Deliberately shading a greenhouse to control the photoperiod is a gardening technique known as light deprivation. Many plant varieties won’t fruit or flower until the daylight hours are reduced past a certain point. Light deprivation enables a horticulturist to limit the amount of sunlight hours per day to initiate a crop’s flowering cycle. This technique gives a grower heightened control over the photoperiod and allows him or her to bring plants to maturity earlier than they would naturally.

Most light deprivation greenhouses have a transparent material (glazing) that is identical to any other greenhouse. Unlike typical shade cloths, which only reduce the amount of light entering the greenhouse, the shade material (also called blackout material) used for light deprivation is designed to keep out as much light as possible. To make the interior of the greenhouse as dark as possible, the shading material used for light deprivation needs to cover the entire greenhouse structure. Each day the shade cover is removed for the desired number of light hours, then manually or automatically positioned back over the greenhouse.

Advantages of Light Deprivation

The advantages of using light deprivation include more control over when to harvest, multiple harvests per year, increased overall yields, and reduced risk.

When to Harvest

A light deprivation greenhouse allows a gardener to control when the flowering stage is initiated and, in turn, when to harvest. This could be advantageous for many reasons, including the ability to harvest the crop at a time of year when other growers still have plants in the ground. That means a higher price for the product and a higher return on investment for the grower.

Multiple Harvests Per Year

The light deprivation method gives the ability to harvest several times per year, much like an indoor garden. If a horticulturist could do the vegetative growth in another greenhouse or indoor garden, he or she could potentially reap as many as four or five harvests per year.

Increased Overall Yields

Although the individual harvests of a light deprivation greenhouse are smaller than a typical outdoor harvest, the overall yield for the season will end up being comparable or even larger. That’s because some of the plants are flowering under the mid-summer sun, so they get stronger, more direct sunlight with a higher level of UV light.

Reduced Risk

By breaking up the outdoor season into multiple harvests, a horticulturist reduces the chance of complete failure. Because of multiple harvests in a light deprivation greenhouse, the grower will not experience a complete loss if one crop is ruined. In other words, if an entire crop is destroyed, the grower will still have other crops to help negate losses. It isn’t an all-or-nothing situation.

Cooling a Light Deprivation Greenhouse During the Light Hours

Controlling the temperature of a light deprivation greenhouse can be tricky. These greenhouses are closed tight during the peak heat of summer, so overheating is possible. The following are some cooling techniques to follow only when the crops are exposed to light. These techniques don’t apply during the dark cycle because they could create a light leak.

Convection

Convection is the physical movement of a warm gas or liquid to a colder location. Some growers rely on convection to help cool their greenhouses. Strategically placed vents in a greenhouse allow hot air to escape from the top, while cool air replaces it, entering from the lower portion of the greenhouse. This cooling method can only be used in a light deprivation greenhouse during the light cycle. Once the dark cycle begins, the vents need to be closed or covered to ensure no light leaks occur.

Evaporative Coolers

Evaporative coolers work well to cool down smaller light deprivation greenhouses during the light period. They draw air over a wet pad to create an evaporative cooling effect. Most evaporative coolers come with a float valve and are connected to a water spigot so water can be replenished as it evaporates. Evaporative coolers work best when they have access to fresh air, so they should be placed near the greenhouse entry or a fresh air intake vent. If the vent intake is not a light trap, the vent will need to be covered during the dark cycle.

Cooling a Light Deprivation Greenhouse During Dark Hours

The most popular and most effective way to cool a light deprivation greenhouse during the dark cycle is to use motorized fans in conjunction with light traps.

Motorized Fans

The best way to cool most greenhouses is with a motorized fan. A fan in a greenhouse pulls fresh, cooler air through the greenhouse (around the plants) and then exhausts the warm air outside of the structure. Actively drawing cooler air from outside the greenhouse brings the temperature down within the greenhouse. In fact, a powered ventilation system will typically keep a greenhouse’s temperature about 10-15 degrees cooler than if the greenhouse is passively cooled. If a fan system is to work during the dark cycle, it must be used in conjunction with a light trap.

Light Traps

A light trap, also known as a breathable wall, is a bent finned device allowing airflow while blocking all light. Essentially, the wavy fin design of light traps blocks light rays from entering the garden, but still allows for sufficient airflow between the fins, allowing horticulturists to maintain a well-ventilated environment in blacked out conditions. Light traps are a perfect fit for light deprivation greenhouses. They are typically used in conjunction with an intake or exhaust fan, or both. The light trap blocks the light but needs a motorized fan or other device to actively move the air. Light traps can be installed on one or more sides of the greenhouse. Much like an indoor garden, having both an intake and exhaust vent will create the most efficient flow of air. Both the intake port and exhaust port on a light deprivation greenhouse should be equipped with light traps. This enables the ventilation system to operate 24 hours a day, regardless of whether it is during the light or dark cycle. Without light traps, it is difficult to properly ventilate a light deprivation greenhouse and not disrupt the dark cycle.

Shade Material

The shade material, or blackout material, used in light deprivation greenhouses can influence the temperature of the greenhouse itself. A shade cloth that has a white or reflective exterior can reflect a good amount of solar energy, thus reducing the temperature when compared with an all-black or dark-colored shade material. Choosing the right shade material can significantly help with cooling.

Air Circulation

Air circulation, or air movement, within a light deprivation greenhouse, serves many purposes. Oscillating fans are the go-to device for creating air movement within an indoor garden or greenhouse. There are many benefits to proper air circulation, including uniformity of temperature, humidity, and CO2 within the environment. Plants respond well to environmental consistency and proper air circulation ensures each plant within the greenhouse receives the same atmospheric conditions. Proper air movement in a light deprivation greenhouse will only help maximize the efficiency of the ventilation system. Air movement is also how growers mimic wind in an enclosed environment. In nature, wind strengthens the cell walls of a plant’s stem and directly influences a plant’s structural integrity.

Keeping Cooler Temperatures during Hot Summer Months

Protecting the dark cycle of a light deprivation greenhouse is extremely important and keeps the crop’s flowering cycle unhindered. By implementing convection, mechanical fans, oscillating fans, and evaporative coolers during the unshaded period of the day, a grower can vastly reduce heat build-up in the greenhouse. During the shaded or dark period, growers must rely on light traps in conjunction with mechanical cooling equipment, ensuring things stay cool and that the dark cycle will not be breached.

After all, if the ability to provide the crop with a true dark cycle is compromised, it defeats the purpose of light deprivation. Light deprivation greenhouses that use light traps to protect the dark cycle will not only produce prolific yields but will also more closely replicate the sought-after results of highly controlled indoor gardens.