A SHOW FOR SPRING On Friday, 1 to 4, and Saturday, 10 to 12 and 1 to 3, there'll be a flower show at the Army Distaff Hall, 6200 Oregon Avenue NW. These blooms are sponsored by the Garden Club of Chevy Chase and the Trowel Club. BETHESDA PLANT SALE Buy a vegetable plant or even a wild flower at the Bethesda Community Garden Club's annual sale, Thursday from 9 to 2:30 at the Farm Woman's Market, 7155 Wisconsin Avenue. Growing instructions and friendly advice from club members are included in the deal, and the club will use the proceeds to benefit community landscaping projects.

Q - I can get four bushel baskets of sawdust from an oak tree I cut down. Can I use it for mulching, or will it make the soil too acid?

A - Research has shown that over a five-year period sawdust has no appreciable effect on soil acidity. As it breaks down, it helps maintain or slightly increase the alkalinity of the soil.

Used as a mulch, about an inch deep, it reduces crusting of the soil, improves the germination of seeds and emergence of the young plants, and helps control weeds. It also helps rain water move into the soil and keeps the soil from freezing deeply during winter.

If plowed or dug into the soil after it's partially decayed, it makes heavy soils lighter and easier to work, and light soils have a higher water-holding ability.

Q - We have a flat tar-and-gravel porch roof, and after a rain the water stands in the gravel until it evaporates. The roof is shaded most of the day, and moss has become a problem, almost covering the entire roof. Is there something that could be used on the roof to kill the moss?

A - There are more than 13,000 different mosses. They are adapted to a wide range of environmental conditions, but an infestation is usually associated with low fertility, poor drainage, too much shade, soil compaction, wet conditions, poor air circulation or a combination of these factors. The only permanent control of moss is to correct the conditions that make it possible.

It can be removed by hand raking, or burning with ammonium sulfate or by spraying with copper sulfate. If ammonium sulfate is used, apply a thorough covering when the moss is damp and do not wash it off by watering. However, the moss probably will soon return.

Q - We have a problem with mimosa webworms on our honey locust tree and bagworms on the arborvitae. Is there any treatment not dangerous to people and animals?

A - Sevin can be used for webworms, but spray only if they are present. Directions on the label should be followed closely. Spray in late afternoon to avoid damage to bees that provide pollination.

Pick the bagworm bags off in late winter or early spring before the eggs hatch, or spray as soon as possible with sevin or spectracide (diazinon) after the eggs hatch, because as the worms grow larger they become more resistant to the insecticide.

Q - Last year my blue grapes ripened unevenly: When a few grapes on a bunch were perfectly ripe, others were shriveled and still others were still green. Is there anything I can do to synchronize them this year?

A - Some varieties of blue grapes ripen more evenly than others. Concord and Worden are poor; Delaware, Niagara and Buffalo are fair, and Portland, Seneca and Fredonia are good. The best remedy is to replace the vine with one that ripens more evenly. Clipping some of the bunches and pruning and fertilizing to keep the vines in moderate growth may help but can't be depended on.

Q - I have a minature orange tree almost two feet tall, with a branch spread of about one foot. It's in a five-inch pot; does it need a larger pot?

A - Probably. Turn the plant upside down, tap the rim of the pot on the table to loosen the soil ball and let it slide out (instead of pulling it out with your hand). If roots can be seen and they are matted together and crowded, the plant needs repotting. Put it in the next size larger pot, in this case a six-inch pot. Water it thoroughly about 12 hours before repotting.

Q - When is it safe to plant tomatoes and lima beans outdoors?

A - The planting dates outdoors suggested by University of Maryland specialists for the College Park area, which is about right for Washington (two days later for Baltimore) are: tomatoes (plants) May 1; lima beans May 10; snapbeans May 5; cucumbers May 15; cantaloupes May 15; eggplants (plants) May 15; pumpkins May 20; summer squash May 1; winter squash May 15; and watermelon May 15.

Q - My candytuft doesn't do very much; what can I do to perk it up?

A - The best treatment for perennial candytuft is to cut it back to about six inches soon after the flowers fade in the spring. Hedge shears can be used to do the pruning. If it isn't sheared, the flowers go to seed and a lot of energy is uded up in the process, often resulting in poor flowering the following spring. Don't cut below six inches, and after pruning fertilize it with a little 5-10-5.