Why climate matters as much as it does
Plants exchange water and gases through their leaves, and the rate depends on temperature and humidity together. A well-managed climate lets the plant feed efficiently and holds off mold and pests. A poor one can't be corrected with nutrients.
Targets by stage
These are starting ranges, measured lights-on. Bringing humidity down as the plant matures is the most important climate adjustment over a grow.
- Seedling / clone: 72 to 78°F, 65 to 70% humidity
- Vegetative: 72 to 80°F, 55 to 65% humidity
- Early flower: 68 to 78°F, 45 to 55% humidity
- Late flower: 65 to 75°F, 40 to 45% humidity (lower humidity reduces bud rot)
What VPD means
VPD (vapor pressure deficit) combines temperature and humidity into a single number for how strongly the air pulls moisture from the leaf. Low VPD (cool and humid) slows transpiration; high VPD (warm and dry) speeds it up and can stress the plant. Common targets are roughly 0.8 to 1.2 kPa in veg and 1.2 to 1.6 kPa in flower.
Grow & Tell calculates VPD from the day temperature and humidity you log each week, so you can watch it trend without doing the math.
Holding it steady
Start by measuring with a thermo-hygrometer, then correct from there. A humidifier raises low humidity, which is common in veg; a dehumidifier or more exhaust lowers it, which is common in flower. A controller handles this automatically, switching a humidifier or fan against a probe so the climate stays on target while you're away.