As cannabis plants typically receive most lighting from above, many growers may be familiar with the disappointment of an uneven, lop-sided yield come harvest time. Whereas in an outdoor setting, plants are bathed in adequate sunlight from the side during those long spells where the sun sets and rises, and treated to ample natural overhead light through the middle hours of the day; in indoor settings where artificial light is typically limited to overhead canopy lighting, plants will tend to offer larger, denser buds on the upper areas of the plant, and smaller budding further down where less light reaches. This makes for a top-heavy yield, bearing less fruit in the lower areas of each plant.
Fortunately, this can be easily remedied with the addition of side or supplemental lighting, which when used correctly and set up effectively, can produce more bountiful results.[i]
Types of Lighting
There are three types of lighting growers can implement in order to enhance gains:
1) Side Lights – These are often positioned vertically around the perimeter of the plants, and can be added to increase secondary and tertiary (undergrowth) flower yield by increasing the light availability to the non-apical (lower) flowers.
2) Sub-canopy lights – These are lights positioned at the base of the plant to increase light availability to the lower parts of the plant (similar to side lights in terms of the goal they try to achieve)
3) Supplemental lights – these can be extra lights e.g. in a greenhouse to manipulate day length, but can also be included to simply add more photons to the growing area, again usually for the purposes of yield enhancement.
In many cases, the type of additional lighting used will likely be dictated by availability of free space in the growing environment.
Training your plants, and increasing the number of apical buds by topping (the practice of cutting the main stalk to force bushier growth) etc, has been shown to increase the yield capabilities, and as such is beneficial in helping to produce a consistent crop, however, auto-flowering cultivars lose the capacity to automatically flower independent of day length when they are cut, and so in this scenario, adding illumination to the lower parts of the plant may be beneficial to the overall yield of the plant.
Yields from sub-canopy lighting
The use of sub-canopy lights has been tested in controlled experiments which resulted in a claimed yield enhancement of between 19% and 24%.[ii] The study concluded that this was the result of increased light reaching mid and lower canopies, which are shaded by the top part of the plant. While this study did not address the quality of the flowers produced on the lower canopy, it was observed that they flowered at a surprisingly low temperature. However, it should be noted that sub-canopy lighting could potentially cause conflicting signalling cues, as plants detect the direction of light and will typically grow in favour of the strongest light source. Despite this risk, the test concluded that, in addition to increased yield, sub-canopy lighting also returned an increased bud-to-leaf ratio, and minor changes in terpenes.
Why You Shouldn’t Use More Overhead Lighting
To strengthen the case for sub-canopy or side light supplementation, consider this: Using only overhead static lights means the lower parts of your plants will receive less intense light levels due to shading, but any attempt to solve this by adding additional overhead canopy lighting will increase the risk of inducing high light-stress responses, likely causing photodamage – which includes bleaching of leaf colour and faster-occuring necrosis among its destructive powers. Sub-canopy and side lighting help to avoid this by distributing light evenly and in a controlled manner across the body of the plants.
Research suggests that, while upward-facing sub-canopy lighting is useful, it’s something of a second-best alternative and should only be employed if side-facing lights are not viable[iii]. LED lights make a great option for cannabis grow rooms, as they allow the cultivator to choose a precise light spectrum. In addition, they provide great lighting for photosynthesis, while at the same time consuming less power, and as such are cost-effective. Better yet, good LED grow lights feature cooling systems such as fans or heat sinks to eliminate the risk of overheating, and avoid leaf burn [iv]. As such, LED lights are deemed a great option for side-lighting.
It’s worth mentioning that photosynthesis in plants can be limited by the following: Intensity/quality of light, and CO2/Water/Nutrient availability. The saturation limit for photosynthesis is typically around 1800 μmol·m-2·s-1.[v]
Improving yield in the lower parts of the plant may be achieved by increasing intensity of light to the mid and lower tiers, toward the 1800 μmol·m-2·s-1 saturation limit – but avoid increasing overhead light intensity. As with any adjustments to the growing environment, small, incremental changes are easier to manage, and if possible, always run a trial before introducing on a large scale.