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Light Is Essential for Plants
Light Is Essential for Plants
Light is the energy source that power plant life. It drives photosynthesis, which is how plants turn light energy into chemical energy necessary for growth and reproduction. Plant growth is controlled by the absorption of light in a specific range of wavelengths from 400 to 750 nanometers, which is called the photosynthetically active range or PAR.
Blue light (450 to 495 nanometers) is needed for vegetative growth and impacts plant structure. It also controls the stomata, the pores that are responsible for gas exchange. Red light (620 to 750 nanometers) is the most efficient range for photosynthesis and triggers flowering and fruit production. Source: Do Plants Grow Better in Sunlight or Artificial Light?
Light intensity and duration are also important factors to conisder. Intensity is used to classify plants as needing low, medium or high levels of light for best growth and flowering. Plants grown in too little light are often spindlky with pale green leaves. The direction of exposure affects the amount of light plants receive. Southern exposures have the most intense light. Eastern and western exposures get approximately 60% of the light of southern exposures, while northern exposures get only 20%. Reflective, light-colored surfaces provide more light than dark, non-reflective ones. Source: Light, Temperature, and Humidity.
Duration of light exposure is important as well. Plants can be divided into two categories depending on light duration. Short-day plants will flower with less than 12 hours of sunlight, while long-day plants need more than 14 hours. Onions are an excellent example; their varieties are divided between short- and long-day categories. Source: What Are Short-day Versus Long-day Plants.
These two factors are combined to create a measurment of total usable light called the daily light interval, or DLI. This measurement is critical to comparing light sources and is often found in specifications for artificial light sources.
Sunlight Vs. Artificial Light
Sunlight Vs. Artificial Light
The question of which is better is common among gardeners, but a better question is "Which light source is better suited to my plants and growing conditions?" That is almost always as simple as "Am I planting inside or outside? Inside needs grow lights; outside needs awareness of sun intensity and duration at the planting location. For a detailed discussion of the differences between natural sunlight and artificial grow lights, see Do Plants Grow Better in Sunlight or Artificial Light?
Sunlight Terms Defined
Sunlight Terms Defined
Full sun, part sun, part shade, dappled shade, full shade. You see different combinations of these terms all the time. What do they really mean? We think we understand full sun and full shade and probably have an idea about dappled shade, but are there definitions that explain exactly what these terms mean?
The most agreement seems to come from biologists and horticulturists at universities. Plant growers and seed vendors are more likely to use a shortened version of their list. Most sources agree on the definitions below.
Source:Planting in Sun or Shade. Penn State.
Photograph source: Picdadilli Days
Full sun is 6 or more hours each day of direct sunlight. It can be broken up with three or four hours of morning sun, some shade midday and 3 or 4 hours in the afternoon.
Partial sun is between four and six hours of sun a day.
Partial shade is two to four hours of sun a day.
Photograph source: Garden Know How.
Shade is less than two hours of sun a day.
Some other terms you may see include light, moderate, and heavy or dense shade.
Photograph source: JR P. Dappled Light and Shade in Atlanta’s Druid Hills. CC BY-NC 2.0.
Light shade is partially filtered sun like that under trees with open canopies. This is sometimes called dappled shade or intermittent shade.
Photograph source: Pinterest.
Moderate shade is mostly reflected light like the bright areas under the eaves of buildings or near walls.
Photograph source: Garden in deep shade of trees. Sharon Sable.
Heavy or Dense shade is an area with no direct sunlight like in the shadow of a building or on the floor of a dense forest. Enough light filters through to allow some low-light-adapted plants to grow, but many other will not do well there.
Grow Lights
Grow Lights
Not All Grow Lights Are Equal
Not All Grow Lights Are Equal
Incandescent light bulbs are ineffective because less than 5% of the energy is released as visible light, and the amount of heat produced is high. Source: Does Incandescent Light Help Plants Grow?
LED lights (Light Emitting Diode) are probably the best choice. They are very energy efficient, release very little heat, and provide nearly full spectrum light that mimics sunlight.
Flourescent tubes were standard for many years and can still be used, especially for seed starting or growing low light plants, but they are fragile, use a lot more energy than LEDs, and contain mercury making them a disposal hazard.
Blurple lights give off a combination of red and blue light wavelengths in an effort to maximize photosynthesis and growth. They work, but they make it much harder to see pest infestations and disease symptoms because they distort colors. Many people also find them glaring and unpleasant.
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