Lowest temp in grow room plants can handle

I’m just curious as to what’s the lowest temps plants can handle in a grow room. With winter upon us I’m just curious as to how low they can go? I run a HPS so when the lights on they will be good but in the dark when the lights go out and the days are chilly when do I need to supply heat and what methods have you used?

I keep my room at a minimum 72 degrees F with the lights off. I use a Rinnai propane heater with a dehumidifier to keep the humidity low. I keep the intake and exhaust fans running 24/7 circulating air from the tent into the room, with a second exhaust fan to exhaust excess heat/humidity from the tent directly outside thru and exhaust hole in the wall.

The plants can handle lower temperatures but you are risking the dreaded Powdery Mildew with temp swings greater than 10 degrees between lights on and off. This I learned the hard way.

I use terrarium heat mat under my pots to keep root zone warm when air temps drop to 16-18C at lights of time.

Driver733 said:

I keep my room at a minimum 72 degrees F with the lights off. I use a Rinnai propane heater with a dehumidifier to keep the humidity low.

The plants can handle lower temperatures but you are risking the dreaded Powdery Mildew with temp swings greater than 10 degrees between lights on and off. This I learned the hard way.

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Ive dealt with that shit. Best thing is to burn some sulfur to kill it. I also have a dehumidifier. I haven’t grown for years indoor during winter and I can’t remember how low I let them go. I do remember thinking about getting a propane heaters since they would produce carbon monoxide which quickly oxidizes into carbon dioxide. Fit I never did and don’t remember what I used or how I went about it. I have a patio where I have done numerous test runs just fucking around but the high humidity along with the cold have at times turned them into everything from furry mold to slime. Since I’m indoors and the heat from the 1000w hps will warm it up 12 hours of the night I only have to worrry about extremely cold days

Had a grow last winter where the temp fell to 11 degrees C when the lights were off. No PM but they didn't grow to full potential. Not a bad harvest, but could be better.
As far as I know, the girls stop to grow when under 14-15 degrees C.

BodegaBud said:

I’m just curious as to what’s the lowest temps plants can handle in a grow room. With winter upon us I’m just curious as to how low they can go? I run a HPS so when the lights on they will be good but in the dark when the lights go out and the days are chilly when do I need to supply heat and what methods have you used?

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Try to keep it around 65 to 68 lights off. When it goes lower the roots start going dominant especially when it dips below 55. Warmer environments keep the plants metabolism up.

I think the safest heat solution is an oil filled heater. I have used one for years now and it's great. Put in a corner, have a fan that circulates it around, done.

I use a oil filled heater with a inkbird temp controller. $30 ace hardware for the heater and $25 for the controller

Wizzlebiz said:

I use a oil filled heater with a inkbird temp controller. $30 ace hardware for the heater and $25 for the controller

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Any issues working it with an inkbird? I would like a solution that doesnt require me programming it daily.

ComfortCreator said:

Any issues working it with an inkbird? I would like a solution that doesnt require me programming it daily.

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Zero issues. Works perfectly

Yup oil filled heater for the win / provides even, gradual heat

Wizzlebiz said:

Zero issues. Works perfectly

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Thank you!
How do you have it set to trigger?

ComfortCreator said:

Thank you!
How do you have it set to trigger?

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It has a thermometer you set up in the tent. Set the desired temp to turn on and off. It does it for you at that point. Extremely straight forward. It even has ability to set different temps at different times of the day.

Wizzlebiz said:

It has a thermometer you set up in the tent. Set the desired temp to turn on and off. It does it for you at that point. Extremely straight forward. It even has ability to set different temps at different times of the day.

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I use the same set up - don’t even think about it lol just listen to Wizz and go buy the heater and Inkbird, then thank him in a couple weeks when your shit’s dialed in. The other thing I’d suggest is to pick up an Inkbird humidity controller while ur at it, same money well spent.

lakesidegrower said:

I use the same set up - don’t even think about it lol just listen to Wizz and go buy the heater and Inkbird, then thank him in a couple weeks when your shit’s dialed in. The other thing I’d suggest is to pick up an Inkbird humidity controller while ur at it, same money well spent.

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Use in conjunction with this and it becomes extremely dialed in with no effort coming from outside of programming it 2 times. Once for veg and again for flower.



