Q - Plant lice are an awful nuisance in my vegetable garden. Is there any way to get rid of them without having to spray every week?

A - Aluminum foil laminated to paper and arranged to cover at least 50 percent of the area around plants will reduce the number of aphids (plant lice) and the virus diseases they spread. The foil strips provide an effective barrier by reflecting ultraviolet rays from the sky. This causes the flying aphids to stay away. Promising results have also been obtained with aluminum foil against thrips and other insects on rose bushes, dahlias, tomatoes, cucumbers and snap beans.

Q - I told a grower that I had taken sawdust from a nearby lumber mill to use on my garden. She told me not to use sawdust as a mulch because it would have turpentine and creosote in it. She claimed it would kill my plants. I think the sawdust is mostly oak but am not sure. is it safe to use it?

A - It's possible that the sawdust came from wood that had been treated with crosote. If so, it could do a lot of damage to your plants. Why not ask the lumber mill about it?

Q - When my neighbor backs his car out of the garage, for about a minute the exhaust is about six feet from my azalea plants, which are two feet high. Will it hurt my plants?

A - Just one time might not produce any visible effect, although some damage could result. Repeated at regular intervals it probably would destroy the azaleas.

Q - My forsythia had long stalks this spring, flowers only at the top, and now only leaves, with no new growth at the bottom. Can anything be done about it?

A - The forsythia is badly in need of pruning. It can be done anytime in the spring. With forsythia, mock orange, deutzia and some other spring-blooming shrubs, a dense top is created that shuts out light from the base of the plant. Consequently the new canes that start from the base are likely to die. Remove about half of the old canes this year and half next year. This will permit new canes to get started. Then each year take out the oldest canes soon after the plant finishes blooming. The bark of a young forsythia stem is bright in color. The first year it bears flowers up and down the stem. After the third year the bark becomes dull in color.

Q - Which is better, to stake my tomato plants or grow them in wire cages?

A - Research by specialists at Virginia Tech has shown that with cages it is possible to get 30 to 40 pounds of good fruit per plant from varieties available today. The cages cost more, but they can be used over and over again year after year.

It is unncessary to prune tomatoes growing in cages, you do not need to stake or tie them, and the fruit is kept off the ground which prevents a lot of ground rot. Research has shown that the length of harvest with cages usually is longer than for those staked or allowed to sprawl on the ground.

The cages should be about five feet high and 24 inches in diameter, and made with six-inch mesh concrete reinforcing wire. The plants should be spaced about three feet apart in rows five feet apart.

Q - Our maple tree has a couple of holes in the truck about the size of a 25-cent piece. Is this due to borers, and if so is there something I should do about it?

A - Almost always such holes are caused by poor pruning and not by borers. When a branch is removed and a stub of one to two inches or more is left, healing of the wound is slow. Decay-causing organisms become established in the wound and as decay progresses, the dead stub falls out, leaving the hole. When a branch is removed, the cut should be made flush with the collar that has developed where the branch enters the tree. This type of would should heal in a single growing season.

Q - I have a gardenia plant that was given to me at Easter three years ago. It was in full bloom and since then has had only a few flowers, one or two at various times of the year. What am I doing wrong?

A - The gardenias sold as potted plants for Easter usually are forms of Gardenia Veitchi. These gardenias develop flower buds only when the night temperature is below 65 degree F. The day temperature is not important as long as it is higher than the night temperature. Flower buds may form at 70 degree, but invariably drop off. For best flower bud development, specialists recommend a night temperature of 62 degree and a day temperature of 70 degree.

Q - I want to grow garlic in my garden this year. When is the best time to plant it?

A - Garlic should be planted in the spring about the same time as onions. Use only large cloves. They should be planted as soon as possible after separation from the bulb, because they tend to deteriorate after separation. Garlic is ready to harvest when the tops begin to turn brown.

If you have a question for Tom Stevenson, send it to him care of the Weekend section, The Washington Post, 1150 15th St. NW, Washington 20071.