Grow Lighting Fundamentals

Plant growth (photosynthesis) is determined by exposure to photons from the blue to the orange/red part of the spectrum (expressed in nanometer units of wavelength: 400nm - 700nm). The energy contained in light is absorbed in the chlorophyll of plants. Not all wavelengths of light are utilized with equal efficiency. The light sensitivity curve shows that orange/red and blue light are the most effective for growth. Blue spectrum light is most important in the active, vegetative phase of plant growth. Orange/red spectrum light is most important in the flowering, fruiting stage.

The spectral distribution charts below show the differences between daylight and typical grow lamp types.

Comparison of Daylight and Grow Light
Spectral Distribution

Guidelines

Photoperiod: Many flowering plants use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in night length, or photoperiod, which they take as signals to flower. Photoperiodic flowering plants are classified as long-day plants, short-day plants and day-neutral plants. The regulatory mechanism is actually governed by hours of darkness, not the length of the day.

Long-day plants requires fewer than a certain number of hours of darkness in each 24-hour period to induce flowering. Examples of long-day plants:

• Aster
• Nasturtium
• Dahlia
• Begonia
• Clover (Trifolium)

Short-day plants flower when the night is longer. Examples of short-day plants:

• Chrysanthemum
• Poinsettia
• Strawberry
• Coffee 

Day-neutral plants are not limited by photoperiod. Examples of day-neutral plants:

• Carnation
• Rose

HPS grow lamps: HPS is often selected for photoperiod manipulation when a significant amount of daylight is also available. In situations where there is little daylight, HPS lamps are best used in conjunction with Metal Halide, specifically at the flowering phase of growth. Plants grown solely under HPS lamps have a tendency to stretch and become "leggy".

Metal Halide grow lamps: Strong in the blue portion of the spectrum, metal halide lamps are best used at the propagation, vegetative, early phase of plant growth.

Fluorescent lamps: Generally not as intense a light source as HPS or Metal Halide, fluorescent lamps are a good source when space is limited or the area to be illuminated is relatively small. Standard fluorescent lamps provide a good broad spectrum of light. Modified fluorescent lamps are available that emphasize the red and blue ends of the spectrum. However, experiments done at the Canada Department of Agriculture and at the University of Connecticut show that there is no plant growth advantage in using blue-red fluorescent lamps compared to standard high output cool, white fluorescent bulbs.