"Amazon.com: AC Infinity CLOUDLINE T4, Quiet 4” Inline Duct Fan with Temperature Humidity Controller - Ventilation Exhaust Fan for Heating Cooling Booster, Grow Tents, Hydroponics : Patio, Lawn & Garden" https://www.amazon.com/AC-Infinity-CLOUDLINE-T4-Thermostat/dp/B06ZXWN3BG/ref=mp_s_a_1_2_sspa?adgrpid=61797575251&dchild=1&gclid=Cj0KCQjw5JSLBhCxARIsAHgO2Sf0LZtltpMfBo0x5KEXdp0YOoOhykzi_FxXo3JlIIypQY-yukQxeR0aAnxxEALw_wcB&hvadid=274839474234&hvdev=m&hvlocphy=9032045&hvnetw=g&hvqmt=e&hvrand=15595402413007073657&hvtargid=kwd-302097316291&hydadcr=12163_10197799&keywords=grow+tent+exhaust+fan&qid=1634059177&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFKRldJOTVFVDMwNEImZW5jcnlwdGVkSWQ9QTAwMjE1MzRIVDhLWVIzQTg0WTImZW5jcnlwdGVkQWRJZD1BMDMzMjQ1NzMyN00yRkxJSTc5Q1Imd2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

Wizzlebiz said:

Use in conjunction with this and it becomes extremely dialed in with no effort coming from outside of programming it 2 times. Once for veg and again for flower.
View attachment 5008247


"Amazon.com: AC Infinity CLOUDLINE T4, Quiet 4” Inline Duct Fan with Temperature Humidity Controller - Ventilation Exhaust Fan for Heating Cooling Booster, Grow Tents, Hydroponics : Patio, Lawn & Garden" https://www.amazon.com/AC-Infinity-CLOUDLINE-T4-Thermostat/dp/B06ZXWN3BG/ref=mp_s_a_1_2_sspa?adgrpid=61797575251&dchild=1&gclid=Cj0KCQjw5JSLBhCxARIsAHgO2Sf0LZtltpMfBo0x5KEXdp0YOoOhykzi_FxXo3JlIIypQY-yukQxeR0aAnxxEALw_wcB&hvadid=274839474234&hvdev=m&hvlocphy=9032045&hvnetw=g&hvqmt=e&hvrand=15595402413007073657&hvtargid=kwd-302097316291&hydadcr=12163_10197799&keywords=grow+tent+exhaust+fan&qid=1634059177&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFKRldJOTVFVDMwNEImZW5jcnlwdGVkSWQ9QTAwMjE1MzRIVDhLWVIzQTg0WTImZW5jcnlwdGVkQWRJZD1BMDMzMjQ1NzMyN00yRkxJSTc5Q1Imd2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

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Lol this is my exact set up - inkbirds (temp/humidity/CO2) with T6 on a Controller 67.

If u pick up a T series fan then I’d strongly encourage picking up the Controller 67 - supports Bluetooth and the app that runs it is slick - you get way more control and a nice historical data display

I got down to about 55f without issues, lower than that and things got wonky and i did as others have said and busted out the oil filled heater

I’m super paranoid about starting a fire so I’m looking for the least risky way to keep em warm. I’m hoping that the lights will warm it up enough where they are ok when the lights go out. I always set the 12/12 with lights to come on at night. The reason being is during the summer it’s too hot to run during the day and in the winter it will be colder at night so they will need warmth. It’s insulated where I have them so I’m hoping it works without adding additional heat.

BodegaBud said:

I’m super paranoid about starting a fire so I’m looking for the least risky way to keep em warm. I’m hoping that the lights will warm it up enough where they are ok when the lights go out. I always set the 12/12 with lights to come on at night. The reason being is during the summer it’s too hot to run during the day and in the winter it will be colder at night so they will need warmth. It’s insulated where I have them so I’m hoping it works without adding additional heat.

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What kinda temps are you talking?

lakesidegrower said:

Lol this is my exact set up - inkbirds (temp/humidity/CO2) with T6 on a Controller 67.

If u pick up a T series fan then I’d strongly encourage picking up the Controller 67 - supports Bluetooth and the app that runs it is slick - you get way more control and a nice historical data display

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I have the ac infinity too...but the older one that doesnt allow for the wifi controller. I may switch them out at some point soon simply to get the wifi feature. Man that would be nice I could leave for days and still be fine esp in veg. Thanks guys!