Q - I'd like to grow mushrooms if it is possible and practical for me to do so. Then I can have them fresh. Can you tell me how to get started?

A - Mushrooms are hard to grow, and unless you have know-how and ideal conditions, you may not be very successful. You need a cool basement with a temperature of 60 degrees or lower, humidity must be maintained above 70 per cent, the room must be well ventilated, and a special compost is required. For a start, go to the public library and borrow a book on how to do it.

Q - I am enclosing two leaves from a viburnum. Would you believe they came from the same plant?

A - Scientists have known for a long time that a plant can have two kinds of leaves - sun leaves and shade leaves. Sun or highlight leaves are found on the exterior and top of the plant. These actually have a different internal structure from those that are lower and in the shade of the upper leaves. Leaves produced in the shade have less difficulty adjusting to lower light conditions.

Q - What makes my tomatoes rot while they are still green? The vines look very healthy, but the tomatoes start rotting at the center and the rot consumes the whole tomato. I lose about half of my crop.

A - This is a physiological disorder of tomatoes called blossom-end rot. The latest research indicates it is caused by a deficiency of calcium. The deficiency is either the result of a low calcium level in the soil or inability of the root system to provide enough from the soil for satisfactory growth of the plant.

Very often the disorder is caused by improper watering by the gardener.

During dry weather, tomato plants should be watered thoroughly about once a week. The soil should be moistened at least six to eight inches deep. If it isn't, the deep roots absorb no moisture and therefore no calcium.

If the soil is kept continuously wet, the roots get no air, they are damaged and unable to absord, and there is no calcium for the plant.

If you have a question for Tom Stevenson, write to him at the Weekend section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071.