What is too hot for cannabis plants? A very good question indeed. One of those questions that has that bothersome little answer of: “That depends.” Recently in an email, I was told that when I say that tops of plants shouldn’t exceed 86 deg. f. for more than a few hours, I am a crazy-man! LoL—duh—but that’s beside the point. They point out their outdoor plants grew in 111 deg. f. outdoors and did killer-good. I have heard this hundreds of times, heh heh.
In today’s article I want to clarify your basic temperature guidelines for growing cannabis. Every garden is different. There are many styles of growing. Whether or not your plants are in the ground or in containers matters HUMONGOUS! However, many lessor dynamics come into play in various environments. I’m just going to run these down and explain why it is the way it is.
Buckle up, it’s bound to get deep up in here, underground deep baybee. That’s our first example up next. Why plants in the ground are completely different than container plants when it comes to ambient temperatures; including humidity, and air movement/circulation as other relevant factors. Let’s go—wheeeeeeeeee…
Too Hot for Cannabis Plants Outdoors in the Ground vs. Indoors
A very good way to take your otherwise top-shelf cannabis down a big notch in quality, is to let your plants get too hot for too long during flowering. Outdoors in the ground, even if it is above 100 deg. out, your plants have like half of their total biomass underground. A lot of it deep underground. Do you know how cool the soil is like 6 inches down during a 100-deg. day? About low to mid 60’s. Several inches deeper, and it gets even cooler, like 55 degrees. So, via the plants circulation it can send all that cool stuff upwards into its above-ground self.
Let’s also not forget about water availability. Plants in the ground have deep water-roots. These roots are way down where the soil is moist most of the time. This gives the plant a constant slower tap to a water source that lasts a long time after the last rainfall even.
82 – 86 deg. at the tops is optimal for container plants, because their rootballs are only about 5 to 10 degrees cooler than the ambient temps. Not to mention, when a container dries up in high heat, it dries up fast. There is a very small grace period involved. Your window can last like literally 2 hours from them being fairly dry, to bone dry. Bone dry stresses plants HARD in containers. Some genetics deal with it better than others.
Too Hot for Cannabis Plants, Regarding Other Considerations
Humidity, air exchange and air movement all come into play. What we are talking about here essentially, is the plants’ metabolism. Specifically, how jacked-up it is in high heat. Add super low humidity, a lot of air extraction, decent “wind” from fans, and you have a situation that can get uber-shitty. During flowering if your lights are also a little too close during times of jacked-way-up metabolism, you can super-bone yourself. You may not realize some of the worst damage. To the resin profile. If all that stuff happens during flowering, your resin gets boned.
Plants that suffer from extreme heat during flowering, in containers, have their resin properties all mushed up into the “Tar Ball” type; as I call it. There are no facets to this high type, it is just a plow-you-down, make you super hungry, and very sleepy type. The flavors and smells get all mushed together as well. There is no real background to the exhale, just tar baybee.
Don’t get me wrong. Plants can take a lot of heat. However, if you have your plant tops running at something above 90 for a couple or a few days with lights too close, during the 2nd half of flowering, that damage to the resin will have begun. Back your lights off to like 2 feet in times of high heat. This is an essential move! Do a grow next time keeping close watch on the temps, and you’ll see what a difference it really makes.
Plants are Peeps Too
“Okey-dokey Rev,” LoL… Not peeps, but plants do have some very relatable issues with heat. High heat to you and I will bring on high stress. Same goes for your plants. Plants dehydrate just like peeps, only way-way-way faster when grown in containers. Dehydration puts you into uber-high-stress survival mode, same for plants. The huge problem here, unlike peeps, plants can have very bad results (for you) if they stress really hard during flowering. Hermaphrodites, re-vegging, and growth mutations. That’s all on top of the stuff above regarding the resin properties. Heat stress is a huge bummer! 😉
What you have is a fundamental problem of supply and demand. In the ground when it’s 111 deg. f. outside, in full sunshine, the roots can circulate cooler water to the tops from the roots. Deep roots in the ground also have constant access to some level of water/moisture. Plant doesn’t stress out bad at all. Deeper roots are uber-well insulated from above ground extremes in either direction.
Indoors, the roots and the plant aren’t that much different in ambient temps. So, there’s little help coming from there. The tops of the plants at high temps (above 89 deg. f. for a few hours) need more water and nutrients than the roots can possibly supply. High AF stress levels happen. Tops are hyperactive; metabolically speaking. Due to ambient conditions, exacerbated by light distance, and contained roots that are not insulated.
Rev’s Tips if it’s Too Hot for Cannabis Plants in Your Garden
- Raising your lights, as I mentioned above helps a lot. If you are lucky enough to have dimmable lights, that’s the ticket right there. Problem solved!
- A standalone portable A/C unit can work great. However, they are spendy, and they eat a decent chunk of power/amps. Never aim the cold air coming out anywhere near actual plants or Powdery Mildew will happen.
- Humidity additions, something as simple as spraying the walls of your tents with a bit of mist, or using a swamp-cooler. Be very-very careful with either one. Watch humidity closely. Many molds LOVE hot & wet conditions! But, it will decently cool things down.
- Micropond style and self-watering containers are good options to use here. Self-watering pots allow you to just add a little water in the catch-tray during high heat. Micropond style offers similar, as you can just give the plant some water in the tray to use during hella heat.
- Bluetooth thermometers are the bombdillio for keeping tabs on things. A laser thermometer can keep you exactly informed regarding temps right at the plant tops, where it matters huge.
- Hydroponically grown plants can take very high heat (well above 100 deg. f.) due to their constant access to water.
In today’s article I have come at the situation via several approaches, all leading to the same place hopefully. For those of you that just didn’t get that whole containers vs in the ground thing, now ya do—yay you! I encourage you to try out things for yourselves and see what you think. My methodologies are time tested, but they won’t work for absolutely anyone, tons of variables—but almost anyone, heh heh. The horrible changes to the resin profile, per effects, is tragic once you recognize it. You forever will recognize it from then on, LoL.
I only recently discovered this stuff. For the last couple years or so I have endeavored to keep my plant tops from ever going critical hot. Critical hot may be less hot than you perceive it to be, just sayin’. Keeping them right at or below 86 deg. f. is not too hot for cannabis plants; in containers. Keep that rule and pull off a harvest of some variety you know and love—you’ll SEE what I mean baybee!
Check out some more Rev stuff here if ya wanna: MILF Rev’s Rave: Chunky Cherry Thai. Stop by over at Kingdom Organic Seeds and gaze at some beans baybee. I’m skipping out of this dimension briefly, but I’ll be back next Tuesday, with another thing, you can peruse if you like, heh heh. L8r G8rs…
- REv 😊
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I'm The Rev, and I have been with SKUNK for about a decade now. I hail from Southern California, spent mucho time in Northern California, and now reside in Southern Oregon; always coastal. I am an all natural style cannabis grower and I have written a couple books on the subject - check out True Living Organics 2nd Edition on Amazon - I have been growing for over 45 years, and I have been breeding cannabis for over 30 years. Check out kingdomorganicseeds.com to see some exotic selections. Growing connoisseur cannabis is what I teach mostly, growing it in living soil without using liquid organic nutrients to feed the plant. I am also a highly skilled synthetics grower, hydroponics, aeroponics, DWC/SWC/NFT, Ebb and Flow, and soilless, but I cringe when smoking synthetic grown herbs, so for the last 15 years or so I preach the artisan style of all natural growing, specializing in container growing. Cheers and welcome aboard.