3 Reasons to Consider Light Dep in Cannabis

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Light deprivation can help greenhouse growers harvest year-round and produce plants even in the hottest climates.

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April 18, 2021

Kevin Carden; Azat Valeev | Adobe stock

Light deprivation, or light dep, is a powerful tool that provides greenhouse growers with the ability to flower cannabis plants year-round and in the hottest climates in places like Arizona and New Mexico.

Light deprivation is the practice of shortening or manipulating natural sunlight with the use of artificial controls like a blackout cloth. Blackout cloths are tightly woven materials designed to block any light from penetrating beneath its surface. The use of light deprivation technology allows you to maximize the yield per square foot of your greenhouse by increasing the number of harvests you can have in a year.

Shading cloths are another tool you can use to manipulate the amount of light your canopy receives. Similarly to blackout cloth, shade cloth is made of woven or meshed materials and filter various percentages of light from reaching the canopy surface. In combination with blackout cloths, shade cloths also can allow you to control the temperature and environment without using a massive amount of electricity, water, or any other costly resource.

While outdoor farmers are at the mercy of Mother Nature’s whims, greenhouses decked with blackout and shade curtains offer farmers the opportunity to leverage the sun while still generating multiple crops every year. Here’s how you can get the most from your structure.

Types of Systems

With the use of greenhouses becoming more widespread throughout the U.S. and the rest of the world, this technology exists in many different forms. On the high end of the cost spectrum, some greenhouses have shade and blackout cloth installed in the walls and along the inside of the roof. The shade cloths typically have multiple layers and are all motor- and cable-driven. These internally installed systems are integrated into automated greenhouse controls, and can work on timers and sensors to best manipulate the environment. Automated systems are crucial to saving money on time and labor in larger operations, but come with a higher upfront cost. On the low end, growers can use a simple tarp on the exterior of small hoop houses to block out light. This may be a less expensive investment, but moving tarps and shade cloths multiple times a day will cost valuable time and money on labor that could be spent instead on more beneficial tasks. Manual tarps and shades, unless monitored correctly, can have adverse effects with limiting light too much or causing humidity fluctuations. One middle-range option includes installing a blackout and shade cloth for your roof, and using a solid blackout wall material in flowering zones in the greenhouse. These cable-guided cloths can be mechanical with a switch and motor or manually drawn.

Shade curtains filter a given percentage of sunlight, reducing the amount of light that reaches your cannabis crop.
Photo courtesy of Heartland Industries

Reduce Day Hours For Flowering

If you have ever tried to grow cannabis indoors, then you know in order for plants to start their reproductive cycle, you have to change the light cycle to 12 hours of light and 12 hours of darkness. This extended dark period makes the plant think it is now heading toward the fall and winter months and must get ready to reproduce to keep its genetic lineage alive. Because most of the time, indoor grows already are in a sealed room, complete darkness is easy to achieve. A multi-acre greenhouse facility, or even a hoop house that can be more than 100 square feet on the small end, is a whole different beast to try to get complete blackout.

Complete darkness is ideal because even small amounts of light reaching the canopy can cause plants to revert back to a vegetative state or create hermaphrodites (plants that present both male and female flowers). This stress can create a multitude of problems, many of which are detrimental to the crop, including producing pollen, which can then pollinate your crop, induce the plants to seed and stop flowering. Larger light leaks can cause bigger issues such as plants reverting back to the vegetative stage or plants stopping flowering altogether.

The install or system in place needs to be able to completely block any sunlight from getting in. Therefore, operators using smaller hoop houses often will throw a tarp over the top to completely block out the light. The light dep systems installed in larger, state-of-the-art greenhouses, however, often have many moving parts. Depending on which company or style you go with, they can run in different directions and close different ways. Your greenhouse build will often dictate which manufacturers can work with you and how these systems will actually operate. The most common style uses the greenhouse’s truss as a starting and stopping point along the ceiling and walls. The trusses create a great seal when the cloth is pulled against the flat side, leaving little room for light to get through. These systems need to be maintained and checked, as the moving parts can wear and come out of calibration, causing the shades to not close properly or system failure. Simple eye checks on the cables, pulleys, drive shaft, and light sealing edges should be done daily while walking through. More extensive checks should be done at least once per quarter, and, depending on your climate and how often you use your machine, a good greasing with the manufacturer’s recommended lubricant at the same time is a good idea. Don’t go overboard, though, as using too much grease can result in globs falling on plants and floors.

