How much light does a plant require? First, consider where and how it grows best in its natural environment. Most vegetables grow best in full sunlight and need as much light as possible to grow indoors. Foliage plants like the Philodendron grow in full shade and grow with little artificial light. Exotic plants grow in varying conditions; some grow in deep jungle shade, others grow in bright sunlight. The amount and type of lighting to grow indoors depends on the specific needs of the plant you are growing.
Light (called photoperiods) and dark periods and their lengths affect plant maturity. Plants also need dark periods. The dark period affects flowering and seeding of most plants. Many plants grow under continuous light, but nearly all plants prefer a dark period for normal growth. All plants need some darkness to grow well or to trigger flowering.
Three factors for successful horticultural lighting are: color, intensity, and duration of light. When these factors are properly balanced, artificial lights, such as fluorescents (CFL, High Output T5's and LED's), high intensity discharge (HID) such as the Metal Halide (MH) and High Pressure Sodium (HPS), can help yield spectacular gardens.
The sun's spectrum contains visible light in every color of the rainbow. Blue and violet rays appear at one end of the visible spectrum and red light appears at the other end of the spectrum. If an object absorbs all of the colors of the spectrum, it will appear black. If all of the colors are reflected, it will appear white.
Plants appear green because they reflect green light, while absorbing the other colors of the spectrum. Chlorophyll, the green pigment in plants, is essential for photosynthesis to store energy. Chlorophyll absorbs light energy from the blue and red portions of the spectrum to convert carbon dioxide and water into sugars used by the plant. Without red and blue light rays, the plant weakens or dies prematurely.
Because of their color spectrums and high wattage outputs, MH and HPS lamps are the most popular artificial growing lights.
HPS lamps emit light in the orange and red spectrum bands that is sometimes compared to that of the harvest sun. These colors promote flower production and stem elongation. When using an HPS lamp, flower production may increase by 20% or more.
MH lamps have a fairly balanced color spectrum, with a higher percentage of blue and violet light rays than HPS lamps. MH lamps are excellent during the vegetative growth stage. MH lamps help keep plants shorter and stockier, with more compact vegetation.
Most growers alternate using MH and HPS lamps, to provide a balanced light spectrum during the plants complete growing cycle. Blue light in the MH promotes strong vegetative growth, and the red light in the HPS lamp promotes strong fruiting and flowering.
Light intensity is dependent on two factors: the brightness of the lamp and the distance to the plant.
The brightness of a lamp is measured in lumens. One lumen is the amount of light emitted from one candle that falls on 1 square foot, one foot away from the light. For example, a 1,000-watt standard metal halide emits 110,000 initial lumens, and a 1,000-watt HPS lamp emits 140,000 initial lumens, measured one foot from the source.
Place HID lights as close to the growing tips of the plants as possible, without burning the leaves. Ideally, HID lamps should be 12 to 18 inches above the garden. Tender clones, seedlings and transplants need about 2 feet or more.
If the plants become tall and spindly, they are S-T-R-E-T-C-H-I-N-G for more light. Bring the light closer to the plants. If the leaf tips become "sun bleached" or begin to curl, back them away from the light. High light intensity keeps plants short and stocky, with smaller but denser and heavier leaves.
Duration of light and dark periods is also important to a plant's growth and reproduction.
During the propagation stage, 24 hours of lighting gives young plants a strong initial growth for plants that are healthier and resistant to disease, ultimately providing better yields.
Most plants do well with 14 to 16 hours of light per day. With supplemental lighting, plants are grown all year long, making out of season fruits and vegetables possible.
Plants take in oxygen during darkness and give off carbon dioxide in a process called respiration. Now the plant uses energy that was stored as sugars during photosynthesis to grow. Over 18 hours of light per day for mature plants slows down photosynthesis and too many daylight hours delays or prevents flowering in some crops.
Long-day plants require 14 to 18 hours of light to flower and reproduce. Short-day plants require 10 to 13 hours of light. Too many hours of light can prevent short-day plants from blooming.
Artificial lighting is well worth the effort. When the color, intensity and duration of light are properly balanced and controlled, plants are healthier, the growing season is extended, and productivity is increased.
Don and Sandy Landers are owners of Dream Garden Hydroponics, LLC, 26380 State Route 7 Marietta. The Year-round Gardener appears every other Monday.