Ask The Gardeners; Q&A.

Q. Our neighbor, who does woodworking as a hobby, gave us several pails of sawdust which we used to mulch our tomato plants. Since our soil is claylike, we also worked a small amount of it into the soil at the time we planted the tomatoes. Now they have turned yellowish and we wonder if the sawdust is responsible.

Indirectly, the sawdust is responsible, but it's still a good mulch and soil amendment. What happens is that soil fungi and bacteria that are decomposing the sawdust are also consuming so much nitrogen that there is a temporary shortage of this element, which is essential for green growth.

You can offset this shortage by adding a cupful of nitrate of soda per bushel of sawdust. Or you can use any balanced liquid plant food (1 gallon per bushel, mixed according to directions) every three or four weeks until the condition is rectified.

Q. There is a problem spot in our lawn that has moss instead of grass. Thinking the soil was acid, I raked off the moss, added lime, and then sowed grass seed. What little came up has been overtaken by moss. Oddly enough, the area is exposed to the sun. I always thought moss grew where it was shady, the soil acid and poorly drained. The area is sandy and well drained, but we have noticed there are few, if any, earthworms, compared with areas where the lawn is healthy. Is this condition due to something toxic in the spot?

Since the area is sunny and well drained, the moss would indicate a lack of nutrients. Lack of earthworms indicates a dearth of organic matter.

First of all, work some organic matter into the soil, such as rotted compost, manure, or leaf mold. If you don't have these you can use peat moss. Then add a balanced fertilizer (either liquid or dry), according to the directions on the container.

Sow grass seed and put a light covering of straw over it, but not enough to shut out the light that is needed for good germination. Keep the area moist and you'll soon have green grass instead of moss.

Q. A few weeks ago we bought a wandering Jew plant in a hanging basket. It has bright-colored coppery leaves that were striped white, green, and pink, with a botanical name that sounds something like zebra. The florist said the plant did not need sunlight. At present, the plant still looks healthy, but the colors have faded to a muddy, bronzy green. We have it in a hallway that faces north. Is there some special plant food we could use to bring back the color?

You have a Zebrina pendula quadricolor. Even though it does not need sunlight , it does require bright indirect light to define and bring out the leaf colors. Move it to another spot with more light. It can be near a sunny window but shielded from the direct rays by a thin curtain.

Q. I want to thank you and the Garden Club of America for the splendid leaflet entitled ''Berried Treasures for Your Birds.'' It's most helpful. Perhaps because it focuses on shrubs, there is no mention of fringe-tree. We thought your readers would like to know that this small shrubby tree bears blue grapelike berries in the fall and is very attractive to birds. Its blooms are feathery white, and the oblong leaves turn a beautiful golden yellow in the fall. Also, it withstands our temperatures (which may fall to 15 degrees below zero F.), and the one we gave my brother in Texas proves it can stand the heat as well. We don't know the botanical name, but perhaps you can provide it for folks who want to order one.

This versatile and highly adaptable little tree is Chionanthus virginicus, a handsome addition to any landscape. In addition to its other qualities it tolerates city conditions, including air pollution.

It may grow to a height of 15 to 20 feet, so it should fit into most yards.

Q. Could you suggest some small, hardy flowering bulbs we might plant in the fall among a natural outcropping of rocks, along a small area in the rear of our property? We'd like these to have a succession of blooms from March through May. They would get full sun much of the day. We planted annual pinks (dianthus), blue lobelia, sweet alyssum, and portulaca for summer bloom. We already have some crocuses in the area.

For March bloom, try Chionodoxa (glory-of-the-snow), Puschkinia, Leucojum vernum (spring snowflake), and Scilla siberica. Then for April you could plant blue and white Muscari (grape hyacinths), miniature daffodils, and some of the low-growing botanical tulips, such as the Kaufmannianas.

Completing the spring display, you could use some more botanicals, such as the Fosteriannas, plus wood hyacinths (Scilla campanulata), Ornithogalum umbellatum (star-of-Bethlehem), being careful that the latter plants do not take over, and Leucojum aestivum (summer snowflake).

Q. When I was a youngster living on a farm in the Midwest, my mother used to have some tall plants called ''caper spurge'' which would keep the gophers out of the garden. They would also repel moles, one of our present problems. Where may I find seeds? I can't find them in a number of seed catalogs.

The plant you're looking for is Leuphorbia lathyrus, also called mole plant. Although there are probably others who handle it, we have found it listed in the following catalogs: J. L. Hudson, PO Box 1058, Redwood City, Calif. 94064; Di Giorgi Company, Council Bluffs, Iowa 51501; and Thompson & Morgan, PO Box 100, Farmingdale, N.J. 07727.

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