Most first-grow problems that get blamed on nutrients, light or bad genetics are actually the tent running too hot, too cold, too wet or too dry. The plant 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 targets
| Stage | Temperature (lights on) | Lights off | Humidity |
|---|---|---|---|
| Seedlings & cuttings | 72–80 °F (22–27 °C) | 65–72 °F | 65–75% |
| Vegetative growth | 72–85 °F (22–29 °C) | 65–75 °F | 45–60% |
| Early flowering | 70–80 °F (21–27 °C) | 62–72 °F | 40–50% |
| Late flowering | 65–78 °F (18–26 °C) | 60–70 °F | 35–45% |
Two patterns run through that table. Humidity should fall as the plant matures — seedlings with tiny roots drink through their leaves and want it humid; dense flowers and fruit in humid air rot. And nights can be cooler than days, but not by more than about 10 °F, or the swing stresses the plant.
Measure at canopy height, not at the top of the tent where the light is, and not on the floor. Hang the sensor on a string in the middle of 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 called vapour pressure deficit (VPD). You don't need to calculate it; you need to know that a hot tent wants more humidity and a cool tent wants less, and that the table above already builds that in. If you're at 85 °F, aim for the top of each humidity range; at 70 °F, the bottom.
Too hot: the most common problem
An LED light in a closed tent will push temperatures into the 90s without exhaust. Signs: leaves curling up at the edges, tips going crispy, plants wilting under the light even when the pot is wet.
Fixes, cheapest first:
- Turn the exhaust fan up or put it on a temperature controller set to ~78 °F. This solves most cases outright — the sizing guide is at Sizing a grow tent fan.
- Raise the light a few inches and dim it 10–20%. Slightly less light is far better than heat stress.
- Run the lights at night. Set the timer so the lights-on period covers the coolest hours of the day. 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. Expensive to run, but for a 4×4 in a hot climate there's no substitute.
Too cold
Less common indoors, but a garage or basement in winter gets there. Growth slows, leaves droop, purpling appears on stems and leaves that isn't genetic. Below about 60 °F most plants effectively stop.
Fixes: turn the exhaust fan down (you're throwing away the light's heat), run the lights during the coldest hours, put the tent up off a concrete floor with a mat or pallet, and if needed a small tube heater or a seedling heat mat under the pots. Don't point a fan heater into the tent — it dries the air out and can cook one side.
Too humid
The dangerous one, because the damage — mould in the flowers, botrytis, powdery mildew — is invisible until it isn't. Fabric pots, a lot of plants, and a small exhaust fan will get a tent above 70% without trying.
Fixes:
- More exhaust. A humidity controller on the fan is the fix here just as for heat — set it to 55% in veg, 45% in flower.
- Air movement inside: a clip fan across the canopy stops humid pockets forming between leaves.
- Water in the morning, at lights-on, so the surface dries during the day rather than sitting wet all night.
- Thin the canopy — remove the lowest leaves and anything that's shading itself. Fewer leaves, less transpiration, better airflow.
- A small dehumidifier in the room (not the tent) for a 4×4 in a humid climate. Empty it daily.
Too dry
Under 35% and leaves go crispy at the edges, growth slows, and spider mites thrive. Common with a big exhaust fan in winter when the house air is already dry.
Fixes: turn the exhaust fan down, water a little more often, put an open tray of water in the tent under the light, or a small ultrasonic humidifier in the room for seedlings and cuttings, which need it most. Don't mist the plants in flower.
The kit that makes this easy
- A digital thermometer-hygrometer with a probe or a remote sensor at canopy height. The ones that log a high and low let you see what happens overnight when you're not looking.
- An inline fan with a temperature and humidity controller, so the fan speeds up on its own when either number climbs. This one purchase replaces most of the fiddling above.
- A clip fan for air movement inside the tent.
The one-line version
Aim for 72–85 °F and 45–60% in veg, then let it cool and dry into flower. Put the exhaust fan on a controller so it holds those numbers for you, hang a hygrometer at canopy height, and most "mystery" problems stop happening.
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