A plant is considered hardy if it can survive the winter outdoors without protection. But many hardy plants, particularly roses, suffer considerable damage during the winter because they weren't prepared for freezing weather.

With the shorter days and lower temperatures of last summer and fall, growth usually slows down and stops altogether. Chemical changes occur within plant cells to protect them from injury when temperatures go below freezing. The process is called hardening. A plant is hardy only when it has hardened. If cold weather occurs before hardening takes place, the plant may be seriously damaged or even killed.

During this period of acclimitization, starting about mid-August, woody plants should not be pruned heavily or fertilized; this will interfere with their adjusting in time for winter weather.

Roses have been one of the main victims of fertilizing too late in the season. Fertilizer is applied to encourage more and better flowers late in the summer. The fertilizer stimulates new growth activity and prevents hardening in time for winter.

Studies have shown that heavy pruning during August and September may delay most evergreens from hardening normally even if new growth is not activated.

Boxwood should not be clipped at this time of the year -- it may encourage new growth that would not have time to mature before frost.

Dead, broken and diseased stems can be removed at any season. Dead leaves and twigs lodged in the crotches of branches should be removed before cold weather. Boxwood should never be pruned when temperatures are below freezing. wThe wood is brittle and easily broken when frozen.

Junipers, yews and arborvitae can be trimmed a little at this time of year without upsetting anything, but let the heavy pruning wait.

Older plants of the same variety often are more hardy because they stop growing earlier in the season. That's why rooted cuttings of azaleas and some other plants need winter protection the first year.

Trees and shrubs, notably roses, which lose their leaves because of diseases and insects are likely to make new growth late in the season and this will not have time to harden before winter. That's one of the reasons that roses infected with blackspot often don't survive the winter.

For severely exposed locations, and where alternate freezing and thawing tends to heave plant roots out of the soil, some winter protection, such as a two-inch mulch, is desirable. All winter mulches should be applied after the soil is frozen to allow for proper hardening of the plants. Q: We sprayed our dwarf apple trees and about an hour later it rained hard. Should we have sprayed again when it stopped raining? A: If they don't dry before a rain, the spray will be washed off them, and other kinds of plants. After the spray has thoroughly dried, it will adhere to the plant for a considerable period of time. On a dry, sunny day the spray may dry in half an hour or less; on a cloudy day it may take several hours. Q: Three weeks ago the leaves started to fall from our 40-foot tulip tree; it looked almost like autumn. What can we do to stop the leaves from falling? A: This is due to a non-infectious disease the same as leaf scorch. Sometimes the condition is due to a poor root system. The tree can be helped by watering it regularly during prolonged dry weather and by fertilizing it in late fall after the leaves drop. The tree is not in danger of dying. Q: Three of my rose bushes have developed yellow leaves, although they do not show any black spots. What do you think may be wrong? A: It could be due to spider mites, which are particularly bad during hot, dry weather. They suck juices from the leaves, usually from the underside. Use a magnifying glass and take a look at the undersides of leaves just starting to turn yellow. Some measure of control can be achieved by spraying the plants regularly with water under pressure from the garden hose. Q: this year the leaves of my big oak tree have galls. What is the ailment? Is it fatal? A: Galls, or odd-shaped growths caused by insects, can be seen on leaves and twigs of all kinds of trees and shrubs. They seldom seriously affect the health of the tree. The only way to prevent them is to spray the tree with an effective material at the proper time.Generally, it's not worth the cost. Q: I want to plant some lilies this year. When is the best time to do it? A: Lillies should be planted during September or October, as early as the bulbs can be bought. Select varieties by color, blooming season and height of plant, to satisfy your particular garden. You can have them blooming from June unitl September, ranging in height from two to six feet. Q: Two years ago I purchased a clematis vine that has grown up very nicely but has never bloomed. Should I feed it? A: Clematis will grow in sun or shade, but to bloom it needs at least half a day of sunlight. The soil should be alkaline, at least neutral, and if the soil is acid lime should be applied every year. The soil should be kept cool (a two-inch mulch will do it) and the plant should be fertilized lightly in the spring every year.