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Light Deprivation: How to Implement It and Why You Should

August 25, 2022 5 min read 0 Comments

WHAT IS LIGHT DEPRIVATION?

Light deprivation is a useful growing technique often implemented to encourage or discourage flowering in plants outside their regular seasonal schedule. Using blackout tarps to simulate the seasons' changing, growers can take advantage of up to three harvests: early summer, late summer, and winter.

Growers accomplish this by controlling how much and how often plants receive light. Plants sense the amount of day and night hours using their circadian clock to determine whether they should be working on growing foliage, producing seed, or heading into dormancy.

HOW DO PLANTS KNOW WHEN IT'S TIME TO FLOWER?

Plants collect information from their environment, actually right from their leaves. They are sensitive to light, temperature, and length of daylight (the photoperiod). Plants are categorized by how they react to changes in the photoperiod. These plant reactions to the photoperiod are Short-Day plants, Long-Day plants, and Day-Neutral.

Short-day plants require long nights to flower. An easier way to think of this would be in terms of harvest time. Short-day plants like to release their flowers or fruit in the fall when the days start to get shorter, and the nights are longer. They will not flower if they receive more than 12 hours of light in a photoperiod. This category of plants can be manipulated into flowering earlier using light deprivation techniques.

Here are a few examples of short-day plants:

Chrysanthemum
Rice
Hemp
Soybeans
Hops
Onion
Violet
June-bearing strawberry varieties

2. Long-day plants require short nights to flower. They need more than twelve hours of light a day to flower. Think of summer flowers and garden vegetables. These plants are often tricked with supplemental lighting to extend their daylight hours to encourage an early bloom. They can also be tricked into a continuous vegetative state using blackout tarps to reduce day length.

Here are a few examples of long-day plants:

Aster
Hibiscus
Coneflower
Lettuce
Spinach
Radish
Sugar beets
Potato
Carrots
Everbearing strawberry varieties

3. Day-Neutral plants are not affected by the number of night or day hours. They flower regardless of the day length. They depend more on environmental conditions, plant developments and sometimes vernalization. Vernalization is a brief cold period given to germinating seeds that promotes flowering. Many tree varieties require vernalization before sprouting. Light deprivation techniques are not useful with these types of plants, but these plants do require at least ten hours daylight for an extended time to flower.

Here are a few examples of day-neutrals:

Cucumbers
Tomatoes
Corn
Peas
Day-neutral strawberry varieties (fruit all year as long as temperatures stay between 40-90* F)

HOW DO PLANTS MEASURE PHOTOPERIODISM?

Plants sense light using photosensor cells (phytochromes) in their leaves. These cells detect pigment, and when exposed to sunlight, they send proteins that signal hormonal changes in the plant. Plants measure these photoperiods using the phytochromes in their leaves. Next, they cue their seasonal changes such as flowering, budding, and tubering with this information. Light deprivation techniques manipulate this cue, giving the grower the ability to control the harvest time frame.

WHAT ARE THE BENEFITS OF LIGHT DEPRIVATION?

Using this technique could give you quite the edge as a grower. Being able to manipulate harvest times according to demand or ease of harvest is quite the advantage. The most considerable advantage is the opportunity earlier flowering presents for an extra harvest.

HOW DO LIGHT DEPRIVATION TECHNIQUES FORCE FLOWERING ON SHORT-DAY PLANTS?

Rather than producing more foliage and branches, asexual reproduction, plants rush to make flowers, a form of sexual reproduction, because their circadian clock believes it is later in the season. The reduced day length from the typical 18 hour day length of the spring signals to the plant that it is time to spread seed for survival. Plants also begin to release all of those starches they use when growing for foliage, along with hormones that signal the plant to start to form flowers and fruit as a form of self-preservation. Energy and sugars are put into the fruit and flowers to attract pollinators to fertilize and for animals to help spread the seed.

