The National Capital Orchid Society will hold its 31st annual orchid show at the National Arboretum on Oct. 7 from 1 to 6 p.m., Oct. 8 from 10 a.m. to 6 p.m., and Oct. 9 from 10 a.m. to 5 p.m. The public is invited and admission is free.

The exhibits will be some of the finest shown on the East Coast. More than 30,000 varieties will be displayed in natural settings. There also will be exhibits of cut flowers, arrangements by experts and amateurs, plus neddlework, weaving and sculpture with an orchid theme.

Scientists call it photoperiodism. In her scheme of things, nature has fashioned plants that are adapted to a day-length/night-length cycle. Taken away from it, they will not flower or reproduce.

Plants fall into three general categories: short-day, long-day and indeterminate.

Ragweed, for example, sets buds when the day is 14 1/2 hours long and blooms in the middle of August. In Maine, the long summer days do not shorten to 14 1/2 hours until August and buds initiated then cannot produce mature seeds before a killing frost.

During the day, the poinsettia should be given the best light you can provide. A sunny window is best, but don't let it touch the cold window glass. The poorer the light, the less colorful the plant will be.

Protect the plant from sudden temperature changes and keep it away from radiators and air registers when the heat is turned on.

Reduce the amount of fertilizer given to the plant after bringing it indoors. With the lower light intensity, there is much less need for fertilizer.

The plant needs to be watered reguarly indoors. The water should be at room temperature (75 to 80 degrees) during the day. Keep the soil moist but not soggy. Usually this means watering every day or two. Water until it drains out at the bottom, wait about 20 minutes and empty the saucer.

If the plant wilts because you forgot to water, immerse the pot to the rim in water and leave it there until air bubbles stop rising from the soil. Then let the pot drain and empty the saucer.