CANNING CAUTIONS For the next week, the Prince George's County Cooperative Extension Service will be testing pressure canners - the kind you need to guard against deadly botulism when canning such low-acid foods as greens, beets, peas or corn. Call 952-3116 to set up an appointment between 8 and 4:30.

Q - I love fresh mushrooms and would like to grow them, if it's possible and practical. Can you tell me how to get started?

A - Mushrooms are hard to grow unless you have know-how and an ideal environment. You need a cool basement, 60 degrees or lower, 70 percent relative humidity, and good ventilation. A special compost is required, usually made from horse manure and straw. If you can provide these requirements, go to the public library and get a book or two on how to grow them.

Q - Five weeks ago I planted a mixture of Kentucky bluegrass seed, and there's not a sign of anything coming up. I've watered them every two or three days. What could be wrong?

A - Perhaps the seeds were planted too deep. Bluegrass seed needs light and oxygen for germination. If buried in the soil, the seeds will not germinate, and unless they're in good contact with the soil they may dry out rapidly and perish. A light sprinkling of peat moss or straw over the seed can help keep the ground from drying out. When temperature and moisture are favorable, the seeds start to germinate. After germination, unless kept moist they die. This means light sprinkling maybe once, or twice or even three times, a day until a root system has been developed.

Q - Our Chinese holly is getting too big for the place it's planted in. Can I keep it smaller by shearing it once or twice a year?

A - Most chinese holly varieties require frequent pruning to keep them within bounds, especially after they have been planted five or ten years. You can reduce the size of the plant by removing individual limbs. This type of pruning can be done almost any time of the year except late summer and early fall, but the best time is early spring before new growth begins.

Shearing, the removal of everything at the same length, is not recommended. It causes a dense growth that shuts out light from the interior of the plant.

Q - I am growing Tiny Tim tomatoes on my windowsill and the plants keep bending over as if they want to see what's going on outdoors. Is this normal?

A - When a plant is near a window, the stem of the plant bends toward the light. If the plant if turned about one-fourth the way around each day, the stem usually grows upright.