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plantsjacques

Optimizing Your LED Spectrum for Leaf Surface Temperature

By Andrew Myers
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Every detail counts at an indoor grow facility. Indoor growers have complete control over nearly every aspect of their crop, ranging from light intensity to air circulation. Among the most important factors to regulate is temperature. While ambient air temperature is critical, growers will also want to measure leaf surface temperature (LST).

To illustrate, let’s say you keep your living room at a cozy 76 degrees. Then, if you place a thermometer under your tongue – your body is (hopefully) not at 76 degrees but is likely between a healthy temperature of 97 to 99 degrees.

A similar story can be told for cannabis plants grown indoors. A grow facility’s ambient air is often different than the plants’ LST. Finding an ideal LST for plant growth can be complex, but modern technology, including spectrally tunable LED grow lights, can simplify monitoring and maintaining this critical aspect.

Why Should Growers Care About LST?

Temperature plays a pivotal role in plant health. Many biochemical reactions contributing to growth and survival only occur within an ideal temperature range. If temperatures dip or spike dramatically, growers may witness inhibited growth, plant stress or irreversible damage to their crops.

The leaf is among the most important plant structures as it’s where most metabolic processes happen. Therefore, finding an optimum LST can improve growth rate and the production of metabolites such as pigments, terpenes, resins and vitamins.

Because many plants rely on their leaves for survival, it makes sense that leaves have their own temperature regulation system. Evaporation through pores in the leaf – known as stomata – can cool the plant through a process called transpiration. Up to 90% of water absorbed is used for transpiration, while 10% is used for growth.

The efficacy of transpiration is determined by the vapor pressure deficit (VPD), which refers to the relative humidity in the ambient air compared to the relative humidity in the leaf. If relative humidity is low, the VPD can be too high, which may cause plants to have withered, leathery leaves and stunted growth. On the other hand, a low VPD correlates to high relative humidity, and can quickly result in disease and mineral deficiencies. Higher humidity often results in a higher LST as transpiration may not be as effective.

When it comes to LST, growers should follow these basic guidelines:

  • Most cannabis plants’ LST should fall between 72 and 86 degrees – generally warmer than the ambient air.
  • LST varies depending on individual cultivar. For example, plants that have evolved in colder climates can generally tolerate cooler temperatures. The same can be said for those evolved in equatorial or temperate climates.
  • CO2 availability also plays a role in LST; CO2 generally raises the target temperature for photosynthesis.

How Does Light Spectrum Affect LST?

We know that CO2 concentration, specific genetic markers and ambient temperature all play an important role in moderating LST. But another important factor at an indoor grow is light spectrum – especially for those using spectrally tunable LEDs. Growers will want to optimize their light spectrum to provide their crop with ideal conditions.

A combination of red and blue wavelengths is shown to have the greatest impact on photosynthesis and, thus, LST. Photons found along the green and yellow wavelengths may not be absorbed as efficiently and instead create heat.

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Indoor cultivator facilities often use high powered lights that can give off heat

Optimized light spectrums – those with an appropriate balance between red and blue light – create more chemical energy instead of heat, thereby resulting in a lower LST. Using fixtures that are not spectrally tuned for plant growth, on the other hand, can waste energy and ultimately contribute to a higher LST and ambient temperature, negatively affecting plant growth. Consequently, measuring LST doesn’t only indicate ideal growing conditions but also indirectly illustrates the efficiency of your grow lights.

LED fixtures already run at a lower temperature than other lighting technologies, so indoor growers may need to raise the ambient temperature at their grow facilities to maintain ideal LST. Switching to spectrally tuned LEDs may help growers cut down on cooling and dehumidifying costs, while simultaneously improving crop health and productivity.

What’s the Best Way to Measure LST?

There are several tools available for growers to measure LST, ranging from advanced probes to specialty cameras. However, many of these tools provide a reading at a specific point, rather than the whole leaf, leading to some inaccuracies. Temperature can dramatically vary across the leaf, depending if parts are fully exposed to the light or in the shadows.

Investing in a forward-looking infrared camera (FLIR) gives indoor growers a more accurate picture of LST and light efficiency. That being said, growers should not only measure leaves at the top of the plant, but across the middle and bottom of the plant as well. That way, growers receive a complete snapshot of growing conditions and can make changes as needed.

At an indoor grow facility, it’s not enough to only measure ambient room temperature. Of course, this aspect is important, but it will paint an incomplete picture of plant health. Measuring LST gives growers nuanced insights as to how plants respond to their environment and how they can better encourage resilient, healthy growth.

