I've got temp and RH under control but I'm trying to actually use VPD instead of guessing. What targets do you run by stage, and do you correct for leaf temperature or just go off air readings?
4×4, LED, currently ~26°C / 55% RH in early flower.
Dialing VPD in flower — target ranges that actually worked for you?
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TentLifeTina Vegging
Asked 1h ago · Lighting & Environment
Best answer
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DWC_Dana Flowering
Lighting & Environment
Rough VPD targets that have worked across a lot of tents: seedling/clone 0.4–0.8 kPa, veg 0.8–1.1, early flower 1.0–1.3, late flower 1.2–1.5. Higher VPD late helps push transpiration and keeps humidity off the buds.
Yes, correct for leaf temp — leaf surface runs ~1–3°C below air under LED, and that offset changes the real VPD a lot. Grab a cheap IR thermometer, measure a shaded fan leaf, and plug leaf temp + air RH into any VPD chart. That said: don't chase the number if the plant looks happy. Healthy transpiration, no taco-ing, steady growth = you're in the zone. Your 26°C/55% is fine for early flower.
Marked helpful by TentLifeTina
268 replies
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LivingSoilLukeFloweringHelpful· 1h ago
The leaf-temp correction is the part everyone skips. My 'perfect' air VPD was actually 0.3 kPa too high once I measured leaf surface.
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CalMagCarlVeggingHelpful· 1h ago
IR thermometer is $20 and changed how I run my tent. Measure a few leaves — edges vs center differ.
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AutoflowerAlSeedling· 2h ago
What about lights-off? My RH spikes at night and I'm paranoid about late flower.
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DWC_DanaFlowering· 2h ago
@AutoflowerAl run a dehumidifier on a humidistat for lights-off, target <55% in late flower. Cooler dark period + high RH is exactly how bud rot starts.
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TrichomeTomHarvest MasterHelpful· 2h ago
Don't crank VPD too high in veg chasing 'faster' growth — you'll get clawing and stalled plants from over-transpiration. Ramp it up gradually into flower.
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FirstGrowFionaSeedlingHelpful· 1d ago
Honestly for beginners: nail temp 24–27°C and RH by stage, and your VPD will mostly land in range on its own. VPD is the fine-tune, not the start.
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LivingSoilLukeFlowering· 1d ago
Airflow matters as much as the number — stagnant air around the canopy gives you microclimates the chart never sees.
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CalMagCarlVegging· 1d ago
If leaves taco/pray too hard upward, VPD/light is too aggressive. Praying flat-ish and relaxed = comfortable.
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TentLifeTinaVegging· 23h ago
Measured leaf temp tonight: 23.5°C vs 26 air. Real VPD was ~1.15, not the 1.35 my app showed. That explains a lot. Thanks.
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DWC_DanaFlowering· 20h ago
@TentLifeTina exactly. Now nudge RH down a touch as you move into mid-flower and let VPD drift toward 1.3.
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AutoflowerAlSeedling· 18h ago
Cheap fix for night RH spikes: a small AC Infinity controller that ramps the exhaust on humidity. Set-and-forget.
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TrichomeTomHarvest MasterHelpful· 14h ago
One more: VPD assumes a healthy root zone. A plant with root issues won't transpire 'to chart' no matter what your air says.
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FirstGrowFionaSeedling· 11h ago
Saving this. The 'don't chase the number if she's happy' line should be pinned.
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LivingSoilLukeFlowering· 8h ago
Late flower I let RH ride low (45–50%) and VPD high — frostier tops and zero rot for three runs now.
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CalMagCarlVegging· 6h ago
Pro move: log temp/RH/leaf-temp once a day. Patterns over a week tell you more than any single reading.
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