Plants vary in their responses to photoperiodic cues

Plants that flower in response to photoperiodic stimuli fall into two main classes:

  1. Short-day plants (SDPs) flower only when the day is shorter than a critical maximum. They include coffee, morning glory, Maryland Mammoth tobacco, poinsettias, and chrysanthemums. Thus, for example, you see chrysanthemums in nurseries in the fall and poinsettias in winter.

  2. Long-day plants (LDPs) flower only when the day is longer than a critical minimum. Spinach, lettuce, and clover are examples of LDPs. For example, spinach tends to flower in the summer, so for harvesting leaves, it is planted in early spring.

While there are variations on these two patterns, photoperiodic control of flowering serves an important role: it synchronizes the flowering of plants of the same species in a local population, and this promotes cross-pollination and successful reproduction.