Your tent is a tiny weather system, and you're the weather. Plants transpire — pulling water and nutrients up through their roots and out through their leaves — at a rate set almost entirely by temperature and humidity. Control those and the plant runs at full speed.
The targets, stage by stage
| Stage | Day temp | Night temp | Humidity |
|---|---|---|---|
| Seedling | 72–79 °F | 68–72 °F | 65–70 % |
| Veg | 75–82 °F | 68–72 °F | 50–65 % |
| Early flower | 72–79 °F | 65–70 °F | 45–55 % |
| Late flower | 68–75 °F | 62–68 °F | 40–50 % |
The pattern to memorize: humidity starts high and only goes down. Seedlings drink through their leaves; dense late-flower buds rot in humid air. Every week of the grow slides you gently down that RH column.
VPD in one paragraph
Vapor Pressure Deficit is just temp + humidity combined into one number describing how hard the air pulls water out of leaves. Low VPD (cool + humid) = plant barely drinks. High VPD (hot + dry) = plant stressed, gulping. The tables above keep you inside the healthy band automatically — VPD charts are just a precision tool for later grows. Bonus nerd points: leaves run a few degrees cooler than air under LED, which is why LED rooms like it slightly warmer than the old HPS advice.
Cheap climate control, in order of impact
- Exhaust fan speed — your main lever for both heat and humidity
- Circulation fans — even out hot spots, strengthen stems, deny mold the still air it needs
- Lights-off timing — run lights at night in summer to fight heat, days in winter to fight cold
- Humidifier ($30) for seedlings and dry winters; dehumidifier for flowering in humid climates — in Western New York summers, expect to need one by August
- Seal and insulate the lung room (the room the tent lives in) — the tent can only be as good as the air you feed it
Warning signs
- Tacoing/curling leaves + crispy edges → too hot and dry
- Slow growth, damp smell, fungus gnats everywhere → too cool and wet
- Condensation inside tent walls at lights-off → humidity spike; this is the bud-rot danger zone in flower. More exhaust, more airflow, dehumidifier.
A $12 thermo-hygrometer that logs min/max is the secret weapon: the problems happen at 4 a.m. when you're not looking. Check the overnight numbers every morning.