Most first-grow problems that get blamed on nutrients, light or genetics are actually the tent running too hot, too cold, too wet or too dry. Cannabis can't tell you which. A five-dollar thermometer-hygrometer can — and once you know the targets, the fix is usually the fan controller, not another bottle. The stakes go up in flower, because the failure mode there is bud rot, which is invisible until it isn't.
The targets
| Stage | Temperature (lights on) | Lights off | Humidity |
|---|---|---|---|
| Seedlings & clones | 72–80 °F (22–27 °C) | 65–72 °F | 65–75% |
| Vegetative | 72–85 °F (22–29 °C) | 65–75 °F | 45–60% |
| Early flower (weeks 1–3) | 70–80 °F (21–27 °C) | 62–72 °F | 40–50% |
| Mid flower | 68–78 °F (20–26 °C) | 60–70 °F | 40–45% |
| Late flower (last 2–3 weeks) | 65–75 °F (18–24 °C) | 58–68 °F | 30–40% |
The pattern: humidity falls as the plant matures. Seedlings with tiny roots drink through their leaves and want it humid; dense flowers in humid air rot from the inside. And nights cooler than days by up to 10 °F is fine — a cool night in late flower brings out purples and helps terpenes — but a swing bigger than that stresses the plant, and warm nights in flower are how you get loose, airy buds.
Measure at canopy height, not at the top of the tent by the light and not on the floor. Hang the sensor on a string in among the plants.
Why the numbers are linked: VPD in one paragraph
Plants lose water through their leaves faster when the air is warm and dry, slower when it's cool and humid. The combined measure is vapour pressure deficit (VPD): roughly 0.8–1.2 kPa in veg, 1.2–1.5 in flower. You don't need to calculate it — the table already builds it in — but the principle helps: a hot tent wants the top of each humidity range, a cool tent the bottom. At 85 °F and 40% the plant is being pulled dry; at 70 °F and 60% it's barely transpiring.
Too hot: the most common problem
An LED in a closed tent will push past 90 °F without exhaust. Signs: leaves cupping up at the edges, tips going crispy, tops bleaching, plants wilting under the light even with a wet pot. Above about 85 °F in flower, terpenes evaporate and buds go loose.
Fixes, cheapest first:
- Turn the exhaust fan up, or put it on a temperature controller set to ~78 °F. This solves most cases outright — sizing is in Sizing a grow tent fan.
- Raise the light a few inches and dim it 10–20%. Slightly less light beats heat stress every time.
- Run the lights at night. Set the timer so the lights-on period covers the coolest hours. In summer this alone can knock 8–10 °F off the peak.
- Move the fan and filter outside the tent, exhausting straight out of the room. Both add heat when they're inside.
- Only then: a portable air conditioner in the room. For a 4×4 in a hot climate there's no substitute.
Too cold
A garage or basement in winter gets there. Growth slows, leaves droop, purpling appears that isn't genetic, and below about 60 °F the plant effectively stops. Cold roots also lock out phosphorus, which shows as a deficiency with a full pot.
Fixes: turn the exhaust down (you're throwing away the light's heat), run the lights during the coldest hours, get pots off a concrete floor with a mat or pallet, and if needed a small tube heater in the room or a seedling heat mat under the pots. Don't point a fan heater into the tent.
Too humid: the dangerous one
Fabric pots, a full canopy and a small exhaust fan will put a tent above 70% without trying. In veg that's an invitation to powdery mildew; in flower it's botrytis — bud rot — which starts in the middle of the densest buds where you can't see it and turns them grey-brown from the inside. By the time it's visible on the surface the bud is lost, and it spreads.
Fixes:
- More exhaust. A humidity controller on the fan — 55% in veg, 45% in flower, 40% for the last weeks.
- Air movement inside: a clip fan across the canopy stops the still pockets between buds where rot starts.
- Water at lights-on, so the surface dries through the day instead of sitting wet all night.
- Defoliate: strip the lowest, shaded leaves and anything blocking airflow through the middle of the plant. Fewer leaves, less transpiration, air gets through.
- A dehumidifier in the room (not the tent) for a 4×4 in a humid climate. Empty it daily.
Check the biggest buds every few days in the last three weeks. A single grey patch means that bud comes out now, and everything around it gets more air.
Too dry
Under 35% in veg and leaves go crispy at the edges, growth slows, and spider mites thrive. Common with a big exhaust fan in winter when the house is already dry.
Fixes: turn the exhaust down, water a little more often, an open tray of water under the light, or a small humidifier in the room for seedlings and clones, which need it most. Never mist plants in flower.
The kit that makes this easy
- A digital thermometer-hygrometer with a remote sensor at canopy height, ideally one that logs the overnight high and low.
- An inline fan with a temperature and humidity controller, so it speeds up on its own when either number climbs. This one purchase replaces most of the fiddling above.
- A clip fan for air movement.
- In late flower, a dehumidifier if your climate needs one. Bud rot costs more than the dehumidifier.
The one-line version
72–85 °F and 45–60% in veg, then cooler and drier every week of flower, finishing around 70 °F and 35–40%. Put the exhaust fan on a controller so it holds those numbers, keep air moving through the canopy, and check the big buds in the final weeks.
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