Garden & Greenhouse


Using Light Deprivation to Increase Harvests

Posted January 15th, 2018 by Garden & Greenhouse in , ,

Greenhouse growers employ many different techniques and products to increase the production of their gardens. From fully automated feeding systems to high tech heating and cooling equipment, greenhouse growers are always on the lookout for anything that can help achieve optimal results. Like indoor growers, greenhouse horticulturists can realize higher quality crops by having heightened control over temperature, humidity, and air quality. However, one large advantage indoor growers have over greenhouse growers, is the ability to control the photoperiod (the length of light per day). At least that was an advantage they had, until greenhouse growers started implementing a technique that allowed them similar control over the photoperiod. That technique is known as light deprivation.

Controlling the Photoperiod

Light deprivation is exactly what it sounds like: depriving the plants within the greenhouse of light. This is accomplished by using a large shade cloth that covers the entire greenhouse. Why would a greenhouse grower want less light to enter his or her greenhouse? The answer: to have control over the photoperiod. Many plant varieties are sensitive to the amount of light/dark they receive in a 24 hour period. In fact, for many plants, it is the amount of light/dark they receive that actually triggers certain responses, such as the initiation of flowering

or fruiting.

In a typical greenhouse, this natural response to the light/dark periods will happen as the seasons change and the days in the late summer get shorter. The plants respond to the reduced light and increased darkness by beginning to produce fruit or flowers. When implementing a light deprivation technique, a greenhouse gardener is able to mimic the shorter days which occur in late summer and “trick” his or her plants into flowering or fruiting earlier than they would normally. This allows the horticulturist to have ultimate control over when the plants will be harvested and to increase the amount of harvests he or she gets per year.

Black Out Material

Unlike a typical shade cloth, which only reduces the amount of light entering the greenhouse, the covering material used for light deprivation is designed to keep out as much light as possible. The shade cloth material sold for light deprivation is sometimes referred to as “black out” material or 100% shade cloth. There are different types and qualities of light deprivation shade cloth, including varieties that are “breathable” or ones that contain multiple layers. The “breathable” fabrics are typically reserved for internal light deprivation systems (where the shade cloth is within the greenhouse, not outside of the glazing material). A breathable fabric will help avoid condensation drips that may occur after the shade material is removed for the day. External black out cloth is the shade material that covers the greenhouse on the outside of the glazing material.

The temperature within the greenhouse can be influenced by the type of shade cloth material being used. The blackout material which is the most effective at reducing excess heat is one made of multiple layers of fabrics, including a white layer on top. When the greenhouse is “blacked out”, the white layer reflects the sun away from the greenhouse, helping to reduce the amount of heat that builds up within the greenhouse. The shading material used for light deprivation needs to cover the entire greenhouse structure to ensure complete darkness. Each day the blackout material is removed for the desired amount of light hours. After reaching the desired length of light hours, the shade material is manually or automatically put back on the greenhouse.

Manual or Automatic

With a manual light deprivation greenhouse, the shade cloth must be manually positioned over the greenhouse structure. As can be imagined, this can be quite difficult with large structures. Gardeners usually rely on some sort of rope and pulley system to aid in moving the potentially heavy shade material. Aside from the physical labor involved, the biggest disadvantage of a manual system is that someone must be present every day for both the covering and uncovering of the greenhouse. Forgetting just once can cause problems for the greenhouse, including out-of-whack atmospheric conditions or stunted growth due to quickly alternating photoperiods.

An automatic light deprivation greenhouse is one equipped with a motorized system for situating and removing the light deprivation material. These systems are typically equipped with a small electric motor. These motors can be wired for use with a chain of car batteries to operate in locations without access to electricity. Automatic light deprivation systems allow a gardener to automate the system’s motor with a timer. This means the horticulturist does not have to be present every time the covering is removed or put back onto the greenhouse.

Heightened Control with Light Deprivation

The biggest advantage of using a light deprivation greenhouse is the heightened control over initialing the flowering or fruiting of the crop. Before light deprivation, only indoor horticulturists could control the initiation of flowering and when to harvest. As with an indoor garden, light deprivation greenhouses are capable of producing two or more harvests per year. If a horticulturist has the ability to do the vegetative growth in another greenhouse or indoor garden, he or she could get as many as 4-5 harvests per year using light deprivation.

Geographical location can have a large influence over how many harvests a light deprivation greenhouse can produce per year and the type of environmental controls that will be necessary. In other words, light deprivation greenhouses located in warm climates will be able to produce more harvests per year than those located in colder climates.

Larger Yields

Many growers report larger overall yields when using light deprivation techniques. This could be attributed to the stronger, more direct sunlight the plants would receive when flowering or fruiting during mid-summer. If the gardener is capable of vegetating his or her plants elsewhere, he or she can potentially set up a perpetual light deprivation greenhouse which continuously has plants in the fruiting or flowering stage. For gardeners in warm climates, this could mean up to 5 harvests per year and a significant increase in overall yield.

Reduced Chance of Failure

By dividing the growing season into two or more harvests, a horticulturist can reduce the chance of a complete failure. Since he or she will have multiple harvests, a grower will not experience a complete loss if there is a catastrophe, such as a heat wave or a pest insect infestation. Put another way, if one crop is lost completely, the horticulturist will still have other chances to negate his or her losses.

Cooling a Light Deprivation Greenhouse

Light deprivation greenhouses do have some special considerations when it comes to ventilation and cooling. After all, these greenhouses are being closed up during the peak heat of the summer, opening up the potential for overheating. As previously mentioned, using a shade cloth material with a white exterior can help reduce heat buildup by reflecting the sun away from the greenhouse. Due to the fact that the greenhouse must remain dark during the shaded period, the louver vents and fan ports typically used in greenhouses should not be used. This is because they would create a light leak and compromise the darkness of the greenhouse interior.

Due to this, most light deprivation greenhouse growers utilize light traps to maintain proper air flow. A light trap, also known as a breathable wall, is a bent finned device which allows airflow while blocking all light. Light traps allow horticulturists to maintain a well-ventilated environment in blacked out conditions. Light traps are typically used in conjunction with an intake or exhaust fan. Without light traps, it is very difficult to properly ventilate a light deprivation greenhouse without disturbing the dark cycle. For many light deprivation gardeners, light traps are a vital device, bringing in fresh air to cool the garden without compromising the dark cycle.

The heightened control over initiating fruiting and flowering, combined with the idea of multiple harvests per year, are surely what grabbed the attention of greenhouse growers. Today, both commercial horticulturists and greenhouse hobbyists are implementing light deprivation techniques to reap the rewards. As with indoor horticulturists, greenhouse gardeners and farmers who use light deprivation techniques will benefit from increased environmental control, including perpetual fruiting or flowering and producing larger overall yields. With the use of a light trap and a large fan, most light deprivation greenhouse growers can maintain adequate temperatures within the greenhouse, even during the heat of the mid-summer months. As with indoor gardening, the increased control over environmental conditions provided by light deprivation techniques is a gateway to larger, more consistent yields. Light deprivation is one significant way for horticulturists to take their control of the greenhouse to the next level.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at

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