8 minute read
Heat - The Basics
Sure, we can attempt to manifest our concept of “heat” into something objective and physical, such as vibrating atoms and molecules, but, as growers, we would do much better to focus on the primary phenomena that serve to raise temperatures in our grow rooms
Bruce: “How did it feel to you?”
Student: “Let me think….”
Bruce: (Face-slaps student) “Don’t think! Feeeeel! It is like a finger pointing the way to the moon…” (Slaps student again.) “Don’t concentrate on the finger, or you will miss all the heavenly glory!”
Okay—forgive me if I sound like a wannabe botanical kung fu master snatching that opening gambit from Enter the Dragon, but Bruce’s finger is not entirely unlike the heat in your grow room. If you want to gain an understanding, you first need to forget all about it. But wait, what?
(Imagine I just playfully slapped you.)
Don’t look at the finger!
To put it another way, we need to stop thinking about “heat” as if it were a “thing”. It’s not. Allow me to repeat: “heat” does not exist. What we feel and describe as “heat” is a side effect of several other things, and it’s these primary “heat creators” that we need to get our heads around. Sure, we can attempt to manifest our concept of “heat” into something objective and physical, such as vibrating atoms and molecules, but, as growers, we would do much better to focus on the primary phenomena that serve to raise temperatures in our grow rooms.
Radiant Heat
Decades past, one of my lighting mentors—a delightfully crazy genius named Gerald Garrison—would yell at me: “Everest! The sun is not a heater!” (I’m grateful that he did not accompany each of his lesson points with a face slap.)
This salient point could be expanded like this: the sun is 93 million miles away, deep in the vacuum of space. The “heat” that we experience when the sun’s rays reach the nerves on our skin is a product of this radiation. Photons radiate out from the sun—tiny quantum packets of energy—and, as Earthlings, we’re primarily concerned with ultraviolet, PAR, and infrared photons. This energy travels through space at the speed of light, and, when it hits our faces, we absorb some of that energy. Nerves sense this, and our brains declare, “HEAT!”
Grow lights vary considerably in the type of radiation they emit. It’s interesting to note that many LED grow light companies used to tout the lack of infrared in their products as a selling point. More recently, some LED manufacturers are incorporating infrared diodes into their lighting arrays. After all, incident solar radiation is made up of nearly half infrared, so it makes sense that plants have gotten accustomed to it.
Moreover, radiant heat increases the internal temperature of your plants, and this sets the tempo for all sorts of metabolic processes. However, radiation is just one heat-generating phenomenon.
Convection Heat
Convection heat differs from radiant heat in that it requires gas or liquid as the transport mechanism. Convection heat, by definition, cannot travel through a vacuum, unlike radiant heat. In our grow rooms, air molecules bounce off one another and, in so doing, they transfer energy. We also use oscillating fans to force the bulk flow of air to homogenize temperatures in our grow rooms—and there’s also free convection, which occurs naturally in response to differences in air density. Our grow lights produce lots of convection heat, and reflector design can play a significant role in how this convection heat is distributed or not.
We can feel convection heat when we hold our hands above the heat syncs in LED grow lights, or an electronic ballast, or an HPS lamp. Sure, hot objects will emit radiant heat too—but when my old lighting mentor Gerald declared “the sun is not a heater!”, what he meant was “the sun is not a convection heater!”.
Conductive Heat
Conductive heat is the transfer of heat within a material— or between materials that are touching. Think of a copper pan sitting on an electric stove. Molecules bump into each other and transfer some of their energy to others nearby. An example from our grow rooms: plant pots transfer conductive heat to the growing media and roots within them.
Now, while the sun admittedly generates its energy by nuclear fusion, I don’t want to discuss this particular form of heat, as it’s not relevant to our grow rooms. Suffice to say, radiant, convection, and conduction are enough to consider. The internal temperature of plants is where it’s at; think of it as the aggregate effect of different types of heat generation. So, if our grow lights lack infrared, we may well need to dial-up another kind of heat to compensate— that’s why some LED aficionados advocate for higher air temperatures in their grow tents and grow rooms. It also explains why plants sometimes seem to do better under LED grow lights in “side by side” trials (where HPS and LEDs are in the same room) than when just LED grow lights are used in the same space. In the side by side trial, HPS lights could be doing a great job of heating the air (and thus providing convection heat to compensate for the lack of IR). In contrast, LED grow lights used solo, especially by growers in colder climes, fail to produce the goods without heaters.
Another “quick fix” for lack of infrared/radiant heat, especially in the vegetative cycle, is to add calmag. The low transpiration rates associated with cooler internal plant temperatures lead to less calcium being up-taken. Of course, too much calcium in the nutrient solution causes its own issues, such as competing with phosphorus, which is why growers who rely on this band-aid often become unstuck during the flowering phase when phosphorus is in higher demand. Similarly, merely dialing down your extraction fans to keep some of the heat in a grow tent, for example, might work okay in veg when plants are small and CO 2 needs low, but you’ll come unstuck in the flowering stage.
It’s important to remember that “heat” is neither good or bad— entirely appropriate for something that doesn’t exist! It’s all about finding the right balance. So, just because a grow light produces less heat (in whatever form), it doesn’t automatically make it better. Sure, it’s a beautiful thing if more electricity is being converted into potentially plant usable photons. Still, if we also need to plug in a 750-watt heater to raise air temperatures sufficiently to generate an internal plant temperature of 25°C (77°F), surely this energy also needs to be factored in. Of course, you won’t find it mentioned in any LED grow light’s system efficiency claims.
I’ll finish with some practical advice. We all know that summertime can be a particularly challenging period for us indoor growers, especially those of us growing heat-sensitive species. This is a great time to consider LED lighting, or maybe a mini-split air conditioner. If big investments are financially out of reach, try increasing the humidity in your room to lower the transpiration rates. Dim or raise grow lights to reduce radiant intensity. Run them at night—obviously. Lower nutrient concentration to offset the higher transpiration rates. Consider adding sea kelp extract or silicon to your nutrient solution to help your plants deal better with heat stress and strengthen plant cells. Tomatoes don’t up-take much silicon at all, but many other plant species do. Resist any temptation to point oscillating fans directly at your plants. Yes, we humans like to sit in front of a fan to cool off on super hot days, but the air movement will only serve to exacerbate the transpiration pressure on your plants. Move the air between the lights and the canopy, and also underneath the canopy. Chill your nutrient solution to 18 – 20°C (64°F-68°F). Try more air exchange, but remember— ventilated grow rooms can only cool as much as the temperature of the input air allows. Meanwhile, over-ventilating your grow room, especially if you live in arid climates during the vegetative period, can dry your plants too much and cause high transpiration stress.
Remember, focusing on just one element in your grow room (whether it’s a heat producer or otherwise) is missing the point here. (You’re not still looking at my finger, are you?). Think about your plants and grow room as an interconnected living system—the heavenly glory. Most importantly of all, grab as much data as you can along the way and never stop learning!
By Everest Fernandez
Everest is a well-respected industry educator, veteran hydroponic grower, and grow light enthusiast based in France. He works primarily as a marketing and cultivation consultant and was the founding editor of Urban Garden Magazine in the UK, US, and Canada. He also writes and researches for the popular hobby horticulturist YouTube channel, Just4Growers.
