Sometimes house plants you love, such as dracaena and dieffenbachia, lose their bottom leaves and become leggy. You can restore them to attractiveness by air-layering, and now is a good time to do it.

Air-layering, which can cause roots to form on the stem just below the lower leaves, is not a new method -- it was practiced centuries ago by the Chinese and was called marcottage. But it's much easier to do now because of the development of plastic films. It can be used also to propagate some hard-to-root trees and shrubs.

With your house plants, use a sharp knife to make an upward- or downward-slanting cut 1" to 1 1/2" long and about a quarter to half the way through the stem. On larger stems, the cut may be made on both sides of the stem. On larger stems, the cut may be made on both sides of the stem. Insert a toothpick into the wound to prevent it from healing over, and dust a small amount of rooting hormone, such as rootone into the incision. The rooting hormone usually hastens rooting, although some plants root just as well untreated.

On bark-covered stems of woody plants, make two cuts around the stem an inch apart and remove the bark down to the wood.

Soak sphagnum moss in water, squeeze out excess moisture, and place the moist ball of sphagnum all around the cut strip.

Cover the moist ball of sphagnum with plastic film, overlapping the edges, and tie it securely at the top and bottom with rubber bands. Electrical tape can be used to seal it.

In the past, the main problem with air-layering was keeping the sphagnum moist, just moist enough and not too moist. With too much moisture, oxygen would be excluded; with not enough rooting would be seriously impeded. Polyethylene holds the moisture in and the peat stays just moist enough without requiring constant attention.

Depending on the kind of plant, it may take three to six weeks, or even a year, for roots to develop.

After roots have formed and you can see them through the plastic, the layer can be cut off just below the moss and roots. Remove the polyethylene before planting, but do not disturb the ball. If the ball is dry, moisten it. Handle it carefully while planting -- it can easily be injured.

Before starting, assemble the equipment you'll need. After the bark it cut, the wound should be wrapped quickly to prevent drying out and possibility of failure. Q: My bearded iris is getting crowded. Is it a good idea to divide it, and if so, when is the best time to do it? A: Bearded iris usually needs to be dug, divided and replanted every four or five years. The best time to do it is soon after it finished blooming. It's a semi-dormant period for these plants, and lasts three or four weeks. Q: The roots of a large oak near the house are pushing up the bricks of a 20-year-old old walk that passes near it. Is it safe to cut back the roots under the walk? A: It may or may not be safe; it depends on how much the tree needs those roots to keep it from toppling over and to give it moisture and nutrients. Any tree surgeon or expert can make an examination and give you and opinion. The health of a tree is in direct proportion to the extent and effectiveness of its feeder roots. The damage might not kill the tree, but it might so debilitate it that it would become an easy prey to insects and diseases. It takes a long, long time to replace a big oak tree. Q: What causes my spruce tree to become brown inside while the outside needles are green and healthy looking? A: The dead zone is due to inadequate light, and usually is the result of shearing with hedge shears. At this point if pruning permits better light to reach the interior it does no good, because most of the dead zone is unable to respond. Q: How long does it usually take for tomatoes to ripen after they first form on the plant? Can you furnish a guide for tomatoes, peppers and other vegetables? A: Tomatoes mature in 30 to 45 days, depending on variety and time of planting; sweet pepper take 30 to 45 days to reach the green stage and another 15 days to turn red; sweet corn matures 18 to 24 days after silk appears; snap beans, seven to ten days after flowering; cucumbers for slicing, 14 to 18 days; summer squash, five to six days, winter squash 11 to 12 weeks. Hot and dry weather sped up maturity but not quality. Q: The leaves of my rhododendron are turning yellow. Could this be due to an iron deficiency? I transplanted it two years ago from a shady spot to one that gets full sun. A: The yellow leaves do not necessarily indicate an iron deficiency. To make sure, have your soil tested. If a rhododendorn is planted even an inch too deep, it could be in serious trouble. Even is planted all right, the plant could have settled due to decay of organic matter in the soil mix. Q: Can newspapers be used to mulch beans and tomatoes in the garden? A: Newspapers are good for mulching. They break down as the season progresses and can be dug into the soil when they have served their purpose. It's important to use several thicknesses, something like a section of eight to ten pages. These usually withstand rainfall over much of the season. e Q: My neighbors catch their grass clippings in a bag and remove them. I have been letting mine stay on. Is it all right to do so? A: When left on the lawn, grass clippings provide substantial quantities of fertilizer as they decay. Removal of the clippings is believed to reduce chances of bluegrass becoming diseased.