Q - How can you tell when a watermelon is ripe?

A - A good sign of ripeness is the change from white to yellowish color on the underside of the melon. Those with experience can judge by the sound of the melon when it is thumped. If you are not sure whether it is ready to pick, plug it. Be careful not to bruise the skin of the melon when you harvest it and take it out of the field. Do not leave it in the sun.

Q - We had our first crop from our blackberry vines this year. Do they need pruning?

A - Blackberry and raspberry canes are biennial, making their growth one season, fruiting the following spring and then dying. The dead canes should be removed in late winter or early spring.

Q - We are seriously considering planting two Japanese dogwoods to go along with out Americans ones. Do you like them?

A - Cornus kousa, the Japanese dogwood, is indeed a very nice tree. It gets to be 20 to 25 feet tall, blooms about three weeks later than the American, the white petal-like bracts are very similar to the flower clusters of the American, the fruits are not the shiny red berries of its American cousin, but rather pinkish, strawberry-like. The tree develops a beautiful structure of horizontal branches with age, is quite tolerant of shade and can serve admirably as an understroy tree, and is more tolerant of wet and compacted soils than the American.

Q - Newspapers have been recommended for use in mulching garden plants. Is there any risk that the printers ink or pulp would be risky in a vegetable garden?

A - Newspapers have been used for years as mulching material for vegetables and other types of garden plants and there is no indication they contain anything that would be harmful to growing plants or to persons eating the fruit, stalk or foliage. Even if the newspaper contains bad news, it can still be used. To be absolutely sure, you might avoid using the slick sections with a variety of color inks.

Q - How should I clean and sterilize old clay pots before using them again?

A - Scrub the used pots with a stiff brush and warm water. If there are spots of encrusted fertilizer salts, scrape them off with a knife or steel wool. Soaking the pot in boiling water for about 10 minutes will take care of any germs that may be lingering on. If you put the pot in a pan of cold water and the place it over heat, it will not crack. Before using new clay pots, soak them in water for about six hours.

Q - Will caster bean plants keep moles way?

A - Moles feed almost entirely on worms and small insects and castor bean plants do not repel them.

Q - Why do my onions go to tops instead of forming good bulbs?

A - Your onions bolt (form a seed head instead of bulb). Either you planted a variety not adapted to your area or the temperature fluctuated widely after the onions started to grow.

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