I received an excellent question from an engineer named Kirk today about my advice on keeping root temps below 78 degrees that I wanted to respond to in a little more depth than allowed in a comment. Here is the comment:
“Hi Silas, I’m another engineer who decided to grow. I’m really working on root zone temperature using aeroponics. I noticed that you do not want roots to be any hotter than 75F. What are your thoughts on the 1944 Nelson paper on hemp with root temps of 84F that did just fine?”
Kirk is absolutely right. Root temps can go hotter than 75– who knows they might be able to run successfully at 90+ and the root system growth rates might well also continue to increase as they do with the plant — IF all the other factors can keep up. That is the big question.
In an aeroponic system, you have so much air injected into the root system, that very likely the much lower levels of air actually dissolved in the water don’t really matter. And frankly if your media has lots of air space and you let them get dryer consistently you could probably also avoid the potential issues caused by higher temps in the roots. But in this situation, as you let the roots get dryer to increase oxygen you decrease the actual nutrient and water availability to the roots so while your roots stay healthy and grow fast your plant could slow it’s growth due to diminished transpiration or even wilt if the internal pressure drops too low.
So the bottom line is 75-78 is kind of a safe bet for root temps, but not actually an optimal temp for all situations, and I have suggested it as such for simplicity sake. In Aeroponics systems — where you have significant air mechanically injected into the root system and do not rely on the oxygen absorbed in the nutrient solution– I would guess that you might see metabolic gains right up to a high temp threshold just as we see with the plant with high air temps. The only difference is instead of CO2 driving where that shutdown occurs, it would be oxygen.
I look forward to reading the study and paper by Nelson Kirk referenced and I will post any interesting details later.
I hope this clarifies this a bit for you ambitious growers, cause that is what this site is about–how to MAXIMIZE plant growth..
Thanks Kirk for the excellent question!
And to those who grow we salute you!