Cautions on weed killers; helping your trees through the winter

Q Our African violets look very peculiar. The leaves are twisting and curling , and some are bleaching. Buds are deformed. We have used some rotted compost in the soil mix. Could it be weed killer that's causing the problem? Our neighbors used weed killers on their lawn a few weeks ago and some could have drifted onto our nearby compost pile.

It is possible, of course, to have weed-killer injury if any air movement brought the fumes or vapor onto the compost pile.

It is even possible to get weed-killer injury if the potting soil is stored near a weed killer, as any escaping fumes could penetrate the soil.

We do not recommend weed killers to home gardeners; and even in commercial use we are increasingly concerned because of the cavalier attitudes of many users, plus the lack of information reaching consumers regarding the harmful effects on animals as well as humans.

To test for the presence of weed killer, plant several seeds of tomatoes or beans in a pot of the soil mix. If weed killer is present, the seeds may not sprout. If they do sprout, the secondary leaves will be distorted. Q Early last spring we found a lengthwise split in the bark of one of our young shade trees and a friend told us it would probably knit back together again. Looking at it lately, however, we note there is some loose bark where it is split. Would you know what caused the damage and what can be done to prevent further injury? The tree is on the south side of the house.

You have what is called ''southwest injury'' or ''winter trunk injury.''

In your area last winter, severe cold was followed by sunny days, causing the bark, which had contracted from the cold, to expand quickly because of the sun shining on it. This sudden, uneven expansion on the southerly side of the trunk caused the bark to split.

Carefully trim away any loose bark so that only solid bark remains. Good scar tissue will then form.

One way to prevent such injury is to paint the trunks with white latex house paint which has been mixed with equal parts of water. The paint reflects the sun's rays and prevents unequal expansion and contraction.

Newly planted trees should be protected from drying winds and sun by using spiral tree guards, kraft wraps, or burlap. Buy these at garden stores. Q I have just been given a burro's tail in a hanging basket, but I have anything but a ''green thumb.'' Will you give me some information about its care?

Burro's tail thrives on neglect. For those not familiar with Sedum morganianum, it has trailers that bear fat, blue-green leaves one inch long. We have one growing in the same pot of perlite in which it was rooted three years ago.

We give it a liquid feeding every six months and water it just enough so the perlite doesn't become bone-dry. A thorough watering if it becomes dry in summer is fine. The same would apply if it were in potting soil, which needs to be well drained. Our plant gets good indirect light, but it does fine in the sun.

Our burro's tail tolerates 40 degrees F. in the winter and was not fazed by 98 degrees F. this past summer.

As the plant gets larger it will be necessary to water it a little more often , but be careful not to overwater. In the winter especially give it only enough water to keep the leaves from shriveling.