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The plant’s perspective on supplemental lights

With more and more growers growing through the winter months, we wondered what effect the lights have on the crops. One ‘expert’ gave us the scoop.

September 26, 2017  By Dr. Mohyuddin Mirza

October 2017 – Growing vegetable crops in winter in Canada is now an established practice in the sense that we know how many lights should be installed and how much light can be delivered to plants. Good information is available on a number of micromoles needed for leafy and fruiting crops and what is a good Daily Light Integral (DLI). With the arrival of LED lights we heard the term “top lights” and “inter-light.”

I still see crop management problems during winter months and they are primarily related to us not thinking about the plants enough when lighting is applied.

Light spectrum: In plain language, whenever I think about sunlight, I think about a young boy – that is me – sitting on a jute rug on the dusty ground under a tree and a teacher trying to explain sunlight to the class.


After being convinced that we students were not going to remember that light consists of different spectra and wavelengths, he made us memorize VIBGOYR. It was a fancy word not found in the dictionary and we memorized it quickly. Then we memorized V for violet, I for indigo, B for blue, G for green, O for orange, Y for yellow and R for red. So that is it. This VIBGOYR is stuck with me.

Then the teacher asked us why we were sitting under this tree. Our answer was that the tree provides shade and shelter from the sun. Now I understand better the role of green chlorophyll in plants.

Here are the corresponding wavelengths for the colours: See colour chart

From a plant viewpoint, blue and red spectra are the two most important for photosynthesis and some other functions. The contribution of other spectra like green, UVa and UVb are slowly being investigated.

Here is an example of how plants react to blue light. (Blue Light 400-500 nm)

  • Better penetration in leaf tissue.
  • Stomatal regulation.
  • Provide shorter internodes.
  • Thicker and darker leaves.
  • Increased root mass.
  • Flower induction.

Red and far-red also have important functions. The bottom line is when growers use lights they should know the spectrum, the intensity in micromoles, and the DLI. The most important fact is that in a greenhouse the light is called “Supplemental.” And this is supplemental to natural light available.

Growers should know about joules as well. Most greenhouse computers will give readings in joules and watts. Joules trigger irrigation, while watts are used for turning the lights on and off and for screens.

The plant’s perspective when to turn the lights on: Once the supplemental lights are installed then growers have other perspectives in mind. Power is expensive and now with added costs like higher minimum wages, overtime payments and increased costs of labour and inputs, growers want to use lights when it costs less.

For example, growers prefer to turn lights on around 1 a.m. when the power rate is the lowest.

Here is the problem with this approach and this is not a “small” problem. Plants get totally confused and ask the question, what are you doing?

Lights are turned on at 1 a.m. and Vapor Pressure Deficit (VPD) is very low. VPD is measured in millibars or grams of moisture per cubic metre of air. The VPD generally is around less than one gram, which means it is very humid and the plant is not transpiring. Screens are open to save heat.

What we are doing by turning the lights on is sending a signal to plants to open their stomata and start “working.” Yet the plant is saying, “OK, you are not doing me any favours. I cannot start working because I cannot transpire. And if I cannot transpire, and you start irrigating my roots there will be more trouble for me and for you. For me it is pain in every cell. My roots are telling me that they have to get rid of water at 1 a.m. and the leaves are saying, sorry my stomata are not open!

“My entire world goes haywire because my cells started bursting with the root pressure created by not understanding my physiology. I am designed to take up water from roots loaded with all the nutrients and then I retain them in my cells and let the water go into the air.

“However, the VPD was very low at the time you decided to turn the lights on because the price of electricity was cheaper. Once my cells are burst, then my chronic enemy, the dreaded powdery mildew, is waiting there to come and those good foods I made for a different purpose are now used by my enemy.

“Even the fruit I made for you is no good because it started turning yellow and consumers don’t like it. My leaves can become loaded with powdery mildew and Cucumber Green Mottle Mosaic Virus (CGMMV) can attack me. In case if you want to know the name of this disease of cell bursting, it is called edema. This is the same thing when your feet are swollen and kidneys are not removing water from your body. This is how my tomato leaves looked like with edema in winter (photo at right).

“So, I would humbly request you don’t turn the lights on until the VPD is at least three grams/m3 of air. I know your arguments as well. Oh, it is very cold, you say, and you have to keep my vents closed and use the energy screen. You can always start giving me light in different ways as well. You can start giving me supplemental light an hour before sunset and continue for few hours or start before sunrise.”

Winter is coming and by early
October growers start using supplemental lights. Please pay attention to the needs of the plants you are growing. Read the plants carefully and make adjustments accordingly.

Dr. Mohyuddin Mirza is an industry consultant,

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