Q - I bought a banana plant last April. It has grown 4 or 5 inches since then, but the leaves turn brown before they completely unfurl. What am I doing wrong?

A - The banana plant does not do well in the average home because of low humidity. The place for it is in a greenhouse where relatively high humidity can be maintained.

Q - L've planted Irish potatoes for two years. They started out fine, but later the shoots dried up and died. What do I have to do succeed with them?

A - It's almost impossible to get a crop of potatoes without spraying them constantly from the time they're a few inches high until maturity, because many different kinds of diseases and insects attack them. To start, buy certified seed potatoes, which will provide some protection against some diseses. They should be planted in a sandy, acid soil to help prevent a bacterial disease called potato scab. They need to be watered during dry weather and fertilized occasionally.

Q - My sweet corn ears didn't fill out, and some have very few kernels. What would cause that?

A - Most likely it was due to poor pollination. Instead of planting he corn in one or two long rows, it's better to plant it in four shorter rows, side by side, to insure good pollination. An application of fertilizer when the corn is about 10 inches high helps, as does watering during dry weather.

Q - We have managed to destroy most of a very flourishing crop of poision ivy by spraying it with amitrol. Now how do we dispose of the dead leaves and plants? Is it safe to touch them now? At what point does poison ivy cease to be poisonous?

A - The poisonous or sensitizing substance in poison ivy is a sticky resinous compound produced in microscopic ducts that run throughout all parts of the plant. The poisonous resin does not disslove in water; it persists for a long time without losing toxicity. Experiments have shown that leaves of poison ivy stored for five years at room temperature were nearly as toxic as when they were fresh. Branches left outdoors on a garage roof for 18 months still proved highly dangerous. The toxin can remain on the feet and fur of pets for several days and be transferred to humans. The smoke of burning poison ivy can cause severe inflammation.

Your best bet with the dead parts is to cut them off and bury them. Use gloves and protective clothing to avoid coming into contact with the leaves and stems. It can also be burned, but be careful - the smoke harm anyone who comes into contact with it.

Q - This spring my peonies were loaded with buds, but only one flowered; the rest remained tight until they died. The bushes are in a spot where they get full afternoon sun. I have never fertilized them. Should I do so, and if so, with what fertilizer?

A - Peonies need full sun during the morning and at least very good light during the afternoon. With shade during the morning, they may not bloom. They need good soil drainage; they do not do well if water stands around their roots in heavy clay soil. Fertilize them in early spring with 5-10-10 fertilizer and again in early June. Do not let weeds or grass or tree roots compete with them during the summer months, and keep them watered during prolonged dry weather.

Q - Three miniature roses were given to me. I have them growing outdoors in pots. What shall I do with them this winter?

A - They can be grown indoors during the winter on a sunny windowsill or under fluorescent lights. If given good light and good care, they should bloom continuously.

They will survive the winter outdoors planted in the ground - in fact, they're more hardy than standard hybrid tea roses, provided temperatures do not go below zero. In the ground, th soil provides some insulation against cold for the roots.

Q - Our dwarf Japanese red maple is red and beautiful in the spring but turns green in early summer and stays that way the rest of the season. Is there a way to make it stay red during the summer?

A - Most red maples, including some Japanese red maples, are that way. It's their genetic makeup and there's nothing that can be done about it.

Q - I bought prtunias and zinnias in packs in the spring; they were tall and spindly with no side branches. What can be done to make these plants spread out and be attractive?

A - Buy them early before they become tall and spindly and if you can't plant them in the ground immediately, put them in individual pots. Pinching off some of the top sometimes may cause them to develop branches on the lower part of the stems.

Q - Are all kinds of leaves good for making compost?

A - Almost any kind of organic matter can be composted, including leaves, grass clippings, potato peelings and other vegetables scraps. Pine needles are slow to decay and are not as good as hardwood leaves.

Q - Last summer we planted cantaloupes and cucumbers side by side, and the cucumbers tasted a lot like cantaloupe. How can this happen?

A - It cannot happen, or at least specialists do not believe it can. If cucumbers are pollinated by pollen from cantaloupe flowers, it will affect the seed of the cucumbers but not the flesh. The cucumbers will taste the same whether the cantaloupes are nearby or not.

DIAGNOSIS, PLEASE - Bright and early Saturday morning - at 7, to be exact - a member of the Cooperative Extension Service of the University of Maryland will conduct a plant clinic at the Farmers' Market in Silver Spring, at the corner of Pershing Drive and Fenton Street. He'll discuss the diseases that afflict vegetables, fruits an landscape plants. The clinic will also be held on August 11.

PARSLEY, SAGE & WHATEVER - Herbs don't come from little glass jars. On Saturday and Sunday, at 11, 1 and 3, you can tour the herb garden of the National Colonial Farm in Accokeek, Maryland, to see what's growing. On Sunday, if you make reservations early, you can also attend an Appalachian dulcimer workshop at this re-creation of a farm from a bygone time. Call 283-2113.

FREE PLANT CLINIC - The Prince George's County Horticulture Extension Service will hold a free plant clinic Wednesday evening, 7:30 to 9:30, at the Good Luck Community Center, 8601 Good Luck Road in Lanham. Call 552-1093 to register in advance.