Plants have specific requirements for light intensity, quality, and night length. Light intensity refers to the strength of the sunlight. The optimum intensity for plant growth is that at which the plant is most proficient at photosynthesis. Higher than optimum light intensity can damage chloroplasts, whereas lower levels do not yield enough energy for photosynthesis. Shade cloth is often used to protect sensitive seedlings and plants from strong sun by partially blocking light and reducing the intensity.
Light quality is based on the wavelengths of visible light, which ranges from violet (the shortest wavelength) to red (the longest), with all the colors of the rainbow in between. Below violet there is the ultraviolet range, or UV light. Above red there is the far red and infrared. Photosynthetic pigments have evolved to interact with light at wavelengths of blue and red. The shorter blue wavelengths provide greater energy. Far-red light causes long, thin stems and also triggers the transfer from vegetative growth to flowering. Placement in the garden affects the quality of sunlight that reaches the plant, as sunlight coming in at an angle closer to the horizon contains less high-energy blue light than sunlight arriving by a shorter overhead path.
Flowering in angiosperms is triggered by night length. Photoperiod is a term that describes the relative numbers of hours of daylight and darkness. Night begins with far-red light at dusk and ends with far-red light at dawn and these wavelengths are involved in the plant’s ability to keep track of the length of the night. Some plants have specific requirements as to how long the period of darkness must last in order to trigger flowering. It is called the critical night length and it appears to be measured by a pigment found in leaves. Those plants that have evolved to bloom when the days are long and the nights are shorter (as in the late spring and early summer) are called long day/short night plants. Many annual plants belong to this group. Some wait for shorter days with longer nights, like chrysanthemums, which can bloom in the late fall or even winter; these are the short day/long night plants. Some plants, such as roses and geraniums, are not controlled by the photoperiod and are considered day neutral. In this case, flowering is controlled by other factors.
Natural daylight ranges from 8 hours at winter solstice to 14 hours at summer solstice. Ancient calendars marked the solstices with physical structures. If you know when solstice is, you know when to plant vegetables to ensure harvest before winter sets in. Knowledge of photoperiod requirements is also required to design flower gardens that will be in constant bloom from the spring through fall. Photoperiod can be manipulated by using infrared light, which is a strategy used to induce flowering in commercial greenhouses.
Light intensity and light quality are used to separate plants into groups that require full sun, part sun, part shade or shade. Full-sun plants like a full day of high-quality light, part-sun plants require 5 to 6 hours of high-quality light, part-shade plants need to get filtered light all day, and shade plants require full protection from strong sun all day. The light requirements for growth are included in seed catalogs and on the seed packet, as well as on the tag that accompanies a plant for sale in a garden center. The bloom time is also provided and may be reported as a month or season or in terms of the photoperiod.
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