What temperature is too hot for vegging and flowering?

OK fellow growers, how hot is too hot for a grow cabinet? I have a thermometer that logs lows and highs for the 24 hr period. My temps are running between 70 F for lows to 93 F for highs. I’d say it stays at 90-93 while the lights are on, is this too hot? The plants seem fine, I’m just concerned. I have about as much ventilation as I can put in this cabinet, not sure what else to do? Any input would be appreciated!

Temps above 85f slow growth

You need to get that temp down after 85F they pretty much stop growing unless your giving them co2… can you use a cool tube for your lite??? ps if you haven’t …you mite consider Down loading Roberts free grow bible … and read it .it’s got a lot of good basic info … stay in the green H

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Thanks Hammer, I can assure you that they haven’t stopped growing, LOL! I’ve downloaded and read Robert’s Grow Bible. And yes, I am supplementing CO2, but even before I started with the CO2, they were still growing like crazy. If they grew any faster, I’d be afraid! LOL! I really don’t see any adverse effects from the temperature, not wilting or yellowing at all! I’m not arguing, but this is what I’m seeing!

Thanks for the input,


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Yes thats way to hot. You need to cool it down my friend.
Because If your grow room feels warm or cold, humid or dry, that is a sign that you may want to look into changing the temperature and/or humidity of your grow area.
Generally, cannabis plants prefer temperatures in the 70-85 °F (20-30 °C) range during the day when lights are on. When lights are off (or at night), cannabis plants are happy with slightly cooler temps.
Optimal Temps For Growing Cannabis
Vegetative Stage: Young growing cannabis plants in the vegetative stage prefer temps a bit warmer than in the flowering stage, in the 70-85°F (20-30°C) range.
Flowering Stage: In the flowering stage (when cannabis plants start making buds), it’s best to keep temps slightly cooler, around 65-80°F (18-26°C) to produce the best color, trichome production and smell. There should be a 10 degree difference between night and day temps for the best results (this is especially important in the flowering stage for highest quality bud development).

If you want to know anymore feel free to ask



Thanks Will! Not sure how I’m gonna get the temps down. As of now, I’ve eliminated one CFL bulb and I have as much air going through the cabinet as I could feasibly have with it’s location. I’m thinking of switching to LED lights but have read that they put out nearly as much heat as CFL’s do.

A definite head scratcher! LOL

Let me see what I can come up with to help you out. Can you post a pic so that can see what im trying to help you cool down



Thanks Will! You can see some pics of my cabinet under “a watched pot never boils” in the lounge. It has some extra vent holes now, but pretty much the same setup. Without dropping the wattage, not sure how to lower the temperature. Thanks for the help!


nice cabinet.
im shocked you dont see signs of stress with that temp being in the 90’s.

is it possible your temp reading is not accurate? like is the thermometer very close to canopy?

curious what your RH% is.
i use a CoolMist humidifier to get RH% up from low 30’s. was like $35 at HomeDepot.
and the cool mist does help lower temp, a little bit. but not a great deal.

are you able to somehow lower the temps outside your cabinet? maybe seal off windows letting heat in or something like that?

one other thing ive done in the past is, schedule your lights to be on during the coolest part of day. and off during the warmer hours.


Thanks Jay! Yeah, the plants appear to be healthy and growing! I’m pretty sure the thermometer is reading correctly. When I open the cabinet, I can tell that it’s really warm! I keep the thermometer on the floor close to the area the plants are. My humidity is in the 20’s, so a cool mist humidifier might actually help some. I think I’ll try that since I need a little more humidity anyway!

The temps outside the cabinet are the ambient temps in my house since my cabinet is in the spare bedroom. Indoor temps are between 68 - 72 degrees F, and I really can’t manipulate them too much or I’ll freeze the wife out! LOL!

I’m considering switching to LED lights, but if I can’t get the temps down with CFLs, is it really worth a try?

I’m gonna get a humidifier and see if that helps any and I guess we’ll go from there! I’ve been keeping the doors propped open a little to let some of the heat out and the temps are staying in the low 80’s. But I’m only running 2 of the 3 250watt bulbs.

Maybe if I switch to 3 300w LEDs it would run a little cooler? I don’t know! Any input is welcomed!



i used 2 of those u.f.o style led from Ebay for a little while. most of them have an exhaust fan built in. so that should help a bit. unless that fan just blows right inside cabinet heh.

i suppose, if youre not seeing issues. best to leave well enough alone.
i recently threw out a thermometer, after confirming with a second one, that it was off by 6-8 degrees.
dont know what kind you have but, those standard temp/rh meters like at home depot and such…are pretty cheap i guess.

OK, I removed the reflectors from all 3 light fixtures and removed 1 bulb. With both doors propped open, the temperature is running between 78 F and 84 F. I guess I’m gonna have to settle for less light intensity in order to manage the temperature. I guess I seriously underestimated the heat put out by CFL’S! I’m still thinking of switching to LED’s if they really run that much cooler.

Damn, I’m frustrated!


Well, I’ve been experimenting and playing trying to see what brings the temps down and I guess I was wrong. I guess the plants were a little stunted by the heat! With the doors partially open, I’ve been able to get the temps down to between 81 and 84 F consistently. I added a small fan to circulate the air inside the cabinet and I swear that the size of my biggest girl has nearly doubled overnight!

I think I’m going to go ahead and convert to LED lighting. Been looking at the Mars-Hydro reflector series. Everyone who is using them seem to like them! Been chatting with one of their reps and they seem to think that I’ll really like the results and the reduced heat generated! The maximum operating temp of their reflector series is supposed to be 105 F which is considerably lower than my CFLs are! It would be nice to be able to close the doors on my cabinet one day! lol!

Thanks for the ideas guys, you all rock!


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Glad it worked out! Please post something or let me know how the Mars works for you, I want to get one for my new veg-room, I’m also looking at Platinum brand, I can’t decide which one and would appreciate the input - good luck

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I just checked the ambient temps around my CFL bulbs and found it to be between 115 F and 140 F depending upon where you point the infrared thermometer. I’m pretty sure I can lower the temps considerably with LEDs!

Paranorman, I’ll be happy to let you know how they work out!



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Hi StonerGolfer: I have a garage with a couple tents. I also have a major air venting system out through the roof. However, during the day it gets very hot where I live, 100+ degrees, so the inside of my tent is about 93 during the day. This has definitely slowed my bloom. I just can’t grow during the summer until I install an air conditioner in the garage wall. Now, to the lights. I run LED 600 watt lights, 2 banks. These are definitely cooler than my T5 light in the veg tent. That thing gets super hot! Even with the two lights in the bloom tent it is not as hot as the T5. You might consider switching to LEDs. That may give you some cooler air. Seems to me the plants are hotter in veg too, all that growing! Good luck!

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I am so happy you asked this question!! I had the same question and thanks for the great feedback from everyone.

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If you had an inline duct fan to draw the hot air out of the tent/cabinet from above. that should help a bunch man.just just blowing the hot air around ain’t going to cut it bubba.

L E D should be the way to go.
B Safe.

90 will slow growth, and in my experience cause light airy buds of lower potency. It will smoke but not at its highest potential. I keep my tent at 70(day, lights on) and 65(nite, lights off). Thats where ive seen the dense strong results. Lots of people grow warmer tho, some grow colder. I deop tent temp to 60 and hold for last week of light and 48hr dark. Mite be my perception, or just how my setup performs.