Directions on when and how to water assorted houseplants

Houseplants are often watered either too much or not enough -- and it's not always easy to tell in which direction we're erring. It seems reasonable enough that a cactus or succulent doesn't require as much water as, say, a coleus because their thick leaves hold a lot of fluid. But what about some of the other plants?

Water passes through a loose, friable soil mix very quickly. If the soil is mostly heavy, sticky mud, it will take water a long time to penetrate an entire pot. Both these conditions are extremes. So try for a happy in-between.

At least one-third perlite in a soil mix will prevent a gummy, muddy situation. At least one-third peat moss (either Canadian or German), or leaf-mold, will allow a light mix to retain moisture. Add a few spoonfuls of crushed charcoal to every soil mix. The kind used in outdoor barbecues works just fine. If I had to choose between the two soil extremes for the majority of houseplants, I'd go for the loose one.

The smooth-leaved ivies, along with spiders, asparagus, episcias, wandering jew, tolmieas (piggybacks), fuchsias, ferns, and the holiday cactus (not a desert cactus) like even moisture. So use a finger -- it's more sensitive than a thumb -- in the pot, probing about a half-inch deep, to see if the soil is damp down there.

If it isn't, water from the top until the water runs out of the drainage hole.

After checking each one, make sure no water stays in the saucer under the pot unless the plant happens to be a cyperus, which likes to sit in water all the time.

Begonias, peper-omias, geraniums, aralias, and scheffleras need to experience definite dryness before you reach for the watering can. ``Drench and dry'' works best for these plants.

Bromeliads require water in the center of the leaf-cup continually. Rainwater is the ideal choice.

African violets and streps do well in a light soil that keeps them nicely damp, but never soaking wet. The violets are probably the most popular houseplants because they are very tolerant both of moisture and light.

In my largest south-facing window I have a huge jade tree that I water every two weeks, keeping track of the watering dates by marking a calendar. Of course, the plants in small pots couldn't go that long. Clay pots can accept more water than plastic ones because the entire pot is absorbent. Clay pots work well for all plants.

The temperature of the water you use makes a difference, too. If possible, let the water in your watering can stand overnight. Then the chlorine which the water might contain is dispersed into the air. The ``magic'' water temperature is 90 degrees F. Room temperature is not warm enough.

When the weather turns colder, most likely there will be more heat inside the house. At these times watering will probably have to be stepped up.

About two or three years ago the Union Carbide Corporation introduced a product for gardeners called Hydrogel, which is designed to increase the water-holding capability of the soil. The formula is three level teaspoons per 4-inch standard pot, four teaspoons per 5-inch pot, and so on. The product proves handy for the thirstier plants such as azaleas, ferns, coleus, and cyclamen.

If plants had all the humidity they want, everything in the house might be covered with mildew. But it benefits the plants to raise the humidity somewhat by:

Grouping them all together. Single plants on opposite sites of the room aren't helping each other.

Having an electric humidifier.

Keeping the smaller plants on humidity trays.

Misting the plants. This is at the bottom of the list because it's hard on both furniture and woodwork and lasts but a short time.

If your houseplants are thriving, you're doing a great job of caring for them. But if some are a bit under par, think more about how you water them.