Q: I started to put rock salt on my front sidewalk to get rid of the snow and my neighbor warned me that it might ruin my lawn. Is that true? A: Rock salt (sodium chloride) can dissolve and be carried in liquid form to the roots of grass, trees and shrubs and damage them seriously. Although plants are dormant during the winter, the roots are still active. If a salt solution penetrates the soil around roots, it results in the roots' losing large amounts of water. Through osmosis the water in the roots is drawn into the soil because of the higher salt concentration. The way to prevent this damage is to use minimum amounts of salt and be careful in its application near grass, trees and shrubs. Q: I bought a Christmas cactus in bloom, and when I got it home I found it was almost exactly like my Thanksgiving cactus. Are they the same? A: Most of the cacti sold as Christmas cacti are really Thanksgiving cacti, according to specialists. Thanksgiving cacti have stem segment edges that are toothed or jagged; Christmas cacti segments are smaller, rounder lobes. Thanksgiving cacti grow upright, Christmas cacti stems tend to hang loosely. Both originated in Brazil, where they grow as ephiphytes in plant debris trapped among tree branches or on the ground in decaying humus. Q: I'm new at vegetable gardening. I want to try growing lettuce and patio tomatoes indoors this winter. What temperatures are best for seed germination and how long does it take? A: Lettuce seeds do best at a temperature of 68 degrees F. with exposure to light. They should be planted on the surface for sunlight and/or cool white fluorescent illumination. Tomatoe seeds do best at a daily temperature of 68 degrees for 16 hours and 86 degrees for 8 hours. Tomato seeds usually take five days to two weeks to germinate, lettuce seeds a week. Q: I have just rooted the top of a pineapple plant. Can you tell me how to take care of it? A: Pot the plant in a six-inch clay pot and give it full sunlight if possible, but at least very good light. Relative humidity of 30 percent is desirable, 40 percent is even better. Good drainage is essential. Water the plant whenever the soil starts to feel dry. Best temperatures for the plant in the home are 75 at night and 80 during the day. Q: I bought two African violet plants; please tell me how to take care of them. Should I put them in larger pots? A: Usually when you buy a plant the pot is large enough and the soil good enough for several months. The plants do not require large amounts of fertilizer -- they're more likely to be damaged by too much than too little. They need good light, can even take morning sunlight during the winter. When the soil feels dry to your touch water the plants with room-temperature water. Pots without drainage holes are difficult to water properly. A pot full of roots supporting a large plant will require more frequent watering than a small plant that's just been potted. Q: Why do the buds on my African violets turn brown and fall off? The new leaves are also becoming stunted. A: The African violets probably are infested with cyclamen mites. They are so small you may not be able to see them without a magnifying glass. If you find them, give the plants a hot-water dip. Put the entire plant in water at 110 degrees F. for 15 minutes. Treat again a week later. Q: When do you prune a Burbon rose? I planted one a year ago and now it has seven-foot canes. A: The Burbon rose needs very little pruning. Dead stems should be removed immediately. Any other pruning should be for the purpose of improving bloom. If one branch shades another, the less desirable should be removed. Abnormal growth of canes may be due to too much nitrogen fertilizer. These canes can be shortened in early spring to conform to the rest of the plant. Q: I can't get canna seed to germinate. What is the secret? A: Maybe you aren't giving them enough time. They usually need 50 days to germinate and sometimes may not sprout for a year or more. Q: I burn about two cords of wood each winter. Can I keep putting the ashes on the garden? A: Probably yes, but have the soil tested every two years as a safeguard. Q: Is it possible to grow garlic in my garden? If so, when should I plant it and dig it? A: Garlic can be grown in the garden; handle it about the same as onions. Separate and plant individual cloves about six inches apart in early spring. Press the cloves into the soil, base downward, about two inches deep. Harvest in about 90 days when the tops begin to turn yellow. Store loose, like onions, in a cool, well-ventilated place. Q: I have had a bromeliad plant for four years. It had a bloom on it when I received it, but has not bloomed since. How can I make it bloom? A: Some bromeliad species flower each year at about the same time, but many species go for years without blooming. Put the plant in a plastic bag with a ripe apple, close and tie the bag, keep it there for four days and in one to six months, depending on the species, it will bloom. Q: The black-eyed Susans growing along the roadside -- can they be grown in the garden? A: They will grow in almost any kind of soil so long as it does not stay waterlogged too long after it rains. They are easy to start from seeds, which can be collected during the summer after they have ripened. Store them in a cool, dry place for the winter. Q: The needles on my Mugo pine are turning yellow and dropping off. What would cause it? A: Usually it is due to inadequate light. Unless they get good light, the needles will die on most conifers; the same is true for broadleaf evergreens. Many azaleas and boxwood plants that have been shaded have dead interiors because light cannot get in. After a while, the interior is incapable of initiating new growth. In a forest, the branches of pines that are shaded die and drop off.