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My friendly adviser says there are only three ways to water houseplants - too much, too little and just right.

But it's not that simple because there are many differences in plant size, pot composition, indoor temperature, humidity and light that will change water use. So, you have to change the frequency or amount of water supplied.According to Ray Rothenberger of the University of Missouri, "Watering is one of the most controllable factors in growing houseplants, and yet, improper watering is one of the most frequent causes for failure with plants.

"Diagnosis of the problem may be difficult because the symptoms shown by plants that have been overwatered are not greatly different from those of plants that have been underwatered. Both conditions cause plants to wilt. Tips of leaves may turn brown, black or yellow, and eventually drop. Improper watering, whatever the type, results in stunted plants that do not grow, even though they may survive."

If a plant has had too little water, roots around the outside of the soil ball will be brown and shrunken. If the plant has been kept too wet, the roots may also be brown, but more likely will be mushy and decayed. Overwatered plants may also have a soft stem that collapses near ground level. Collapse of the main stem is much less likely to occur in underwatered plants unless the lack of water has persisted a long time. Many fungi can grow in constantly excessive moisture, and some will show their presence by the production of powdery spores.

When watering, keep in mind that indoor conditions can change. Determine plant needs, and don't water on a rigid time schedule. Do not keep the soil surface constantly wet. Even plants that need plenty of moisture should be allowed slight drying at the soil surface. Don't let watered plants sit in a saucer of water for more than a few hours.

Rothenberger points out that plants have different watering needs. "A few, such as the umbrella plant, need to be very wet at all times," he says. "Most houseplants fit into a middle group, which consists of those that need fairly constant moisture. Slight drying between waterings is beneficial, but wilting should never occur."

Before water is added, the soil surface should be dry to the touch. "But, if we scratch into the soil a quarter of an inch in small pots and half an inch in large pots, we should find moisture to the touch or visibly, since moist soil is darker than dry soil."

A third group of plants need to become drier between waterings. These plants should not be watered until signs of moist soil are found one-half to 1 inch beneath the surface.

Water thoroughly each time. Water should run through the soil and drain into a saucer under the pot. It may remain in the saucer for about an hour to allow some reabsorption. After an hour, discard the water that remains in the saucer.

"Another damaging watering situation results when fluctuation between extremes occurs," Rothenberger says. Shifting from periods of too wet to periods of too dry kills even more roots.

"To some extent, plants are able to adjust to a problem that is constantly the same. Erratic or irregular watering allows them no chance to adapt. However, proper care is the only way to grow healthy plants indoors."

For a copy of Earl Aronson's "AP Guide to House Plants," send $1.50 to: House Plants, AP Newsfeatures, 50 Rockefeller Plaza, New York, NY 10020.