Using spectrally tunable LEDs makes achieving LST easier and more cost-effective. Lights with optimized spectrums for plant growth ensure no energy is wasted – resulting in superior performance and efficiency.

Soapbox

The Stress of a Grower

By Carl Silverberg
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Tell me that you can’t relate to this story.

You’re sitting down to dinner at a restaurant about ten minutes from where you work, finally relaxing after a tough day. You’ve set your environmental alerts on your plants; you have that peace of mind that the technology promised and you know that if anything goes wrong you’ll get notified immediately. As you’re looking at the menu, you receive an alert telling you that the temperature in one of your 2,000 square foot grow rooms has gone out of the safe range. Your mind starts to race, “It’s week seven, I’ve got 500 plants one week away from harvest, that’s 200 pounds of cannabis worth about $150,000-$200,000. Oh my God, what am I going to do?”

You’re doing all this at the dinner table and even though you’re not in a state of panic, you are extremely concerned. You need to figure out what’s going on. You check the graphing and see that over the past hour your humidity dropped and your temperature is gradually going up. Within the past ten minutes, the temperature has gone to 90 degrees. Your numbers tell you that the temperature in the room with $200,000 of cannabis is going up about five degrees every three minutes.

adamJgrow
Monitoring a large grow room can be a stressful task.

“I see this trend and can’t figure it out,” the grower relates. “Normally, the HVAC kicks on and I’d begin to see a downward trend on the graphs. I pre-set my trigger for 90 degrees. But, I’m not seeing that. What I AM seeing is the temperature gradually and consistently getting warmer without the bounce-back that I would expect once the HVAC trigger was hit. All I know is I better find out what’s causing all this and I better find out fast or my entire crop is gone.”

You go through the rest of the checklist from LUNA and you see that the lights are still on. Now, you’re starting to sweat because if the temperature in that room hits 130 and stays there for more than twenty minutes, you’re losing your entire crop. You have to walk in your boss’s office the next day and explain why, after all the time and money you put in over the past seven weeks, not only is all that money gone but so is the $200,000 he is counting on to pay salaries, expenses, and bank loans.

This is something you’ve been working on for seven straight weeks and if you don’t make the right decision, really quickly, when that room hits 130 degrees here’s what happens.

“My equipment starts to fail,” our grower continues. “The crop literally burns as the oils dry up and the crop is worthless. At 130 degrees, my grow lights essentially start to melt. All you can think of is that temperature going up five degrees every three minutes and you’re ten minutes from your facility. I need to leave that restaurant right now, immediately, because even if I get there in ten minutes the temperature is going to be almost 120 degrees while I’ve been sitting here trying to figure out what’s wrong.”

You run out to your car and you speed back to the facility. The grow room is now 125 degrees, you have maybe three or four minutes left to figure things out before you flush $200,000 down the drain. The first thing you do is turn off the grow lights because that’s your primary source of heat. Then, you check your HVAC panel and you realize it malfunctioned and shorted out. There’s the problem.

The real toll is the human cost. Once this happens, no grower ever wants to leave and go home or even go to dinner. It’s a horrible toll. It’s the hidden cost we don’t talk about. The grower opens up with his own personal experience.“This system allows the grower to step back and still feel confident because you’re not leaving your facility to another person,” 

“You think about the burden on the person that you bring in to replace you while you’re out of town and then you think about the burden on you if something goes wrong again. And you decide, it’s not worth it. The anxiety, the fear that it will happen again, it’s not worth it. So, you don’t go. I didn’t even see my sister’s new baby for eight months.”

Your desire to see your family, your desire to have a normal life; all of that goes out the window because of your desire to be successful in your job. It outweighs everything.

This is every grower. It’s why many farmers never leave their property. It just becomes a normal way of living. You just repeat it so much that you don’t even think about it. Why go on vacation if your stress level is higher than it is if you’re home. You’re constantly worried about your farm or your facility. The only way to escape it is to not go away at all.

“This system allows the grower to step back and still feel confident because you’re not leaving your facility to another person,” he tells us. “You don’t realize how stressful a lifestyle you live is until you step back and look at it. Or, if you have an alert system that allows you to pull back. That’s when you realize how difficult your life is. Otherwise, it just seems normal.”

As AI technology expands its footprint into agriculture, there will be more tools to help mediate situations like this; more tools to give you a more normal life. It’s one of the reasons we got into the business in the first place.