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Mold and Mildew on Cannabis: The Humidity Levels That Matter at Each Stage
By the Cultivar grow team · published 2026-09-30 · updated 2026-09-30 · how we source these numbers
Mold is the loss that costs a whole harvest and gives the least warning. The good news is that it follows humidity closely, and humidity is something you can measure and control. This page lays out where the risk actually starts at each stage, why the number you see during the day is often not the number that matters, and what to do before and after you find it.
If you log your readings in Cultivar, the app grades every reading against these same lines and tells you when a space crosses one.
The humidity lines, stage by stage
Each stage has three lines. Watch means you are near the edge — worth knowing, nothing to panic about. High means the conditions mold needs are present; act today. Severe means act now.
| Stage | Watch | High | Severe | Main threat |
|---|---|---|---|---|
| Seedlings | 80% | 85% | 92% | damping-off, mold on the medium |
| Veg | 72% | 80% | 88% | powdery mildew |
| Flower, weeks 1–5 | 60% | 65% | 75% | bud rot |
| Flower week 6+ and flush | 55% | 60% | 70% | bud rot |
| Drying | 62% | 65% | 70% | mold on hanging buds |
| In the jar | 66% | 68% | 72% | mold in the jar |
A few notes on reading this:
- Germination domes and cuttings are left out on purpose. They run near 100% humidity by design, and a warning there would fire on every healthy tray.
- Late flower is stricter because the buds are denser. Botrytis usually develops in weeks six to eight of flower, when big colas trap moisture in their centres. The same 62% that is fine in week three deserves attention in week seven.
- Temperature changes the picture. Bud rot grows best between about 15 and 25°C (59–77°F) and slows above 28–30°C. Powdery mildew prefers mild to warm conditions around 20–30°C (68–86°F). Heat is not a fix — it brings its own stress — but it does change which problem you are most likely to see.
The number that matters is the one at night
Most growers check their tent during the day. The dangerous hours are after lights-off.
When the lights switch off, the air cools by several degrees. Cooler air holds less water, so the same amount of moisture shows up as a higher relative humidity. A tent reading 55% at noon can easily sit at 68% at 3 a.m. That overnight stretch is when bud rot gets its start.
Two habits fix most of this:
- Use a hygrometer with min/max memory, or a logging sensor, so you see the peak rather than the reading you happen to catch.
- Keep the dehumidifier running through lights-off, and keep the night temperature from dropping sharply — a smaller drop means a smaller humidity spike.
Condensation: when water lands on the plant
Water forms on any surface cooler than the dew point of the air. Leaves at night, and leaves under LED lighting, often run a degree or two cooler than the air around them. When the air temperature is within about 2°C (4°F) of its dew point, you should assume leaves and buds are getting wet.
Wet surfaces are what botrytis, damping-off and most leaf rots need. (Powdery mildew is the exception — free water actually hinders it.) If your readings show air that close to its dew point, treat it as urgent at any stage past seedling.
How to check for it
Bud rot (botrytis). Start with the biggest, densest colas. Look for a single sugar leaf poking out of a bud that has turned yellow or brown. If it pulls out with no resistance, open the bud: grey-brown fuzz, dry crumbling tissue and a musty smell confirm it. It starts inside, so the outside can look fine. Our bud rot guide covers this in detail.
Powdery mildew. Look at the tops of fan leaves, starting with lower and shaded ones, for round white spots that look like a dusting of flour. Rub one gently: mildew wipes off, trichomes and spray residue don't. More in the powdery mildew guide.
While drying. Smell the room every day. A musty or ammonia smell is usually the first sign. Squeeze the densest buds and look inside any that feel damp.
In the jar. Smell each jar as you open it. Look for fuzz or white spots where buds touch each other. A small hygrometer inside the jar tells you the humidity the buds actually sit in, which is often different from the room.
What to do if you find it
- Bud rot: cut the affected bud out with a generous margin, straight into a bag. Don't shake it — the spores spread. Clean your scissors and hands before touching healthy plants. Then bring the humidity down for the whole space and check every day for a week.
- Powdery mildew: remove badly affected leaves into a bag. In veg, a potassium bicarbonate or sulfur product can knock it back — follow the label, and never spray buds. Mildew on buds means discarding those buds.
- Drying and curing: remove moldy buds with a margin and throw them away. Lower the humidity in the drying space, or open the jars and let the buds dry a little more before re-jarring.
Moldy cannabis is not made safe by trimming, drying or curing. Throw it out.
Prevention that actually works
- Hold flower at 45–55% from week six, lower at night.
- Keep air moving over, under and through the canopy. Still air inside dense growth is where both problems start.
- Defoliate enough that air reaches the middle of the plant, and space plants so leaves aren't touching.
- Water early in the light period, never just before lights-off.
- Dry slowly and cool — about 60°F and 60% humidity is the classic target — and jar only when small stems snap rather than bend.
- Clean the space between grows. Spores persist from one run to the next.
Sources
- Punja, Z. and Buirs, L., on botrytis and powdery mildew in greenhouse cannabis production, Greenhouse Canada.
- UMass Extension Greenhouse Crops and Floriculture Program, "Botrytis Blight" fact sheet.
- UC Statewide IPM Program, "Powdery Mildew on Vegetables and Ornamentals".
- Thermometer + hygrometer with min/max memory — shows the overnight humidity peak, not just the reading you happen to see
- Dehumidifier sized for your space — the direct fix when a flowering tent climbs past 60%
- Mini hygrometers for jars — the only way to know the humidity your cured buds actually sit in
Affiliate links: we may earn a small commission at no cost to you. We only list gear that solves the problem on this page. Disclosures.
FAQ
What humidity causes mold on cannabis?
It depends on the stage. Seedlings tolerate 80% or more, veg starts inviting powdery mildew above about 70–80%, flowering plants are at real risk of bud rot from about 65%, and from week six of flower the safe ceiling drops to about 55–60%. Drying rooms should stay near 60%, and cured buds in a jar near 58–62%.
Is 60% humidity too high in flower?
In early and mid flower 60% is at the top of what is comfortable, not a crisis. From week six of flower, when buds are dense, 60% is where bud rot risk becomes real, especially if humidity climbs further after lights-off.
Why does humidity go up at night in my tent?
When the lights switch off, the air cools. Cooler air holds less water, so the same amount of moisture reads as a higher relative humidity — often 10 to 15 points higher. That overnight spike is when most bud rot infections start.
What is the difference between bud rot and powdery mildew?
Bud rot (botrytis) grows inside dense buds in cool, wet conditions and shows as grey-brown fuzz and dying bud tissue. Powdery mildew grows on the surface of leaves in mild, humid, still air and looks like white flour spots that wipe off. Bud rot needs moisture on the plant; powdery mildew does not.
Can you smoke buds that had mold?
No. Moldy bud should be thrown away, not trimmed, dried or cured. Drying does not make it safe, and inhaling mold spores is a real health risk.
Keep reading
- Bud Rot (Botrytis): Catching It Before It Takes the Harvest
- Powdery Mildew on Cannabis: White Dust That Wipes Off
- Root Rot in Cannabis: Brown Roots, Sour Smell, Stalled Growth
- Automatic VPD Monitoring: Which Sensors Work and What They Cost
- Brown or Rust Spots on Cannabis Leaves: Calcium, pH or Pests?
- Drooping Cannabis Leaves: Overwatered or Underwatered?
- Fungus Gnats: Small Flies in Your Grow and the Larvae That Matter
- Heat Stress in Cannabis: Taco Leaves, Foxtails and What to Fix First
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