Some plants like spinach and wheat produce flowers in summer while others like and dahlia and cosmos flower in winter. Plants that flower in summer require longer duration of light per day than those flowering in winter. The duration of light plays an important role in flowering of plants. This effect of duration of light on the growth of plants is known as photoperiodism.

Photoperiodism is the response in growth, transpiration, photosynthesis, and reproduction (flowering) of a plant to the specific duration of light, which falls on it per day. On the basis of day-length required by the plants for flowering, the plants are classified into the following three categories:

(i) Short-day Plants (SDP): Some plants produce flowers when exposed to a light period shorter than a required day-length. These are called Short-day Plants. Chrysanthemum, Cosmos, Dahlia, Soyabean, are short-day plants.

(ii) Long-day Plants (LDP): They produce flowers when exposed to a light period longer than a fixed day-length. Gulmohar, radish, spinach, are long-day plants.

(iii) Day-neutral Plants (DNP): In these plants flowering is not affected by length of light period i.e. they produce flower in almost all photoperiods. Cucumber,Tomato, and Sunflower, are day-neutral plants.