Graphic of light spectrum.

How Light Affects Plant Growth

Excerpted From: Environmental factors affecting plant growth

  Plant growth is greatly affected by the environment. If any environmental factor is less than ideal, it limits a plant's growth and distribution.


Three principal characteristics of light affect plant growth: quantity, quality and duration.


Light quantity refers to the intensity, or concentration, of sunlight. It varies with the seasons. The maximum amount of light is present in summer, and the minimum in winter. Up to a point, the more sunlight a plant receives, the greater its capacity for producing food via photosynthesis.


Sunflowers in sun.

Light quality refers to the color (wavelength) of light. Sunlight supplies the complete range of wavelengths and can be broken up by a prism into bands of red, orange, yellow, green, blue, i ndigo and violet.

Blue and red light, which plants absorb, have the greatest effect on plant growth. Blue light is responsible primarily for vegetative (leaf) growth. Red light, when combined with blue light, encourages flowering.


Duration, or photoperiod, refers to the amount of time a plant is exposed to light. Photoperiod controls flowering in many plants (Figure 1). Scientists used to think that the length of light period triggered flowering and other responses within plants. Plants are described as short-day or long-day, depending on what conditions they flower under. We now know that it is not the length of the light period, but rather the length of uninterrupted darkness, that is critical to floral development.

Plants are classified into three categories: short-day (long-night), long-day (short-night), or day-neutral, depending on their response to the duration of light or darkness. Short-day plants form flowers only when day length is less than about 12 hours. Many spring- and fall-flowering plants, such as chrysanthemum, poinsettia and Christmas cactus, are in this category.

In contrast, long-day plants form flowers only when day length exceeds 12 hours. Most summer-flowering plants (e.g., rudbeckia, California poppy and aster), as well as many vegetables (beet, radish, lettuce, spinach and potato), are in this category.

Day-neutral plants form flowers regardless of day length. Examples are tomato, corn, cucumber and some strawberry cultivars. Some plants do not fit into any category, but may respond to combinations of day lengths. Petunias, for example, flower regardless of day length, but flower earlier and more profusely with long days.

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