The Dude Grows Show

LED grow day and night temp/humidity?


I grow in a sealed room with co2, 2gal hempys, and 3 LED grow lights. I have AC and a dehumidifier as well.

I heard the guy from spectrum king saying you want to run temp at 85f when growing with LEDs. Is he talking room temp or canopy temp?

Also I never hear people talking about lights off temps. Do I drop temps when lights are of, or keep them the same? I’m trying to follow the VPD chart.

Do I follow VPD all the way through flower or do I dial down humidity towards the end? If I was to run temps close to 85f the humidity would have to be 75-80%. That seems high for flower?

Any help clarifying would be greatly appreciated.


9 responses to “LED grow day and night temp/humidity?”

  1. HD_Chop Avatar

    Hey Mr Burns
    If I recall correctly, optimal air temp for growing cannabis is 78F. Under sunlight or HID lighting, the leaf surface is getting hit with a fair amount of IR light. This IR radiation will raise the leafs surface temp 5-7 degrees above that 78F air temp that is said to be optimal.

    So when the air temp is right on at 78F, the leaf surface is actually about 80-85F.

    Now since LED’s do not emit IR radiation, we don’t get that 5-7 degree bump up at the leaf surface. So we can increase the room temp up to 85F and the plants metabolism will rise accordingly. Basically just like it was in sunlight on a balmy 78F day.

    I just took a reading of my canopy right before lights went out. Room temp was 78F and depending on which leaf I targeted with the IR temp gun, I got 78 to 81F. I’m running 4 CXB3590 COBs and they’re pretty close to the canopy @ about 5-6″. So there is not much diff between the air temp and leaf temp under LED. (I should raise my temps a little. LOL)

    As for VPD, not sure what chart your referring to, but this one
    shows that at 85F (about 29C) you should be at 55-62.5rh while in full flower.. I run that range myself in flower.
    The last couple weeks, I let it start going lower, and will eventually have it 35-40rh. Seems to put on the trichs.

    Your running Co2, so you could take those temps a little higher I believe.

    I could be off on all this, but its what I understand and works for me.

    1. w33zy Avatar

      Wt? U could be off on this? Why write all that and say u could be wrong, u are a goofy. Stop giving advice if u don’t know 100% because everything u said was correct then u through it in the garbage once u said I think. Goofy

  2. HD_Chop Avatar

    Oh yeah, you asked about night temps too.
    I think consistent temps day and night are ideal. If it cools too much at night, humidity will go way up. Not good for plants, good for PM. YIKES!
    During those last couple weeks, when your letting the humidity drop, if you could let the night temps drop too, you may get some colors in the bud if the genetics are there for it. (Strain dependent… LOL)

  3. Elmer_Budd Avatar

    Your plant generally prefers a somewhat cooler temperature during the dark period, and this will encourage your plant to grow as fast as possible. Therefore, aim to maintain a temp of about 65-70°F during lights-out. If using a heater to regulate dark period temp, make sure it doesn’t get too hot. Strive to avoid large day/night fluctuations of more than 10-15 degrees. If your day temp is 80 for example, adjust your heater so that temp goes no lower than 65 at night.

  4. Mr. Burnz Avatar
    Mr. Burnz

    Thank you for the explanation and chart! I was checking the chart over and realized the one I originally looked at did not show what the color ranges ment. I appreciate the info and it all make sense now!

    1. w33zy Avatar

      Be aware dude vpd chart has 2, one for hid and the other led. Make sure u are looking at led. And get that light further away from that canope. U said 6″ away? That’d because u got cheap lights ain’t no way in hell my ROI E780 needs to be 6″ away, at 3 ft my ppfd is like 1688. Buy good lights or quit plan and simple

  5. Mr. Burnz Avatar
    Mr. Burnz

    Thanks for the info! That was my instinct, it usually gets cooler at night outside, so it would be the same indoors. I’ll shoot for those temps when lights are off.

  6. HD_Chop Avatar

    Greetings DOC.
    Just listened to this morning’s Wake and Bake, where Dude and Scotty talked about this grow question.
    Some confusion on reading the COD chart.
    There are3 color bands in the chart. The “key” to what the colors mean is on the top right corner.

    The VPD is the diff between the moisture level in the leaf, compared to the RH of yhe surrounding air.
    The interior of the leaf is 100% totally saturated with water. If the TH of the air is 100% too, the is nowhere for moisture in the leaf to evaporate into. As the humidity in the air drops, the is more room for water to transpire out of the leaf into the air. As this water leaves the plant, more water (along with nutes) is drawn up through the roots.

    You’ll notice the light blue band is for propagation and early veg. It has a higher humidity level in this band. Therefore, less transportation. We want to keep cuttings from evaporating all their moisture and shriveling up and dying. Let them girls develop roots before we smash the gas pedal to the floor.

    In the chart, you’ll see as the plant developed, we can let the RH of the air lower so more water can evaporate into the air. More water, more nites are being pulled into the plant too. During flower, the deficit is even larger.

    Is the deficit is too large, the plant will sense this dry condition and close up the openings in the underside of the leaf (stomata). Water and nutes will slow to a crawl.

    Last few weeks of flower will benefit from a dryer air, and help with trichs production.
    I let my RH drop to 40% the last week and a half.

    The ability of the air to hold moisture changes with the temp. That’s why the chart is a swoop curve.
    That is also why lights out will see a rise in RH. Cooler air holds less water and can condense out of the air and form dew in your buds.

    I don’t run a Big temp drop at night for this reason. Maybe 19 degree temp drop at most.
    Some strains will developed nice color with a cooler night temp, but be careful.

    FWIW, since I have been running my enviro closer to what the latest science says, I noticed an increased growth rate and vigor.

    Much growers live to all.

Leave a Reply

Recent Posts

Dude Grows Show 1643

What’s Growing On DGC? The Dude and Scotty are Hanging Out Talking Cannabis News, Culture and Growing. THIS…

Roots Organic

I’ve been using Roots Organic either OG, 707 soil (or if available for flower transplant Lush) and I usually…