The quality, intensity, and duration of light directly impact plant growth.

Light Quality

Light quality refers to the color or wavelength reaching the plant's surface.  A prism (or raindrops) can divide sunlight into respective colors of red, orange, yellow, green, blue, indigo and violet.  Red and blue have the greatest impact on plant growth.  Green light is least effective (the reflection of green light gives the green color to plants).  Blue light is primarily responsible for vegetative leaf growth.  Red light, when combined with blue light, encourages flowering. 

Light quality is a major consideration for indoor growing.

• Florescent cool white lamps are high in the blue range, and the best choice for starting seeds indoors. 
• For flowering plants that need more red light, 3 cool white to 1 “grow light” makes a good combination.  The fluorescent “grow lamps” have a mixture of red and blue colors.
• Incandescent lights are high in red and red-orange, but generally produce too much heat for use in supplementing plant growth. 

Light intensity

The more sunlight a plant receives, to a degree, the higher the photosynthetic rate will be.  However, leaves of plants growing in low light readily sun scorch when moved to a bright location.  Over time, as the wax content on a leaf increases, it will become more sun tolerant.

As illustrated above, interior light levels in most homes are below that required for all but low light house plants.  Except for rather bright sunny rooms, most house plants can only be grown directly in front of bright windows.  Inexpensive light meters are available in many garden supply stores to help the indoor gardener evaluate light levels.

Landscape plants vary in their adaptation to light intensity.  Many gardening texts divide plants into sun, partial sun and shade.  However the experienced gardener understands the differences between these seven degrees of sun/shade:

Full sun – direct sun for at least 8 hours a day, including from 9 a.m. to 4 p.m.

Full sun with reflected heat – Where plants receive reflected heat from a building or other structure, temperatures can be extremely hot.  This situation significantly limits the choice of plants for the site.

Morning shade with afternoon sun – This southwest and west reflected heat can be extremely hot and limiting to plant growth.

Morning sun with afternoon shade – This is an ideal site for many plants.  The afternoon shade protects plants from extreme heat.

Filtered shade – Dappled shade filtered through trees can be bright shade to dark shade depending on the tree’s canopy.  The constantly moving shade pattern protects under-story plants from heat.  In darker dappled shade, only the more shade tolerant plants will thrive.

Open shade – Plants may be in the situation where they have open sky above, but direct sunlight is blocked during the day by buildings, fences and other structures.  Here only more shade tolerant plants will thrive.

Closed shade – The situation where plants are under a canopy blocking sunlight is most limiting.  Only the most shade tolerant plants will survive this situation, like under a deck or covered patio.

In hot climates, temperature is often a limiting factor related to shade.  Some plants, like impatiens and begonias, may require shade as an escape from heat.  These plants will tolerate full sun in cooler summer climates.

Pruning of fruit trees, shade trees and shrubs is, in part, a light management tool.

Light duration

Light duration refers to the amount of time that a plant is exposed to sunlight.  Travelers to Alaska often marvel at the giant vegetables and flowers that grow under the long days of the arctic sun even with cool temperatures.

Photoperiod

The flowering response of many plants is controlled by the photoperiod (the length of uninterrupted darkness).  Photoperiod response can be divided into three types. 

Short day plants flower in response to long periods of night darkness.  Examples include poinsettias, Christmas cactus, chrysanthemums, and single-crop strawberries.

Long day plants flower in response to short periods of night darkness.  Examples include onions and spinach.

Day neutral plants flower without regard to the length of the night, but typically flower earlier and more profusely under long daylight regimes.  Day neutral strawberries provide summer long harvesting (except during heat extremes).

Additional Information – CMG GardenNotes on How Plants Grow (Botany):

#121 Horticulture Classification
#122 Taxonomy
#131 Plant Structures: Cells, Tissues, and Structures
#132 Plant Structures: Roots
#133 Plant Structures: Stems
#134 Plant Structures: Leaves
#135 Plant Structures: Flowers
#136 Plant Structures: Fruit
#137 Plant Structures: Seeds
#141 Plant Growth: Photosynthesis, Respiration and Transpiration
#142 Plant Growth: Light
#143 Plant Growth: Temperature
#144 Plant Growth: Water
#145 Plant Growth: Hormones

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