Q. I want to keep my poinsettia and have it bloom next Christmas. Am I supposed to give it a rest period now by cutting back on water and keeping it in a dark place?

A: There has been a tremendous improvement in the durability of poinsettias in the last few years. The modern poinsettia should be watered and given good light indoors until warm springtime weather. Then repot it and cut the stems back to 4 to 6 inches above the soil. When night temperatures stay above 60, put the plant outdoors in light shade. Keep it watered and fertilize it about once a week. About mid-July prune the plant to give it better shape and induce branching.

In the fall when temperatures start to go below 60 bring the plant back indoors.

Q: Antitranspirants are supposed to protect plants during the winter during prolonged freezing weather.One brand specifies it should be applied in October and early November. Another says it should be applied before killing frosts. Has anyone compared the effect of high and low air temperatures at the time of application?

A: The purpose of applying an antitranspirant is to prevent dessication of plants when roots are unable to provide sufficient moisture to replace that lost by transpiration. Transpiration involves water loss in vapor form from microscopic pores on plant surfaces, called stomata. Some plants have stomata on both sides of the leaf while others have most on one side or the other.

Stomata open and close in response to environmental factors such as light, temperature and humidity.

Some authorities claim more plants are injured or killed as a result of transpiration exceeding water absorption than by any other cause.

During the winter when the soil is frozen to a depth that includes all the roots of a plant, transpiration continues, particularly with evergreen plants, and if the freeze lasts very long the plant is in serious trouble. Obviously, the roots cannot absort water from frozen soil.

The antitranspirant reduces water loss by transporation by either providing a film that acts as a barrier to prevent water loss, or by preventing the stomata from opening fully, or by reflecting solar radiation back from plant parts.

Antitranspirants should be applied when plant leaves are fully turgid. They should not be applied at times when it is difficult to fully wet the leaves on both sides. They may not be effective if applied during the heat of the day. After application, there must be time for the film to cure or solidify before freezing temperatures. Otherwise, temperature is not a factor.

Q: Every year I plant cockscomb seed and I have yet to get good results. What is the secret?

A: Celosias (which include plume or feather celosias and the crested cockscombs) are one of the garden flowers easiest to grow from seeds. Wait until summer has definitely arrived before planting; they need 70- to 75-degree temperatures to sprout. Cover seeds slightly with sand, sprinkle with a fine spray of water and cover with clear plastic. The plastic will admit the warning rays of the sun and keep the soil temperatures high on cool nights. Seeds should sprout within 10 days. Remove plastic as sprouts begin to appear.

Q: I've seen beautiful ornamental grasses growing at the Arboretum. Are they okay for growing in my garden?

A: At least 80 kinds of ornamental grass are suitable for garden use. They range in heights from 6 inches to 20 feet and are grown for their attractive foliage color, texture, plant form or flowers.

All of the annual types as well as many of the perennials have attractive flowers and are excellent for dried arrangements. They are relatively free of pests and diseases and require very little care.

Seed of ornamental grass are listed in the catalogue of Park Seeds, Greenwood, S.C. 29649, and Stokes Seeds, Box 548. Main Post Office, Buffalo, N.Y. 14240.