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Understand Light’s Effects on Your Plants

January 31, 2017

For plants, and cannabis in particular, light is the primary source of energy. Nutrients, water and growing medium are also important, but a plant will never grow if it doesn’t get enough light. And how a cultivator provides and controls that light can determine yields, cannabinoid content and overall quality of the buds.

The electromagnetic spectrum (Source: NOAA) 

Take a look at the electromagnetic spectrum; it contains various types of light. Visible light, which we can see, falls in a narrow range of the spectrum. Ultraviolet (UV) light and infrared (IR) light possess many of the properties as visible light, but they can’t be detected with the naked eye. IR’s wavelengths are too long, and UV’s wavelengths are too short. Within the UV spectrum, the UV-B type has shorter wavelengths than the UV-A type.

Effects of light on cannabis

What does light do to cannabis? The two main effects are plant height and THC content.

Growing taller: Plants communicate with one another, and cannabis is no different. When grown around other plants, a plant can sense IR signals alerting it to the presence of its peers. An evolutionary mechanism causes the plants to grow taller, presumably in competition with one another to reach the light. Whether growing with a screen-of-green, sea-of-green or vertical growing approach, keeping this competitive growth phenomenon in mind can increase yields. The more plants that are being grown in proximity to each other, the greater the effect.

Increasing THC: Also, cannabis produces more THC when it’s exposed to UV light. This is another evolutionary trick: UV light can damage the plant’s cells, so it responds by increasing THC production. THC can negate the effects of UV radiation, particularly UV-B. Cultivators can take advantage of this survival mechanism. Adding UV-B lights to a grow room can simulate a higher elevation (that is, almost like the plants are closer to the sun) and boost THC production.

There’s a disadvantage to using too much UV-B light. THC is not the perfect sunblock, and high amounts of UV-B will still slip through the plant’s natural cannabinoid defenses. Because UV-B can damage the plant’s cells, this can lead to stress—which will ultimately cause lower yields but more potent buds.

One more thing: If you’re cultivating strains specifically for CBD, avoid UV-B. The more THC a plant produces, the less CBD it makes, and vice versa.

Knowing how plants respond

Light is a physical constant. It doesn’t really change, and neither does its effects. However, cannabis strains have their own traits and characteristics, so each strain varies as to how it responds to visible, UV or IR light. Every grow operation, lighting system, location and cultivar is different, and all of these factors work together to influence how light affects plants.

For example, every plant has its own ideal absorption frequency spectrum. This is determined by cannabinoid content, flavonoid content, terpene content, and flower, leaf, and stem colors. Cultivators will need to determine which plants respond best to which types of light. Taking detailed notes can help keep track of this.

Finally, exposing plants to UV earlier in their life cycle will make them more resistant to UV as they mature. This resistance is structural; plants grown under UV early in life become hardier, sturdier plants. If you’re growing a traditionally indoor strain outdoors, hitting the vegging plants with UV may increase their chances of survival (and their yields) when they’re transplanted outside.

By Randy Robinson
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.
© 2017 CAN Performance Group, LLC. All rights reserved.

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