Temperature, Humidity, and VPD by Growth Stage

Plants ride a temperature–humidity relationship called VPD (vapor pressure deficit). You don't need to obsess over the chart, but the stage-by-stage ranges below keep you in the healthy band. SEEDLINGS & CLONES - Temp: 75–80°F (24–27°C) - RH: 65–75% Small root systems can't drink fast — high humidity keeps them from drying out while roots develop. A dome helps for the first week or two. VEGETATIVE - Temp: 70–85°F (21–29°C), lights on - RH: 50–70% Wide tolerance. Growth is fastest around the upper end as long as airflow is good. FLOWER (weeks 1–6) - Temp: 65–80°F (18–27°C) - RH: 45–55% Drop humidity as buds stack. Dense colas trap moisture inside themselves. LATE FLOWER (last 2–3 weeks) - Temp: 65–75°F - RH: 40–50%, lower if you can manage it This is where bud rot (botrytis) lives. A big cola at 70% RH in week 8 is a rot farm. Dehumidifier beats everything else here. LIGHTS-OFF DROP A 5–10°F drop at lights-off is fine and normal. A big drop (15°F+) combined with high humidity causes condensation inside buds — again, rot. If night temps crash, run a small heater or time the dehumidifier for dark hours. VPD IN ONE PARAGRAPH VPD measures how hard the air pulls moisture from leaves. Roughly: seedlings want 0.4–0.8 kPa, veg 0.8–1.2, flower 1.2–1.5. You hit those bands automatically by staying in the temp/RH ranges above. A cheap temp/RH sensor that logs to your phone is worth more than any gadget in the tent. AIRFLOW IS PART OF ENVIRONMENT An oscillating fan moving air through the canopy (not blasting it) does more against mold and weak stems than most additives. Stems should sway gently, not thrash.