Ask the gardeners

You often give advice to ''cut back a plant,'' but don't say how far. We'd like to know how far to cut back a fuchsia plant and some rosebushes. TIt's impossible to give a general figure for all plants and it's difficult to give a firm figure for specific plants.

Fuchsias should be cut back at least halfway when you bring them indoors in the fall. Geraniums and mum plants (potted) can be cut back to about 4 inches above the pot so new growth will be sturdy. In cold areas, roses should be pruned back to about 18 inches before winter.

Shrubs that are going to be moved should be trimmed back 1/3 (or thinned out about 1/3). Trees that are transplanted ''bareroot'' should usually be top-trimmed back about 1/3 to compensate for root loss when they were dug.

Don't be afraid to do a reasonable amount of pruning, because it's a stimulating process, causing plants to throw out new growth.

Awhile ago you answered a letter from a writer inquiring about plants attractive to hummingbirds. At our home here in Washington State, here is what brings them to our yard: hanging baskets of fuchsias, and apricot-colored tuberous begonias, which they visit daily. Also, we have wild currant, butterfly bush (Buddleia), and Clematis jackmani as well as evergreen clematis. We thank our Western friends for the suggestions.

A few years ago I bought two Manchurian apricots advertised in the Monitor. One has since died but the other is hearty, except there has been no sign of fruit. Do I need two apricots for cross-pollination? Some apricots are self-pollinating, meaning you only need one tree for fruit to set. However, they all produce better if there are two varieties for cross-pollination.

Moongold and Sungold are Manchurian types that need cross-pollination. You must plant one of each. Moorpark and Goldcot and Early Golden are self-pollinating, but even they do better when planted with a variety that blooms at the same time.

Although our tomatoes were late getting ripe, we did finally have enough to can and freeze. However, even though we planted the same varieties we have for the past few years, the fruit did not have as much taste as other years. Can you explain this? Could it be our soil? We've had this question many times this past season.

In many parts of the country, cool nights and rainy periods kept fruit from ripening well and acquiring top flavor. Try the trick our mothers used a few years back.

When you open the cans (or freezer containers) this winter, add a squirt of lemon juice and about a tablespoon of sugar per quart (or a little more if you prefer). We add a few chopped celery leaves, also. If you have a question about your garden, inside or out, send it to the Gardening page, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115.