HOW TO MAXIMIZE YOUR HARVEST OF SHORT-DAY PLANTS WITH LIGHT DEPRIVATION

Light Dep techniques can accelerate flowering in short-day plants. An excellent example of this is with hops. These techniques enable the grower to harvest as early as July, three months before its natural harvest time in October. Not only would you have the product in the offseason, but there would also be an opportunity for a second harvest in the fall.

Light Dep. How to:

Start seedling indoors under a grow light.
Use a light deprivation tarp to shield plants from light 12 hours, while allowing light for 12 hours. This schedule mimics the Fall Equinox, which would typically hit around the end of September. Your plants will switch into flowering mode because it will think it is much later in the season. This reaction ensures that all plants start flowering as soon as you put them outdoors. Continue this technique until harvest.
Start another batch of starts as soon as you bring your flowering plants outside. These will be your second harvest.
Harvest your first batch, replace it with your second set of starts. You will not need to do the light deprivation techniques on the second batch as the days will already be shorter when they head outdoors.

HOW ARE LIGHT DEPRIVATION TECHNIQUES USED WITH LONG DAY PLANTS?

Some growers can extend their root crop and lettuce season with the use of blackout tarps. Restricting the number of daily hours these plants receive to less than 12 daylight hours keeps crops such as spinach, lettuce, and root crops from bolting.

Not only that, but they will continue to put this energy into their foliage and roots. This forced vegetative state brought on by the grower enables the growth of fall crops earlier in the season when daylight hours exceed this critical threshold.

WHAT ARE THE CONS TO LIGHT DEPRIVATION TECHNIQUES?

These techniques, when properly executed, can mean extra profits for the grower. These techniques can also lead to a total loss of crops if not managed correctly. Any inconsistencies with coverage can shock your plants and leave them vulnerable to pests and diseases. So consistency is vital for this to be successful.

For this reason, in a large greenhouse setting, many rely on an automated system and timers to prevent this type of failure. This extra cost is another downside to light deprivation techniques.

Is Light Dep Better than Outdoor Growing?

There is no stronger source of light available than the sun. Growing short-day plants outdoors will often result in large, bushy plants. Plants receive large amounts of light during their growing phase, resulting in excessive foliage.

Light deprivation techniques signal the plants to use that energy for sexual reproduction (flowering) instead of cell division. The advantage you get from this is that plants can utilize the stronger sun rays of the spring/early summer for flower formation instead of foliage growth.

HOW TO INSTALL AND USE A LIGHT DEPRIVATION TARP WITH A HOOP HOUSE?

Light deprivation tarps can easily be installed on hoop houses using shade-cloth clips and ropes. This technique requires the grower to manually remove the tarp to allow for sunlight during light periods. Look for a blackout tarp that is resistant to mold, contains UV additives, and is at least 6 millimeters in thickness for best results.

As stated previously, it is essential that timing stays consistent or more damage than good will come from this method.

RESOURCES:

Poppe, Steve and Esther Jordan. 2014. "Taste of sweet success: Strawberry season extended using low-tunnel production." Organic Broadcaster, March/April 2014. Midwest Organic & Sustainable Education Service (MOSES). https://mosesorganic.org/farming/farming-topics/market/strawberry-season-extended/

Provencio, I. (2010). Shedding light on photoperiodism. Proceedings of the National Academy of Sciences of the United States of America, 107(36), 15662-15663. Retrieved September 13, 2020, from http://www.jstor.org/stable/27862310

Yano, M., Kojima, S., Takahashi, Y., Lin, H., & Sasaki, T. (2001). Genetic Control of Flowering Time in Rice, a Short-Day Plant. Plant Physiology, 127(4), 1425-1429. Retrieved September 13, 2020, from http://www.jstor.org/stable/4280209

Rhythms of Life: The Plant Circadian Clock. (2016). The Plant Cell, 28(4). Retrieved September 13, 2020, from https://www.jstor.org/stable/plantcell.28.4.992

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