Cool Hot Spaces

Depending on what type of roof you have installed in your greenhouse, the temperature can rise significantly throughout the day—especially during the summer months. Glass greenhouses tend to trap the most heat in structures, while poly greenhouses can have anywhere from a clear single layer to cloudy double layers, creating a very wide range of heat and sunlight control. To combat rising temperatures and cool the space, greenhouse growers can deploy many different techniques depending on what systems they have installed in their greenhouse. Most of these systems involve either an enormous amount of electricity (for an HVAC system) or a massive amount of water and ventilation systems (for a water wall). Unless you are running off a solar field or have unlimited water, these systems can get costly very quickly. By simply using a shade cloth, whether it is mechanical or manually drawn, greenhouse temperatures can drop significantly with little to no electricity used. (With 30% shade cloth, we were able to see 15-degree temperature drops in the middle of the Colorado summer.)

The weave or amount of the shade cloth will determine how much light is allowed to pass through. Some automated greenhouse systems will have multiple layers of shade cloth for more control. The different layers allow operators to not only shade some of the light from outside, but actually choose what percentage of light is being shaded. On the other hand, hoop houses will often either hang shade cloth from the inside or drape it over the top for quick deployments. Either way, the shade cloth is usually able to be applied or removed quickly unless you know your canopy light levels are high enough where you only have to worry about temperature.

Having shade cloths that are easily removable is critical for states with less light, high altitudes, or continuously changing weather. If the light level above the shade cloth becomes too weak, then the plants will only receive far red light (that spectrum can penetrate shade cloths), causing them to stretch rather than grow bushier. Having an automated system or manually monitoring all variables will help ensure that everything is receiving the correct amount of light and a suitable temperature for optimal growth. Without automation, this also quickly becomes labor-intensive (and thus more costly) with larger operations.

One of the most common shade curtain styles is to have them mounted on greenhouse trusses.
Photo courtesy of Heartland Industries

More Harvests Per Year

Now, some might wonder why a grower would want to flower plants in a greenhouse more than once a year and go through all the trouble of having one of these systems. Why not just grow bigger plants and have one big harvest?

Well, the short answer is sometimes bigger plants are not the best option. Whether it is environmental controls that cannot keep up with increased humidity, structural size limitations, or the labor required to harvest a massive crop is too expensive, challenging or otherwise not feasible, then light deprivation can help manage plant development.

Having smaller, successive crops that fill out the square footage rather than all the cubic volume of the greenhouse has many benefits. If done correctly, growers can produce more per square foot by having multiple crops a year as opposed to just one. Having multiple crops throughout the year also means that you will be able to hit the market with fresh product at different times in the year. Consumer desires tend to change month to month in the cannabis industry, so having different times to sell at can translate to more profits and more demand.

A smaller crop also means less storage space is needed for drying, curing and shipping. Also, the amount of time the finished product is stored is shortened, which helps minimize natural degradation of the cannabis. Another benefit is that because plants are smaller and younger, they can be less prone to disease and pests.

Also, when applying pesticides or trying to increase the airflow to improve plant health, large plants can be more to account for than you would anticipate. They create more humidity while also having more places for pests and spores to hide. Smaller plants allow more air to pass through the greenhouse while also not affecting the climate as much.

The final benefit that immediately comes to mind is labor. Unless you already have a considerable workforce to tackle a huge crop, it may be worth considering that you can harvest smaller crops with fewer people. By limiting the number of people you hire for your greenhouse, you not only save money in training and experience, but also in background checks and the hiring process as a whole. In general, if you are running a greenhouse and you can afford it, a blackout system can really change your entire business scheme.

When trying to maximize the outputs of your greenhouse, using some type of light deprivation is necessary to be profitable in all seasons of the year in the cannabis industry. At the minimum, having a blackout cloth will allow you to produce at least one more crop per year. Shade cloth becomes almost necessary for nearly any greenhouse in the Southwest, as the light intervals are so strong that some of the light is seen as excess, so shading becomes a great option for heat control. Combining shade cloths and blackout curtains with automation technology is ultimately the key to fully seeing the benefits of these systems to increase outputs across the board.

Gevin Gros is the greenhouse general manager at Heartland Industries.