We are diving deep today into understanding temperatures growing cannabis indoors. It is important that you understand what temperature variations do regarding cannabis growth. Things like nights too warm and humid, can cause serious stretching. Nights too warm and dry during flowering, and many varieties keep trying to make new flowers well past their harvest date. This can result in Hermaphrodites (“bananas”).
Cold temperatures in your grow rooms, too cold, and you will see issues with phosphorous (P), and potassium (K) uptake. 57 degrees is really my global floor temperature (nighttime) as a rule of thumb. However, many varieties of cannabis will turn wicked purple if their nighttime temps are down around 55 for their last week of flowering.
If your plants/garden(s) get really cold at night, it is super important that you bring them up to high temperatures (mid to high 80’s) over the course of a few hours, and not all at once. This is where you can see P, and K issues appear. When the containers are cold, they need some time to warm up. Alrightythen, let’s do this…
Growing Temperatures for Sprouts and Vegging Plants
I like my sprouts hot. I routinely germinate at ambient temps around 88-92 degrees. I find this very efficient, however, at these high growing temperatures you must ensure your little sprouts don’t dry out. Intense lighting, 250w or greater, is critical for high temps here. Once they are above ground for 2 weeks or so, I place them in a vegging zone. Vegging plants I keep in two different zones. I have a hot-zone, my Jarden tents with Eye Blue 250-watt metal halide lamps. Plants that are in Jardin tents are heading for the flowering room soon, or younger seedling plants. Temps are mid to high 80’s in this tent when lights are on.
I also have a cool-zone. I run T5s (daylight/blue lights) in this zone. Mother and father clones hang out here. Slowed down metabolism saves me transplanting work. My hot-zone Jardin tent runs normal growing room temps, mid to high 80’s daytime. My cool-zone runs more like high 60’s to mid-70’s daytime temps. Vegging plants can take temperature fluctuations without hardly a care. Maybe a slight K, issue for a minute. But flowering plants are different, and react “big” to small environmental changes, like temps. Read on.
Rev’s Tips – Light Distance, Humidity, & Rooting Clones
- Light distance is huge. It is also temperature related, because you are “nuking/cooking” your plant tops with too much intensity. This causes hyper-fast metabolism at the tops, and the plants cannot keep up with the needs. Keep high intensity growing lights at least 18” from plant tops.
- In high temps, too much humidity will bone you. Temperature and humidity work together and can have serious combination effects. Likewise, temps too cool and high humidity will bone you. Target your humidity at 50%, and as high as 65% is good during the night. 30% humidity is getting severely dry. Plants in super low humidity will go through more water and need good sources for K, and Ca.
- I like to root my clones in aero-cloners. My standard growing temperature range for rooting clones is 75 daytime, and 57 as my bottom low limit, before heaters kick in. You don’t want it too warm when rooting clones. I run aquarium heaters in my cloners, keeping the water above 68 degrees, always.
Keep in mind that things like super high or super low growing temperatures can cause damage that won’t show itself for at least a week or ten days. It’s a good idea to know how to control your temperatures with things like, air exchange, and light distancing. Thermostatic (digital) switches for heaters are a lifesaver to keep nighttime temps from going too low. Just make sure to set things up so no random lights from the thermostatic switch, or heater, can light-poison flowering plants. Space heater light will for sure do it. I run in tents so it’s easy; I temperature control the rooms the tents are in.
Growing Temperatures for Flowering and Preflowering
I run my flowering plants at mid to high 80’s when lights are on. Growing temperatures for flowering, are like my hot-vegging zone I mentioned above. However, there are two very cool tweaks you can do, that will raise the excellence factor on the quality of your finished flowers.
Two weeks before starting to flower, drop your photoperiod a couple of hours. So, say you ran your vegging photoperiod at 18/6 (I do), you would just change it to 16/8 for two weeks prior to starting to flower. Your growing temperatures should match what your flowering temperatures run at. I do this super easy by just putting my plants in my flowering tents two weeks early, and using a 16/8 photoperiod—boom! Done dealio.
Now, here’s another cool tweak. During the last two weeks before harvest date, drop your “daylight” growing temperature by 2 or 3 degrees. Also, drop the “nighttime” temps by a couple degrees, if possible. Your “daytime” temperatures matter more. You only need a 2-degree consistent change. You don’t have to be exact here, and some temperature fluctuations are not a problem. Just be diligent and consistent at doing this the best you can for the final two weeks.
Let me tell you about those tweaks I spoke of above. “Why Rev, why do I want to do these things?” I’m so glad you asked baybee! Regarding the preflowering tweak… This move ups everything you love. Yields and resin production. In nature plants are aware and can prepare for flowering, as temps and daylight hours drop. This move allows the plants to have a two-week adjustment period to go into flowering at full power and ready to rock. Otherwise, they spend the first two weeks adapting to flowering; ya follow?
The last one-week temperature change really has a greater impact than you likely think. Flavors, and a total finish is what you gain. I actually do this for the last 10 days, myself, but a week works well. This again signals the plant that it is time to finish, Winter is Coming!
Okay my esteemed homeskillets. One last thing here. Geographic origins will matter here, so get a grip on your genetics/lineage. As an example, an African Malawi will handle drought, heat, and low humidity, much better than an African Congo would. A Congo, however, would handle lower pH, higher humidity/higher heat, and even some overwatering. Congo cannabis, from the jungle. Malawi cannabis, from temperate African arid regions. Yes? You see? 😉
Growing Temperature Observations
If your growing zone(s) experience temperatures that are kind of extreme for short periods, keep an eye on your plants for 10 days afterwards. Notice changes to morphology. You can get the hang of this so well, you can manipulate your plant morphology as you see fit. It can be awesome for limited space, accelerating/slowing timelines, and other subtle goals.
For some sexually healthy, organically grown and bred cannabis seeds, check out KOS! I recommend checking out the Chunky Cherry Thai—wowzerz! Here’s another article by yours truly, if you’d like to read another: Letters to Rev – Cannabis Seed Germination Problems – Skunk Magazine. Okay everyone, until next week, Rev out. L8r G8rs.
- REvski 😊
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.