Getting Started: Lighting for Indoor Crops

From http://www.maximumyield.com/

By Erik Biksa

It's tough to beat the intensity and spectrum of naturally available sunlight. However, indoor growers have the advantage of producing consistent light levels and being able to control light and dark periods to promote different growth phases as required.

IN NATURE the sun provides the energy required for photosynthesis (production of plant energy) to occur. Plants absorb the spectrum (colour wavelengths) of the sun. The photosynthetic plant response curve is typically within the wavelength of 400 to 750 nanometres. Interestingly, most plants appear green to the human eye because the green portion of the spectrum is reflected by plants rather than absorbed. Basically, light is solar radiation and it has far-reaching effects on biochemical responses. For many crops more light intensity equals faster growth with heavier fruit and denser flowers. Light is a critical factor for intensive production, but if other environmental requirements such as temperature are not optimal, the plant response will be limited.

Indoor growers provide light levels conducive to plant growth by artificial means. A 1000-W HID lamp provides high light intensities at close distances to crops; however, it also provides increased temperatures. As lamps are moved closer to the plant canopy, the area covered decreases in size. So, the grower must compromise between intensity and practicality. For example, referring to table 1, a 4 x 4-ft. (~122 x 122 cm) area with a 1000-W HID lamp suspended 16 in. (~41 cm) above the canopy with a good quality light reflector is a standard.

Table 1: General Lighting Footprint

A light meter can help the grower provide consistent light levels to all plants in the garden. Remember that the farther a plant is away from the HID lamp, the less light it will receive relative to plants closer to the lamp in the garden. Installing side lighting and raising containers or beds to the outside of the growing area may help border plants to receive more light for uniform growth and yields. As a point of reference, strong natural sunlight is about 5000 lumens. Keep in mind the sun's spectrum is optimal for plant growth, while HID lamps are not. An increase in light intensity from HID lamps helps to compensate for their less-than-perfect spectral output. Table 2 (page 52) provides a general outline for the lighting requirements of a short-day light-loving crop.

Table 2: General Lighting Regimen for Short-Day Plants

The length of light time, darkness received, and wavelength (spectral) ratios of light play a strong role in triggering developmental responses in many crops.

Day-Neutral Plants: These are plants that are not sensitive to light and dark cycles for triggering different plant responses such as flowering. Day-neutral plants generally "auto flower" after getting to a certain size or number of days since germination. The temperature and light spectrum may also play a role in triggering the flowering cycle of this classification of plants.

Long-Day Plants: These are plants that require long day lengths, typically greater than 12 hours of light, to initiate flowering or the reproductive cycle. The amount of darkness received is just as critical. Interruptions in the dark cycle may trick long-day plants to flower early.

Short-Day Plants : The majority of plants grown indoors under light are short-day plants. These are plants that require decreased day lengths, usually 12 hours or less of light to flower. It is especially important for the indoor grower or greenhouse grower forcing flowering to maintain an uninterrupted dark cycle, for example, 12 hours of uninterrupted light and 12 hours of uninterrupted darkness.

The proportions of wavelengths of light the plant receives may also influence plant development and can also play a role in the life of other organisms surrounding the crop. Interestingly, there have been some developments in greenhouse film, plastic mulches, and row covers. For example, the films may absorb a pigment of light required by a diseased organism, but not necessary for plant growth, thus starving the organism of the wavelengths it requires.

In general terms, the colour of light is often measured in degrees kelvin (K). The wavelengths in the photosynthetic response curve are measured in nanometres. Lamp ratings for the colour or spectrum emitted by that particular lamp are referenced as K. The higher the number of K the lamp emits means that a higher ratio of the light available is in the blue spectrum. For example, a Super Metal Halide lamp may emit 5000K, while a Super Red High Pressure Sodium lamp may emit 3000K, which is proportionately less blue and more red in the spectrum. Natural sunlight is about 4200K, but may have a higher K value in early seasons while decreasing to more red wavelengths towards the fall.

Different geographical regions have different light environments in which particular strains of seed plants have been developed. In equatorial mountainous regions there are more UV (low nanometre) wavelengths available. Some growers have also found the cultivars from northern regions tend to produce better where there is more blue light available. For crops that spend most of their life cycle flowering or fruiting, higher levels of red light promote abundant harvests indoors.

Red light helps to trigger germination, rooting, and flowering; these phases are associated with reproductive processes. Blue light promotes other developmental characteristics and is particularly advantageous in the vegetative growth and transitional phases. Plants tend to stay shorter and bushier and may produce denser and more compact flowers when wavelengths are higher in the blue spectrum (higher K). Some growers use different ratios of light in different growth phases. This is accomplished by the type of colour in the lamps they use throughout various growth phases. Specific lighting transformers (ballasts) are required for specific types of lamps. You may not just screw in any lamp to any fixture. Fortunately for the horticulturist, manufacturers are producing lamps with different light temperatures (K) that can be operated with the same ballast.