In a collection of small, potted plants that I bought at a plant society sale, there was a slight, unimpressive "green thing" labeled "elfin herb." Everybody present agreed that it was indeed elfin; otherwise it was nameless.

When the diminutive plant bloomed in a few weeks, its identity was established. It was Cuphea hyssopifolia. In this somewhat cumbersome Latin name, Cuphea is from the Greek word "curved" for the shape of the seed capsule and hyssopifolia means "with leaves like hyssop." It is petite and floriferous charmer nevertheless, and well worth adding to the indoor garden.

Cuphea hyssopifolia comes from a family of about 250 different annuals, evergreen perennials and subshrubs native to Mexico and south into Peru. Among its relatives are the garden flower, Lythrum, and the Crape Myrtle so familiar in this area. The numerous, tiny bell-like, purplish-rose flowers account for its other names - Mexican Heather or False Heather.

The Elfin Herb is a shrubby little plant, rarely exceeding 10 inches in height. Its linear leaves, less than half an inch long, are crowded on the branches, making the foliage almost fern-like. It grows well in any exposure except north and thrives in direct or partial sun. A sunny windowsill is perfect, but in summer the plant should be shaded if it is in a south window.

Under fluorescent lights, Elfin Herb blooms at a distance of 15 inches from the tubes for whatever length of time is provided for other light garden plants. Normal house temperature agrees with it in summer or winter.As a tropical plant, it will not survive outdoors in winter in our latitude. It should be fertilized with water-soluble fertilizer every two weeks and watered so that it is constantly moist.

An attractive compact plant can be grown in a 3- or 4-inch pot. Keep in neat and compact at about eight inches by trimming or clipping branch tips. This pinching back encourages flowering. Groom by shaking off flowers as they fade. A brisk spray at the kitchen sink refreshes and cleanses the plant and is insurance that mites or aphids won't take up residence.

Cuttings of new shoots in the spring can be used to propagate more plants. Place the cuttings in moist vermiculite and shade them until rooted about two weeks; then pot the new plants in regular potting mix. Seed propagation is easy, although cuttings are a quicker means of acquiring vigorous extras. Collect the little black seeds by shaking a plant that has bloomed over a sheet of white paper.

Another Cuphea frequently seen along with Elfin Herb in greenhouses specializing in houseplants is the Firecracker Plant or Ciga Flower, Cuphea platycentra (Cuphea ignea). A variety called "Firefly" is considered especially desirable.

Although closely related, the Cigar Flower is very different from the Elfin Herb. Its dark green leaves are 1 to 2 inches long and pointed at both ends. The abundant tubular flowers are bright red with a black ring at the tip backed at the backed up by a white ring. It too can be grown on the windowsill or in the light garden, but it is a larger plant, reaching 12 inches tall and equally broad. It sometimes is used outdoors as a bedding plant and is suitable for hanging baskets. Its normal flowering period is from April to the end of November, but if it is well grown, it will bloom indoors most of the year. It requires a great deal of water but little fertilizer.

These two easily grown Cupheas are becoming more familiar and more available as indoor gardeners seek out plants small enough to be accommodated in limited quarters and still be able to give profuse and continuous bloom.

Indoor gardening questions may be addressed to Jane Steffey at The Weekly, The Washington Post, 1150 15th St. NW., Washington, D.C., 20071. Please include your address and telephone number.