Things to consider when choosing a greenhouse lighting system:

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Greenhouse lighting for vegetative growth or flower production?

Vegetative growth is supported by a blue light spectrum. Blue light is put off by LED, fluorescent, and metal halide bulbs. This light is crucial for leaf growth, root development, and node spacing. Flower production is stimulated by a yellow light spectrum. Yellow light is put off by LED, incandescent, and high pressure sodium (HPS) bulbs.This spectrum encourages root and plant stretch, as well as heavy flower production.

Is it the only source of light or will it be used for supplementation?

Indoor cultivation methods require all the light within the greenhouse to be produced artificially. In this scenario it is important to do an energy vs. infrastructure cost analysis:

  • Is there enough power to fill the growing area with lights?
  • Is there room in the budget to upgrade to highly efficient lighting system such as LED’s with solar panels?

Typically when used as a primary source, a metal halide or high pressure sodium light will cover a 5ftX5ft section of canopy.

Greenhouse cultivation methods utilize the sun as the primary source of light energy. In this case, artificial greenhouse lighting is used as supplementation. In the vegetative stage, metal halides are used to extend the daylight hours to keep plants from flowering on short days. When flowering, HPS lights are used on cloudy days to maintain high levels of plant production. Under these conditions, lights can be spaced out more to cover and 8ftX8ft section of canopy.

In most cases a lighting controller is used for timing, light intensity, and it’s automatic safety shut off features. Simple controllers will keep the lights on a cycle while more advanced ones will do all of the above.

How Plants See the Light

Plants can differentiate light based on its electromagnetic wavelengths. Light that is not visible to humans, like ultraviolet (UV), are visible to plants. This ability also lets plants sense the time of day because the wavelengths of light change from sunrise to sunset. Plants can also sense brightness and intensity of light, light exposure length, and even location of light sources. While plants lack a nervous system to translate light into pictures like humans, they are very much able to “see” in a way that is as complex as human sight.

Blue-green light like that found during the day does not affect flowering in plants. Both plants and humans have light receptors called cryptochromes which absorb blue light. While blue light does not play a role in plant growth, it does give cues to the plant’s internal clock.

Red light specifically (which plants only respond to in darkness) will trigger flowering. Far red light, light at the extreme red end of the visible spectrum between red and infra-red light in the region between 710 and 850 nm wavelength, will turn off flowering. Phytochrome is the plant molecule that causes this. It is first activated by red light, which primes it to recieve far red light afterwards. It is thought that because far red light is the last color in the spectrum that a plant sees at night, that it is a signal for the plant to "turn off" for the evening. These days, it is possible to manipulate light quality thanks to recent technological evolutions such as the development of photo-selective films and light-emitting diodes. Manipulating the red light within a greenhouse is able to improve both crop yield and quality.

Length of Light

Cannabis is known to be a short day plant. Plants have the ability to sense day length, and in short day plants this means that flowering is induced when a shorter length of day is sensed. This phenomenon is scientifically known as photoperiodism. A 12 hour day length will trigger flowering in cannabis, longer light exposures of 18-24 hours will keep it in a vegetative growth stage.

Light is absolutely vital to plants. Since it is so integral to food production and survival, plants in the wild must be able to compete for it. In order to do this they have an advanced sight-like system where phytochrome in the leaves acts like the photoreceptors of the eye, the tip of the plant acts like the eye itself, and the shoot of the plant moves signals like the brain. Providing cultivated plants with high quality lighting with proper intensity, distance, temperature, and exposure time is critical in ensuring proliferation.

Greenhouse Lighting Quote

Hortitech has expert knowledge on lighting conditions for greenhouse cultivation. Contact us to discuss lighting systems for your home or commercial operation. Call us to get a greenhouse lighting quote at (541)480-9392.

References

  1. Chamovitz, D. 2012. What a plant sees. In What a plant knows (9-26). New York, NY: Scientific American/Farrar, Straus and Giroux.
  2. Chandra, S., Lata, H., Mehmedic, Z., Khan, I. A., & ElSohly, M. A. (2010). Effect of Light Intensity on Photosynthetic Characteristics of High Δ9-THC Yielding Varieties of Cannabis sativa L. Planta Medica, 76(05), P11. DOI: 10.1055/s-0030-1251773 Retrieved from: https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-0030-1251773