Winter is a stressful time for ferns indoors. Special attention to their care will keep them fresh and green. Probably most important to their health is the relationship between temperature and humidity.

The air in a house or apartment is often hot and excessively dry. As temperature increases, relative humidity decreases. The resulting dryness is injurious to most ferns. You can increase humidity around plants by setting pots on a tray of pebbles which are kept moist or a tray of damp sand. Misting, with caution, will increase humidity, but remember that fungus growth is accelerated by over-moist conditions - a special risk for ferns with finely dissected fronds on which droplets of water are likely to collect.

Temperature requirements depend somewhat on the origin of the species. Most warmclimate ferns grow well in the average winter warmth of the home. You should learn to know the preference of the species you have.

If it is difficult to maintain a high humidity-temperature relationship during the winter, keep your ferns in a cool room - 65 to 70 degrees. Avoid cold drafts from windows or doors and hot air form heating ducts.

The size and type of pot and humidity of a room influence the amount of water used by the plant.Some ferns require more water than others. No hard and fast rule can be given for watering.

Ferns require light, either natural or artificial. The amount of light depends on the type of fern. Winter sun, which is of short duration and weaker intensity, is not too bright for most ferns. Where the window faces north, almost any fern will do well. Natural light may be supplemented with fluorescent light, or the ferns can be grown entirely with fluorescent light, allowing 14 hours daily.

Inspect both sides of the fern fronds weekly for scale. Remove the scale with fingernails or toothbrush.

To keep your ferns looking their best during the winter, remove any brown fronds which happen to develop because of low humidity.

Horticultural Short Courses.

Care and propagation of foliage houseplants will be the subjects of classes scheduled for early this year at the U.S. Botanic Garden. Dates and subjects to be covered:

Saturday through Jan. 14 - Ferns, plams, Norfolk Island pine, banana, spider plant, Schefflera, and Ficus species.

Jan. 21 through Jan. 28 - Rex begonias, grape ivy, croton, coffee, velvet plant, wandering jew, Swedish ivy, German ivy, Peperomia.

Feb. 4 throu Feb. 11 - Chinese evergreen, Marantar species (Prayer Plant), coleus, Dieffenbachia, Aralia, Dracaena species, Philodendron species, Pothos.

All courses are free and open to the public. Advance registration is not required. Each class lasts about an hour and will be repeated at 10 a.m., noon and 2 p.m. during the weeks indicated.

Classes are at the Botanic Garden Conservatory, First St. and Maryland Ave. S.W. For additional information, call 225-7099.