This post was written by Steve Grossnickle on part of Tantalus Labs’ community of contributors.
Cannabis production in North America has been largely conducted indoors for the last 50 years. As this species gains mainstream acceptance, the face of its production infrastructure must modernize alongside the political landscape. To optimize its intake of light, water, and nutrients in a consistent and environmentally-controlled way, there is simply no better cultivation infrastructure than the modern Greenhouse.
Cannabis can be grown with varying efficacy utilizing a number of different lighting sources. The simple fact is, however, that not all lighting sources are created equal. To preface the position of this piece, ask yourself, “Why is commercial-scale food crop cultivation not conducted indoors?”
This article addresses lighting options, as well as provides a treatise on light, its importance in plant growth, and the differences in light quality and quantity between synthetic light sources and natural Sunlight.
Also provided is a discussion regarding different artificial light options. The advantages of using primarily natural light for growing specifically Cannabis in a greenhouse-based industry will be presented, as well as the ways that artificial light can be used to supplement natural light in such a greenhouse environment.
Natural light is a form of energy that is transmitted from the Sun as solar radiation. Light is measured in wavelengths, with the length of the wave determining properties of the particular ray. Solar radiation enters the biosphere within an approximate wavelength range of 300–100,000 nm. The solar radiation visible to the human eye is called visible light. The human eye is sensitive to only a narrow range of wavelengths. Humans see light in the violet/blue (380 nm) to orange/red (580 to 740 nm) spectral range, with greatest sensitivity peaking in the yellow/green (555 nm) spectrum. Light comprises 40–50% total incident solar radiation, and is very important, as it has a major impact on the biological processes of plants, specifically Cannabis.
Light and Plant Development
Light in wavelengths measuring from 400 to 740 nm directly affect the photosynthetic processes of plants, and is called photosynthetically active radiation. Photosynthesis is the biological process wherein plants absorb light energy, and along with carbon dioxide and water, convert this energy into photosynthates. Photosynthates are the primary product of the photosynthetic process. These products (i.e., sugars, starch, carboxylic acids, amino acids) are the energy storage compounds and the basic organic substances that plants utilize for growth.
Plants do not effectively use all regions of visible light for photosynthesis. They absorb light from the violet/blue and orange/red regions of the spectrum, while only absorbing minimal light in the yellow/green region of the spectrum. It has been observed that the orange/red region of the spectrum has a greater relative value for photosynthesis; meaning that light in this region has a stronger action than the same quantity of incident blue light. Since plant and human sensitivity to light are not in the exact same range of the visible light spectrum, what we see as bright light does not always translate into plant growth.
Photosynthesis is not only driven by the quality of light, but also the quantity of light. Photosynthetically active radiation received at sea level under full sunlight conditions is 450 W m–2 or 2000 μmol m–2 s-1. Plants receive solar energy as direct light from the sun, or as light that has been modified as it passes through the atmosphere or is reflected off objects.
The actual amount of solar radiation reaching a plant on any given day is dependent upon time of the year, weather conditions, and particulates in the atmosphere. All plants have a similar generalized response to increasing light intensity (Figure 1). In the dark, photosynthesis does not occur in plants. Under very low light conditions (<5% full sunlight) photosynthesis is below the light compensation point; meaning the plant is not converting enough light into photosynthates available for growth. As light intensity increases, leaf-based photosynthesis rises rapidly up to 25% to 33% of full sunlight, and reaches the light saturation of photosynthetic processes at around 33% to 50% of full sunlight.
Anecdotal evidence has indicated that Cannabis plants effectively use light up to 650 μmol m–2 s-1, which is ~33% of full sunlight. Thereafter, increasing light results in only a slight increase in leaf-based photosynthesis. Further increase in plant-based photosynthesis occurs as partially shaded leaves reach complete light saturation. Maximum plant-base photosynthetic capacity is not reached until every foliage photosynthetic unit is saturated.
This point of maximum plant-base photosynthesis depends on the shoot architecture and foliage density causing ‘within-plant-shading’ and the ‘between-plant-shading’ that occurs depending on plant spacing. Medical cannabis has a plant canopy that contains many broad horizontal orientated leaves within a shoot system of many leaf layers. This dense canopy explains why there can still be a gradual increase in plant-based photosynthesis at light that is normally considered above the leaf-based light saturation level.
Natural Light versus Artificial Light
Artificial lights are used by the greenhouse industry in the growing of horticultural plants, vegetables and forest tree species. Typically these plant-related industries grow plants in controlled greenhouse environments and utilize artificial light as a supplemental light source, but not as a sole light source. The reasons for this operational practice are twofold; light quality and light quantity.
Currently there are a number of lamp types available to provide supplemental light for crop growth in the greenhouse industry. High-pressure sodium (HPS) lamps create a light spectrum that peaks sharply in the yellow bands; very close to optimum for photosynthesis. Metal halide (MH) lamps have a broader light spectrum meaning more light energy is outside of the photosynthetically active range. The broad spectrum of MH lamps produces a balanced ‘white’ light making plants appear more true to color to the human eye; which is why they are used predominantly in the retail side of the industry.
Fluorescent and LED lamps are light sources used on a limited basis in the greenhouse industry. Fluorescent lamps are used because they are inexpensive, though they are not desirable light source because of low output of light in the photosynthetically active range. LED technology offers promise because it is possible to choose a very precise light color spectrum, though it is a new and costly light source that is still evolving.
It must be recognized that none of these supplemental light sources exactly duplicates the desired natural light spectral range. Studies show that plants grow best under natural sunlight due to the color distribution. Even HPS lamps, which are the dominant supplemental light system used in the greenhouse industry, only convert 30% of electrical energy into photosynthetically active radiation. Thus, all artificial light sources have recognizable limitations as the sole light source to grow plants, as compared to natural light.
Artificial light sources could effectively replace sunlight only if they can provide enough energy within the desired light spectrum required for plant growth. Plants require at least 33% of full sunlight to come close to efficient growth. If the intent of the plant-based industry is to maximize productivity, while minimizing costs, then the logical approach is one that efficiently grows crop plants. Light requirements must therefore be based on maximizing plant productivity, and enabling achievement of genetic potential.
Figure 2 provides an example of the number of HPS lamps required to achieve certain light intensities of photosynthetically active radiation at the top of the crop. This example demonstrates that it is unrealistic to expect that a synthetic light source can provide sufficient light within the desired spectral range to maximize plant growth. To meet the plant requirements for light, the crop plants would most logically be grown under sunlight; in order to amplify these effects by optimizing environmental control, sunlight would be best applied within a greenhouse complex. Artificial lights should be provided as a supplemental light source to deliver moderate light intensities that increase plant photosynthesis and crop growth during times of the year when the additional artificial light will augment natural sunlight to achieve maximum plant growth.
Plants require light within a specific spectral range for photosynthesis to occur, and result in growth. Natural sunlight is the most effective means to provide plants with this light source. Technology has developed artificial light systems that can provide supplemental lights that are used in the greenhouse industry to augment natural sunlight. However, these artificial light sources are limited in their spectral quality and ability to provide a sufficient quantity of light to fully replace natural sunlight. Thus artificial light sources have limitations to being the sole light source to grow medical cannabis plants when compared to natural light.
The position of Tantalus Labs is that maximum productivity within a plant-based Cannabis industry will likely be best achieved by growing plants in controlled greenhouse environments that provide natural Sunlight and utilize artificial light as a supplemental light source.