what's wrong w/ a hot growroom?

conventional wisdom has it that, ideally, one should keep one's growroom somewhere in the low 70's. temps over 80 are considered unacceptable, long term.

wtf? cannabis grows all over the world, often at temps way into the 90's w/ no problem...

what gives?

Cannabis can handle high temperatures, especially those varieties which have adapted to such climates. It is just that the open atmosphere of the outdoors, where the air is constantly flowing in all directions and bringing more carbon dioxide to the plants makes such temperatures bearable. Inside of a grow room not only do you have heat building up, but stagnant air that is depleted of carbon dioxide.

Such factors hinder photosynthesis, as the plants' stomata close up and gas exchange just isn't happening. Constant temps in the upper 80's or above will lead to stretching, airy buds, decreased yield. You can either vent and/or cool your room properly, or you can supplement carbon dioxide.

Great post Nullis, had this very problem, here's a real-world example:

My grow room is situated in a basement with little ventilation, temps average in the low 90's. Working to find a solution to the heat issue however in the interim supplementing the CO2 allows the plants to tolerate the higher temps w/o displaying symptoms of heat stress as Nullis described (watched it disappear like magic). It's not a matter of WILL it grow but attempting to create an optimal environment to maximize yield. As an additional benefit the plants grow at a faster rate with reduced inter-node spacing (bushy), my scrog has over 20 bud sites per square foot of canopy. ANY increase in the CO2 concentration helps, the goal being 1500 - 2000 ppm as "optimal" for the plant (air normally averages ~300ppm). There are all sorts of devices to test CO2 levels or you can save a few bucks and do a little simple math to determine how much CO2 to deliver. In my case it worked out to about .03 cubic feet, due to the heat it was not practical to shut off the vent fans during CO2 injection so a little more math is needed to calculate how long it takes to exchange the room air volume and ensure you can inject the appropriate volume of CO2 in an equivalent period of time. Don't know if that helps any and it's certainly possible that I'm full of crap but my results have been tremendous.

"What we obtain too cheap, we esteem too lightly; it is dearness only that gives every thing its value. " Thomas Paine, 1776