Short Day Plants, Long Day Plants, and Indifferent

The growth and flowering of many plants depend on their daily hours of exposure to light—either natural or artificial. Without going into technical detail, this, roughly, is called photoperiodism, a phase of which is called day length. Plants are informally classified into three groups.

1. Dahlias, delphinium, pansies, tuberous begonias, and
other plants which come into flower more rapidly during the
long summer days of maximum light—or under the stimulus of
artificial lighting—are called long-day plants.

2. Chrysanthemums, gardenias, poinsettias, etc., which start
to flower when their light-exposure period goes down, are called
short-day plants.

3. A third group contains plants—including African violets,
carnations, and roses—which are unaffected by day length; these are called indifferent.

Science is now trying to determine whether plant growth and
flowering are actually governed more by the daily period of uninterrupted darkness than by the length of exposure to light. Thus the plants that are now classed as long-day and short-day eventually may be called short-night and long-night plants.