Amanda Stern   |   4 days ago

Mastering Light Recipes and Cannabis Grow Spectra

Getting the right light recipe for a cannabis grow can be tricky, and taking a one size fits all approach to lighting is detrimental to yield and creates energy waste. Considering photoperiods and spectra is a meticulous must.
Amanda is a freelance writer and cannabis industry worker living in Southern Colorado.

Ask any indoor cannabis grower how important their lighting formula is to the success of their harvest, and they're likely to say it can make or break a growing season. Growers also attest that a one size fits all approach to indoor cannabis grow lighting results in wasted energy and money.

When it comes down to it, every cannabis cultivar has individual needs and responds to the light they receive. Sativas, for instance, are generally thought to be more "light hungry" than indica cultivars. This creates an opportunity for the most dedicated grower to fine-tune their lighting recipe to the strains they are growing and let the test-zone adjustments begin. Finding master recipes begins with making minor adjustments and documenting every step.

What's a Light Recipe?

A light recipe is the schedule of lighting provided to the plants, how much, when, and how long. Growers must imitate the sun and manipulate the lighting spectrum during the natural growth cycle of cannabis.

Grow spectra is an evolving science. Different colors within the spectrum serve different purposes within a developing and growing cannabis plant, and knowing and choosing which ones to use and when opens the door to higher dry flower yields and increased cannabinoid and terpene percentages.

Light Recipe Variables

●      Timing or photoperiod: the frequency and length of time the lights are on. This differs for each stage of development, as the sun changes position and intensity in the sky.

●      Light Level: the measurement of light shed upon a plant is referred to in micromoles (μmol), the number of photons passing through a target area. 1μmol (of light) is equal to just over 602 quadrillion photons.

●      Uniformity: how evenly light is distributed across all cannabis plants, not just those that are centrally located. Uniform light is essential for uniform plant growth.

●      Distance: the distance between the canopy of the cannabis grow and the lighting source. This will vary greatly depending on what type of lights are used and whether or not the grow has reflective materials on the walls to ensure maximized lighting potential.

●      Spectrum: the different range of colors seen and recognized by the human eye. Visible light makes up only a small portion of the electromagnetic spectrum and ranges from 400 nm to 700 nm.

●      Photosynthetically Active Radiation (PAR): Ultraviolet to infrared, all radiation between 400-700nm that cannabis and other plants "see" and use for photosynthesis.

●      Photosynthetic Photon Flux Density (PPFD): measures the amount of light (PAR) a plant receives over time. This measurement is vital for helping growers accurately measure light intensity for photosynthesis at the canopy level.

Types of Lighting

The type of light used in a grow can drastically impact lighting variables such as PAR and light level. HPS delivers a good balance with plenty of red light but lacks blue and can produce lanky plants. Metal Halide provides ample blue light, which is much cooler, and whole plant and root performance are proven to be higher quality with red light. Blue blood standards use CMH or fluorescents for veg and HPS for flowering.

For the sake of simplicity, this article will focus on cannabis LED lights, used by a majority of cultivators for one or more growth stages due to efficiency, improved affordability, and ease of operation through automation and apps.

Though some are fixed, many LEDs deliver different spectrums to allow for fine-tuning. Better quality LED setups have multiple settings delivering veg or grow spectrums and a flower or bloom spectrum. Dimmable lights eliminate the need to raise and lower the lights so the proper intensity is delivered. The best of the best LEDs are fully programmable, allowing the grower to specify the timing and the spectrum, automating the whole light cycle.

Seedling:

●      24-36" between seedling canopy and the light source is recommended during this phase.

●      Lighting should be kept on for 18 hours (seedlings can tolerate 24 hours of light, but do so gradually over a couple of days to stress the crop). This occurs for about two weeks.

●      Full spectrum light (veg or grow setting) will ensure seedlings get all the necessary wavelengths. Plenty of red light encourages root growth, while blue light ensures the plants don't stretch out, keeping them compact and robust for the coming growth stages.

●      Signs of discoloration, bleaching, and burning are all indications that the light source is too close and requires adjustment by dimming or raising

●      It should be noted that clones need more intense light to begin. Depending on the power of the light and maturity of the clones, a 14-36" distance is ok.

Vegetative:

●      12-24" is as close as lights should be for photosynthesis.

