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Twenty years ago we transplanted a 2-foot mugho pine from Minnesota to New Mexico. It's grown to 5 1/2 feet tall and about 5 feet wide and has never been pruned. Now it has become gnarled like a huge bonsai. Could you give suggestions on how to prune it?

Drastic pruning of any pine at this age and size would leave only ugly stubs. If you want to keep it at the same size, just prune off the tips of branches where new growth appears.

Two years ago I planted some grapefruit seeds and now have a 30-inch tree growing in a small pot in my apartment. Some of the leaves have started to curl quite tightly. Is this a sign it needs repotting? What time of year should this be done?

Quite likely your grapefruit tree needs repotting after having reached about 30 inches in height. Most potted plants can be transplanted at any time of year. Exceptions are tuber and bulb plants.

The curling leaves may be an indication of root crowding or they may have aphids on them. These insects, sucking the juice from leaves, cause them to curl. They also deposit a sugary substance which you can see.

If aphids are present, mix 1 tablespoon of Tabasco sauce, 1 tablespoon of liquid dishwashing detergent, and 1 pint of rubbing alcohol in 1 gallon of water. Spray on the tops and bottoms of the leaves and stems, being sure not to breathe the fumes. Tabasco spray is ''sneezy.'' Repeat in seven days.

We are troubled with a weed called ''great bind weed,'' similar to a morning glory with white blossoms, except that the blooms and leaves are smaller. A friend says you mentioned some time ago that it can be controlled with white vinegar.

The best way to apply the white vinegar is with a sprayer, covering the foliage and stems. We use it full strength. We are not certain that it is harmless to other plants, but we have applied it near grapevines and fruit trees with no harm.

We've also applied it near our vegetable garden, but taken care to do it on a calm day so there was no drifting of the mist. You will probably need to make two or three applications about a week apart.

For years we've used what we call ''spikes'' in our urns and porch pots. They have gotten so expensive in the garden stores that we would like to raise some of our own. Can one buy seeds? I cannot find them listed in any catalog.

Spikes would be listed under Dracaena indivisia in seed catalogs.

One of the reasons they are expensive is the length of time it takes to grow them from seeds. Bedding-plant growers who want to sell medium-size ones in May must start them in August. For large spikes they must sow the seed abut 15 months ahead of time (March of 1983 for large spikes by May of 1984).

Last year we started marigolds in peat pots for planting along our sidewalk. They grew fine indoors but never did very well after they were set in the ground. When we dug them in the fall we noted that the roots were mostly confined in the peat pots and the walls of the pots were dry, even though it had rained only a few days before. Can you explain this?

Peat pots (and anything containing peat) tend to dry out if not watered consistently and thoroughly. Once they have dried, they are almost impervious to water.

When planting, carefully skin away as much of the pot as possible, and be sure none is remaining above the ground. Any sticking out acts like a wick and draws moisture to the top, where it evaporates, the roots get stunted, and the plant struggles along all season.

Watch peat-moss mulches. If they dry out, water rolls right off and the ground underneath becomes so dry the plants cannot survive. Peat moss is a boon to gardeners if watered properly.

We've tried raising sweet potatoes several times but they never seem to mature before frost hits them. Are there any varieties that can be grown satisfactorily in southern Michigan?

Our experience was similar until we discovered Dudley Sanders of Gleason, Tenn., who assured us we could grow ''sweet taters.'' In fact, he sent us some plants to try out, along with complete instructions for growing and storing.

By using some bottomless plastic jugs for protection from an unexpected late-spring frost, we did get a respectable crop.

Dudley tells us Vardaman and a variety called All Gold, grow with short vines , making them adaptable to small-space gardening.

If you're interested in sweet potatoes, write to Dudley Sanders, Steele Plant Company, Gleason, Tenn. 38229.