if it works for you."

Horticulturists say concerned owners have given Vitamin C, birth control pills or Jell-O to dying plants, practices they don't recommend.

"They want to believe," says Stewart, "success is just one ingredient away." When you have an ailing plant in your home, he advises, "don't look for a magical, chemical, or quick-fix reason why it's not doing well."

And bear this in mind, he says: "You, not the plant, should be the boss."