ASK THE GARDENERS. Q&A.

Q. For three years we have bought hardy mum plants from a nearby garden store. They are always in bloom when we plant them, but they never seem to live through the winter. We even tried mulching them with leaves, but it didn't work. Our soil is rather heavy even though we did work some peat moss into it last year. What do you say?

Hardy chrysanthemums should not be mulched with leaves as the leaves tend to pack down and smother the plants. They should have a well-drained soil. Peat moss and other organic matter can help a heavy soil become more porous, but it will be of little value in wet spots.

Also, in climates where there is heaving and thawing during the winter, mums do not do well. Laying some evergreen boughs over the plants will lower the chances of heaving.

Some people dig the clumps and place them in a cold frame or set them in a protected spot near a building for the winter. In the spring they divide and replant them.

Q. We grew cucumbers and gourds on our back fence this year. While we got a large crop of each, many of the cucumbers tasted slightly bitter. A neighbor says it's because they are planted with the gourds. Is the neighbor right?

The only way flavor from the gourds could have an effect on the cucumbers is if you saved the seeds and planted them next year. The bitter flavor from cross-pollination might then be transmitted through the genes.

The cause of bitterness during this season would be sharp changes of temperature, such as a drop of more than 20 degrees, either between day and night or a sudden drop after a warm spell.

Q. A week or so after a lawn-care company sprayed our lawn with weed killer, the petunias and impatiens began to show signs of wilting and finally dried up. Because it was a windy day, the spray apparently blew onto the flowers. What should I do now to prepare the soil so the flowers will thrive next year?

We receive questions such as yours all too often. The slightest breeze can cause drift. Often, more serious than the loss of plants is the danger from the runoff into the water supply.

We think that a few weeds in the lawn (which produce just as much oxygen and cooling effect as grass) are far more desirable than polluting the environment with harmful chemicals. We use no weed killers at all, except a sharp hand tool called a ''spud'' and our trusty lawn mower.

You can, of course, add organic matter to the soil, since it will encourage the activity of micro-organisms that diligently work to break down chemicals in the soil. Each spoonful of soil has billions of those microscopic creatures working for man's benefit. Detoxifying water is much more difficult.

Q. I have never been able to store dahlia tubers so they would come through the winter. I cut off the stalks and dig the tubers after the first hard freeze, then break them apart and set the tubers in a box of peat moss. What do you suggest I do?

Dahlia toes should not be divided in the fall. Rather, they should be dug carefully, leaving some soil around the clumps. Turn them stem-end down and let dry for about half a day in a dry, airy place. This process lets any accumulation of fluid drain from the stem. They can then be stored in boxes with shredded paper or peat moss. Store at a temperature of between 45 and 55 degrees F.

Some shriveling will not hurt them, but if they are getting too dry and shriveled, you can sprinkle them with water in February.

Q. A friend gave me a cutting of a plant that she bought at a variety store. It has bronzy, pointed leaves (3 or 4 inches long), somewhat serrated, and orange tubular blooms. It was labeled the Sunbird plant. Neither of us knows much about its care, nor the full name of it. Can you help out?

We've had several letters about this member of the Gesneriad family (a relative of gloxinia, episcia, and the African violet). We called on the Bailey Hortorium at Cornell University, which identified it as Chrysothemis (kris-o-the-mis) pulchella.

It needs a humusy soil, bright window (not in direct sun), and a temperature that is no lower than 65 degrees F. Overwatering will cause it to expire, so keep it ''just moist'' as with other members of this family.

Unfortunately, some amateur plant hunters, interested in monetary gain alone, constantly find specimens which they label with any name that suits them, disregarding the need for identification. It is particularly hard for florists who are sometimes asked to tell a customer if a plant is toxic or not. Without the scientific name, it would be impossible to tell the poison control center or other knowledgeable person what the plant really is.

We often have parents of small children call us for plant identification when a part of one has been ingested.

If you have a question about your garden, inside or out, send it to the Garden Page, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. Doc and Katy Abraham are nationally known horticulturists, authors of several books on gardening, and greenhouse operators for more than 25 years.