The difference between winter sunlight and summer sunlight is dramatic: Less-intense winter sunlight enters windows at a very low angle and reaches far into your rooms. Even the most tender plants aren't adversely affected by exposure to direct winter sunlight. Summer sunlight comes from almost directly above and is much more intense and potentially damaging to certain plants. Both the intensity and the duration of light should be taken into account. Duration affects a plant's blooming cycle. For example, Thanksgiving cactus (Schlumbergera truncata) (figure A) flowers reliably in November, triggered into bloom by the shortening days. Christmas cactus (Schlumbergera bridgesii) blooms later in the year, and Easter cactus (Rhipsalidopsis gaertneri) (figure B) is triggered into bloom as days grow longer. Poinsettia (Euphorbia pulcherrima) is also encouraged to bloom by a change in the duration of sunlight. It requires long periods of darkness to set buds. Fosterella pendulaflora (figure C) is a reliable bloomer at the very beginning of spring. When you see flower spikes beginning to form, you know spring can't be far behind.
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