●      18 hours on, 6 hours off (uninterrupted). Should the grower move into 20+ hours of light - which the plants can tolerate now – it must be done gradually to prevent stress.

●      The lighting spectrum should include different blues to mimic the high and bright sun during a long summer day. 450 nm is ideal.

●      As in the seedling stage, if stretching is noted, lower the lights. Cannabis stalks and branches must be strong enough to support the upcoming flowering stage.

●      Cannabis can be held at veg indefinitely, so as a general rule, when the plants reach about half of their end flowering height, then flowering can be initiated.

Flowering:

●      12-18" is the average minimum height for this stage. But for each of these suggestions, check with the manufacturer's recommendation.

●      12 hours on, 12 hours off. Dark time should remain uninterrupted. Light leaks cause plants to return to the veg stage or even turn hermaphrodites, pollinating the rest of the crop.

●      Ideally, time is reduced over a few days, again, to eliminate plant stress. Taking them from light exposure at 18/6 to 16/8, then 14/10, and finally 12/12.

●      Mimicking the shorter days of summer and the sun's lower position in the sky with more concentration of the red spectrum. Photosynthesis peaks at 660 nm.

●      Approximately two months as a minimum should achieve vigorous, potent buds.

Green-yellow spectrum for green cannabis

For a long time, the green spectrum 510-565 nm wasn't a part of the lighting recipe. It's been proven to penetrate leaves exceptionally effectively. Red and blue light is devoured entirely in the top two layers of the light-hungry cannabis canopy. Green light penetrates deep into the foliage, driving photosynthesis throughout the lower portions of the plant. Commonly, the green light was introduced as a byproduct of white light, but with the advancements in LEDs, including full spectrum LEDs, a percentage of green light can now be programmed into the lighting schedule.

In addition, green light helps balance the red/blue lights in human eyes, making for a more comfortable working condition. This also improves growers' abilities to detect nutrient deficiencies, leaf burn, pests or pathogens, or other issues.

Introducing FR, NIR, and UV lights as nature intended

Some growers have found great benefits in adding near-infrared (NIR) and ultraviolet (UV) light, specifically UV-B. While unseen by the human eye but felt in the form of warmth on our skin, these rays are received by the photoreceptors covering cannabis plants. These photoreceptors read these invisible wavelengths and react in ways to help them grow and maximize their growth potential.

UVR8 is the category of photoreceptor that is responsible for UV-based responses. Cryptochromes and phototropins regulate mainly blue and green wavelength responses, and phytochromes are red and far red sensing.

During the vegetative stage, the responses generated by UVR8 receptors encourage robust growth, helping in the development of bushier plants. In the flowering stage, UV light promotes growth in terpenes and cannabinoids, including THC. Near-infrared light provides skin with a painful sunburn if unprotected and exposed to the sun's rays; it is used by cannabis to penetrate the canopy deeply, keeping plants strong to support the heavy buds forming.

One must consider the origins of today's cannabis cultivars. Where on the planet did each strain originate in nature? Some of the world's highest potency strains originated in places near the 30-degree N parallel latitude in high altitude locations, all with a higher UV index, where the full spectrum provided by the sun is naturally stronger. Think of the verdant mountain ranges in Morocco, Lebanon, and Pakistan. The natural light cannabis plants thrived upon in these potent and popular cannabis strains become compelling evidence for the addition of purposeful, balanced FR, NIR, and UV treatments.

The light at the end of the tunnel

While advancements in lighting continue to change indoor cannabis cultivation as we know it, perhaps the most important thing to remember is the documentation of each step taken and the results achieved with those steps. Recording the growth stage, light levels, distance, spectrum, etc. along with the observed effects, both good and bad, eliminates problematic settings, aiding in developing a master light recipe. It takes a lot of time, patience, and hard work to build a light recipe from scratch.

However, once this is done, there’s no real need to continue local experimentation on a large scale, potentially damaging or losing some of the crop. The recipe can be relied upon, again and again until a change is made to the light system or cultivation method. Even starting with a basic lighting recipe such as those mentioned above will take some time and adjustments to be the best it can be for any given grow location.

Whether it's LEDs, HPS, metal halide, or a combination of lighting types, documentation is absolutely necessary, providing cultivators a reference for what works versus what doesn’t. A master lighting recipe ensures that optimal light settings are used for each stage of cultivation, eliminating the guesswork, and saving both time and money while maximizing yields.

 

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