BEGINNING BEGONIAS Ed and Millie Thompson, authors of a begonia guide, will appear at the National Arboretum, Bladensburg and R Streets NE, Sunday at 2. The program, presented by the Potomac Branch of the National Begonia Society, is free. For more information call 703/837-1327.

Foliage houseplants, such as philodendrons, pothos, rubber plant and Chinese evergreen, usually don't need to be watered as often during December and January as during spring, summer and early fall. The days are shorter, with only 9 or 10 hours of daylight, and the plants use less water.

This is something to watch because too much water can cause serious damage to the plant. Oxygen moves slowly through water-logged soil, and root-rot may develop. Bottom leaves turning yellow and dropping is one of the symptoms.

On the other hand, if given too little water the plant wilts and its growth slows. Leaves turn brown or black at the edges and die progressively up the plant. Feel the soil about half an inch below the surface: If it's dry, it's time to water. Use room-temperature water, neither hot nor cold.

Give the plant enough water to moisten the entire soil mass and drain out. Water until it comes out at the bottom. Wait 15 or 20 minutes for excess water to drain and empty the saucer. If the pot stands in water for several hours oxygen will have a hard time getting to the roots.

Some people believe that small amounts of water should be applied rather frequently, because they've read or been told that the soil must be kept uniformly moist. This is not correct. If the soil is kept constantly moist, there won't be enough air in the soil for proper root function.

The roots are down toward the bottom, and if not enough water is applied to wet all the soil, the roots may not get any.

The soil in a clay pot will dry out faster than in plastic and some other kinds because of evaporation from the sides.

If the pot has no drainage hole, watering can be a problem -- usually you don't know how much water to apply. It should be enough to moisten all the soil but no more, because water would accumulate in the bottom of the pot. In general, a pot that holds about a quart of soil will probably require half a pint of water for adequate moisture.This is merely a guideline, since variations in the soil mix can make quite a difference in how much water it can hold.

To see if you've put in enough water, turn the pot on its side on the kitchen sink. What runs out is excess, and will give you some idea how much to apply next time.

When plants are in large containers with no drainage holes, you can put a 1/2" copper tube in the soil, someplace where it won't show, with one end at the bottom and the other about an inch above the soil. Then you can stick a bamboo strip or heavy wire down the tube to see if too much water has been added, as you'd use a dip-stick to check the oil level in your car's motor.