Section 5: Plant Growth Requirements

Glossary

Lesson 2: Light Quality, Intensity, and Duration



That plants require light to grow is obvious, since the process of photosynthesis depends on light. But different plants require different amounts of light.

For example, it is well known that all vegetable plants and many flowers require large amounts of sunlight, but did you know there are some plants that will not flower or bear fruit if there is too much light? They are known as short-day plants. Read on!

There are three factors pertaining to light requirements that directly affect plant growth. These are:

 

  •  light quality

  • light intensity

  • light duration


 


    Does a tree in a parkade, directly under a light, have sufficient light? NO!

    Light Quality

     
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    Light quality refers to the colour, or wavelength, of light.
    The colour of light that reaches a plant's surface will significantly affect its growth, since it determines how effectively photosynthesis takes place.
    Light quality is a major consideration when growing plants indoors.



    Light Intensity

    Light intensity must be sufficient for photosynthesis to take place. The more sunlight a plant receives, the higher the rate of photosynthesis, and the faster a plant will grow. Indoor lighting in most homes is usually below what most plants need, except for some low light house plants.
    Outdoor or landscape plants vary in their adaptation to light intensity. Many gardening texts divide plants into sun, partial sun, and shade. However, the avid gardener likes a finer differentiation, and understands the following seven degrees of sun/shade:
    Direct sun for at least eight hours a day, including 9 a.m. to 4 p.m.
    Plants receive reflected heat from a building or other structure. In such sites, temperatures can be extremely hot, and choice of plants is very limited.
    This southwest and west reflected heat can be extremely hot, and very limiting to plant growth.


    This is an ideal site for many plants; the afternoon shade protects plants from extreme heat.
    Dappled shade filtered through trees. The constantly moving shade pattern protects plants from heat, but in darker dappled shade, only the more shade-tolerant plants will thrive.
    Plants may be in the situation where they have open sky above, but direct sunlight is blocked during the day by buildings or fences. Only more shade-tolerant plants will thrive there.
    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.



     

     Â©iStock

    Light Duration / Photoperiod

    Light duration refers to the amount of time that a plant is exposed to sunlight. It can have a significant effect on plant growth and development. For example, travelers to Alaska often are surprised by the giant vegetables and flowers that grow under the long days of the arctic sun even with cool temperatures.

    The flowering response of many plants is controlled by the photoperiod . Plants can be categorized into three groups according to photoperiodism:

    These flower in response to short days and long periods of night darkness. Longer exposure to light actually inhibits flowering e.g., Christmas cacti, poinsettias, and chrysanthemums.
    These flower in response to long days and short periods of night darkness. Long periods of darkness inhibits flowering e.g., onions, spinach, cucumbers, tomatoes, and roses.
    These flower without regard to the length of day and night. But, they typically flower earlier and more profusely under long daylight hours e.g., roses, cucumbers, tomatoes, and day-neutral ever-bearing strawberries provide summer long harvesting, because they are not affected by the shortening or lengthening of the days.




    Red Poppies

    The red poppy is an annual that grows quickly from seed each spring.
    It is a full-sun plant which is also day-neutral.
    Consequently, it is quite adaptable to a variety of sites and locations.

    Like all poppies, it develops a tap root and, therefore, requires light soil for the root to develop in time for a good bloom.
    The only condition it will not tolerate is a heavy clay soil.