Run the vegetative stage at 70 to 85°F (21 to 29°C) with lights on and 5 to 10°F cooler at night. Keep relative humidity between 40 and 60%, landing near 50 to 55% once the canopy fills. Fresh clones and seedlings want it warmer and stickier, closer to 65 to 75% RH, then you step it down.
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Quick targets for veg
- Lights on: 75 to 80°F is the sweet spot. Anything from 70 to 85°F grows fine.
- Lights off: 65 to 75°F. A 5 to 10°F drop is enough. Bigger swings slow growth and stress the plants.
- RH, early veg: 60 to 70%.
- RH, mid veg: 50 to 60%.
- RH, last week before flip: 45 to 55%.
- Root zone: 68 to 72°F. Cold roots stall nutrient uptake even when the air is right.
- VPD: roughly 0.8 to 1.1 kPa. That is the range where transpiration moves without the plant shutting stomata.
Why temperature and humidity are one number
I stopped chasing temperature and RH separately years ago. What the plant actually feels is vapor pressure deficit, which combines air temp and humidity into one reading. Warm, dry air pulls water out of the leaves fast. Cool, wet air barely pulls at all.
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During veg you want steady, moderate pull. Transpiration is how calcium and other poorly mobile nutrients travel up the plant. Push VPD too high and the stomata close to protect themselves, so growth stalls and leaf edges crisp. Push it too low and the plant sits in wet air, transpires little, and you get condensation on leaves, slow uptake, and a welcome mat for powdery mildew.
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Temperature details that matter
Leaf surface temperature is not room temperature. Under LEDs, leaves often run 2 to 5°F cooler than the air because there is less radiant heat. Many LED growers run rooms at 80 to 82°F to hit the same leaf temp a 75 to 78°F room gives under HID. If your plants look cold and slow under LED, raise the setpoint a couple degrees before you change anything else.
Night temps do real work. A modest drop speeds respiration and helps keep internodes from stretching. A 15°F swing, though, invites stress, purple stems from cold, and uneven growth. Insulate the room and use a controller with a deadband so the exhaust is not kicking on and off every minute.
Humidity by stage
Think in phases, not one fixed number.
- Clones and seedlings: 65 to 75% RH with a dome or a humidifier. Roots are small and the plant loses water fast through soft leaves.
- Early veg: 60 to 70%. New roots are established and the plant can handle more pull.
- Mid veg: 50 to 60%. The canopy is closing and airflow between leaves is dropping.
- Late veg: 45 to 55%. Tighten up before the flip. Dense colas and high RH are a bad pair.
Once the canopy touches, airflow matters more than the exact percentage. I keep a fan moving air under the canopy and check the corners, not just the middle. Stale pockets at 55% RH cause more rot than a well mixed room at 65%.
Where to put the sensor
Hang the hygrometer at canopy height, in shade, near the middle of the room. A sensor on the wall reads the wall, not the grow. A sensor in direct light reads radiant heat and runs several degrees high. Check min and max once in the morning and once in the afternoon. Those two readings tell you more than any single snapshot at noon.
Common veg mistakes
- Chasing a single number while the daily swing runs 20°F.
- Misting leaves in late veg to raise RH. That is a mildew shortcut. Raise humidity at the room level instead.
- Sealing the room and adding a humidifier without exhaust. CO2 and moisture build while the air goes stale.
- Ignoring leaf temperature. A 78°F room with LEDs may have 74°F leaves, which is cool for fast veg growth.
- Letting roots sit in cold water. Root zone below 65°F shows up as slow growth and dull leaves.
A simple daily routine
Log the high and low for temp and RH. Adjust exhaust first, since moving air fixes both heat and moisture. Add heat, cooling, or humidity in small steps and give the room a day to settle. Watch the plants, not just the meter. Praying leaves, steady node spacing, and fast new growth mean your numbers are working.