How the Right (or Wrong) Lights Affect Cannabis Growth

How the Right (or Wrong) Lights Affect Cannabis Growth
Scott Lentz
Growing your own cannabis indoors can seem intimidating to amateurs, partly because of the hot, glaring lights required to keep the plants in peak condition. But those blinding bulbs can pump out some serious heat in more ways than one, with indoor flower routinely looking, tasting and smoking better than anything grown outdoors.

Now that growing pot isn't as taboo as it once was, lights intended for cannabis are starting to receive the attention they deserve, from both a performance and a sustainability perspective. As more growers look to scale their output more efficiently, LED lights have emerged as a long-term option. To learn more about the evolution of growing lights and their effect on the cannabis we smoke, we caught up with Kevin Frender, chief science officer of Black Dog LED lights. A horticulturist since he was a toddler, Frender has grown everything from water lilies and orchids to mangoes, cacti and cannabis, and much of that bounty was cultivated indoors.

Westword: What can growing lights do that the sun can't?

Kevin Frender: All plants evolved to be nourished by the sun, but they have different requirements for the intensity of light and the number of hours of light they receive, depending on where on the globe they evolved. Different plants have different total light requirements for each day, as well as different hormonal responses to getting shorter or longer periods of light. If you want to grow something that wants more hours per day than the sun can provide in your location, or if the total amount of sunlight in your location isn’t enough in the middle of the winter, you need grow lights to provide supplemental light even when growing in greenhouses.

However, beyond just supplementing the sun, by carefully choosing the colors of light plants are given, we can control how they grow. Plants given low light intensity levels or excessive red light levels will grow larger-than-normal leaves and longer-than-normal stems. While this may make them look like they’re growing well (bigger leaves and taller plants make many people think the plants are actually happier), it is actually the plants’ signal that they're searching for the assurance of seeing “blue sky” and uninterrupted sunlight.

By manipulating the intensity, duration and spectrum/colors of light a plant receives, we can control many aspects of plant growth, such as the production of leaves versus stems, and leaves and stems versus flowers and fruit. It is even possible to keep plants more compact than they would be in nature so that you can get incredible yields on shorter, smaller plants and be able to fit in more plants and more yield per square foot. This applies to cannabis, mangoes and, really, any flowering or fruiting plant. By providing them with light of a carefully tuned spectrum, you can keep the plants compact but incredibly productive of flowers and fruits.

How much have growing lights evolved since medical and recreational cannabis legalization took hold?

When the first medical legalization occurred in California in 1996, there were only three viable plant-growing lights: fluorescent (only suitable for very small plants), metal halide (MH) and high-pressure sodium (HPS) lights. Combinations of MH and HPS lights could allow you to grow almost any plant indoors or in a greenhouse, but not in an ideal way.

In 1998, Phillips introduced ceramic metal halide (CMH, later also known as LEC), which provided a much broader spectrum of light that grew plants better. Like fluorescent, MH and HPS lights, CMH is also limited in being able to optimize the spectrum of light generated, simply by the chemistry of the mixture of compounds required to generate the colors, so none can be “tuned” and made ideal for plant growth. These lights still worked well enough to prove the utility of using artificial light for plant growth. Unfortunately, all of these lights also contained significant amounts of mercury, enough that if a bulb ever broke, you should really throw away all of the plants underneath to avoid mercury poisoning. All fluorescent, HPS, MH and CMH bulbs still need to be disposed of as hazardous waste for the same reason.

Although LEDs first became commercially available in the ’70s and the first practical blue LED was introduced in 2002, it wasn’t until about 2010 that all colors of LEDs required for ideal plant growth became bright and intense enough to be used for growing plants. The first LED lights weren’t as efficient at generating light as older fluorescent, MH, HPS and CMH lights, but they offered something no previous technology had: complete control of light spectrum, allowing it to be tuned for what plants really need. As LED technology has continued to advance, it has surpassed the efficiency of the older technologies while retaining the ability to create a better spectrum for growing plants, as long as the right combination of LED colors is used.

How can growing indoors become more sustainable as commercial cannabis and legal indoor growing becomes more popular?

Through continued technological advances, LED grow lights continue to set new records for efficiency and yield per watt, far surpassing previous lighting technologies. Because LEDs don’t contain any mercury and require less energy to create the same amount of light, there’s much less environmental impact in the manufacture and disposal of the lights for growing.

Besides being more efficient at just creating light, LEDs also allow for complete spectral tuning to get the plants to grow how we want them to, [which is] to maximize the production of flowers instead of putting energy into growing large leaves and long stems. When a properly tuned spectrum of light is used, it also heats up plants’ leaves less than a “works-but-not-ideal” spectrum, allowing for warmer temperatures in grow rooms. This saves significantly on cooling costs and energy requirements, as well as making the plants grow faster with less stress.

Do all plants need the same light? Is cannabis different from most plants, or relatively easy to grow from a lighting perspective?

Different plants have different light requirements in terms of intensity, duration and spectrum — at least, to maximize the traits we want from them, such as more leaves or more stems or more flowers and fruit. NASA has done studies on lettuce with LEDs to determine how to maximize the weight of lettuce grown, although they weren’t checking on the nutritional value of the leaves. The ideal spectrum for maximizing lettuce weight turns out to grow really leggy, floppy, larger plants, including cannabis, tomatoes and fruit trees. Like many — but not all — plants, cannabis requires full sunlight in the wild to perform best, and that means it needs high-intensity light, including significant ultraviolet, blue and red light to perform at its best. Mangoes and starfruit require the same thing, but lettuce not as much.

Why are so many cannabis grows lit in certain shades of colors?

The orange or yellow-ish lights are HPS lights; they’re good at pumping out high-intensity light, but unfortunately lack blue light in their spectrum, which is why they look yellow or orange to us. Plants see light differently than we do; they mostly reflect green light, which is why they look green to us, and our eyes are more sensitive to green and yellow lights than red or blue. Plants absorb and utilize mostly red, yellow and blue lights, although yellow light heats up leaves more than red and blue. The yellow-looking HPS grows you’ve seen had to keep their growing temperatures down with lots of air conditioning, below the temperature cannabis plants would encounter in the wild; the plants can use yellow light to grow, but it heats up their leaves more than even natural sunlight would. Under HPS lights, you have to keep the temperatures below about 80 degrees Fahrenheit to prevent the plants from dying due to overheating. Cannabis can be grown at higher quality, temperature and yield while using less electricity for light and cooling with a spectrum tuned to the plants’ needs.
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Thomas Mitchell has written about all things cannabis for Westword since 2014, covering sports, real estate and general news along the way for publications such as the Arizona Republic, Inman and Fox Sports. He's currently the cannabis editor for
Contact: Thomas Mitchell