Bud rot: how humid, for how long, and do I chop early?

The short answer

• There is no cure. Once a bud is rotting inside, that bud is lost; the job is to stop it spreading and to decide about the rest of the plant.
• The threshold is not the whole story. Botrytis needs damp tissue and still air for long enough to get established. A few hours at 70 % overnight is a risk; a week of it is a near certainty in a dense late-flower canopy.
• Chopping early is a trade, not a rescue. You give up the last of the ripening to keep what you have. Whether that is worth it depends on how far along the trichomes are and how many sites are already gone.

What it actually is

Bud rot is Botrytis cinerea, the grey mould, and it behaves differently from most things that go wrong on a plant: it starts on the inside of the flower. The dense interior of a late-flower bud stays damp long after the outside has dried, and that is where the fungus establishes. By the time it is visible from the outside it has usually been going for days.

The first sign is small and easy to miss. A single leaf or bract on a bud turns brown or yellow and, if you pull it gently, comes away with no resistance. Open the bud there and you find grey fuzz and brown, crumbling tissue in the middle of what looked like a healthy flower. The field’s reference text describes exactly that sequence, and once you have seen it once you do not mistake it again.

Two things it is not: pistils turning brown or orange on a maturing bud is ripening, not rot — the tissue underneath is firm and the bud smells like the plant. And white, powdery patches on leaf surfaces are powdery mildew, a different fungus with different conditions; it does not start inside the bud.

Why the number everyone gives you is not the answer

Ask on any board and you will get a relative humidity: keep it under 55, closer to 40 in late flower. Those are reasonable targets. But notice the shape of the question that prompted them. Nobody asks “what is the safe RH?” They ask “it was at 70 % for a few hours last night — was that enough?” That is a question about duration, and a threshold cannot answer it.

The published picture is that Botrytis needs high humidity, damp or wet tissue, and moderate temperature, and that dense flowers are the vulnerable structure because their interior does not dry out. It does not give you a clean “X hours above Y %” because it depends on the density of your buds, the air moving through them, and whether the surface actually reached the dew point and got wet. What the literature is clear about is the direction: more hours damp, more risk; free water on the flower, much more risk.

Here is what we model on our own device, given as what we do rather than as a published threshold: risk starts rising around 62 % RH, is high above 75 %, is worst between roughly 15 and 25 °C, climbs sharply when the canopy comes within about 2 °C of the dew point, and is weighted almost entirely to flower and late flower. Those are our synthesis of the greenhouse literature, not a measurement of your tent.

The hours you are not seeing

The damp hours that matter are mostly the ones you are asleep for. When the lights go off, the tent cools, and relative humidity climbs even though not a drop of water has been added — the air simply holds less at the lower temperature. A room that reads 55 % at midday can sit at 70 % or more from lights-off until the fans and the morning warm it back down. This is first-party observation from our own tent, not a citation, but it is easy to check: put a logging hygrometer at canopy height and read the overnight trace.

Outdoors and in greenhouses the same thing happens with the season. Late September in a humid climate is when the nights get long and cool, dew forms on the flowers before dawn, and the rot that had been waiting all summer gets its window — which is why the question spikes on every board at the same time each year.

How to check this yourself

• Open a bud. Pick the densest, most shaded cola. Gently tug any leaf that looks even slightly off-colour; if it comes away clean, part the bud there and look inside. That is the only reliable inspection.
• Log the night, not the day. A hygrometer read at noon tells you almost nothing. Put one at canopy height, read it at lights-off, two hours later, and just before lights-on. If it is above the mid-60s for most of that window, you have your answer about “how long.”
• Feel for wet, not humid. Free water on the flower surface is the multiplier. If the buds are damp to the touch at lights-on, the tent is condensing overnight regardless of what the number says.

Should I chop early?

It is a trade, and the way to make it is to price both sides.

What you keep by chopping now: every bud that is not yet infected, at whatever ripeness it has reached. What you give up: the remaining ripening — the last one to three weeks in which the trichomes finish and the flower fills out.

So look at the trichomes first, on the buds that are still clean. If they are already mostly cloudy, you are giving up very little and the decision is easy. If they are still largely clear, you are choosing between an immature harvest and a gamble — and the gamble is worth taking only if you can change the conditions tonight: air moving through the canopy, humidity down, and the infected sites removed.

Either way, remove what is already rotting now, carefully — cut well below the affected tissue, take it out of the room, and disturb it as little as possible, because a rotting bud is a spore source for everything around it. A middle path many growers take is to harvest the affected colas and any dense tops immediately and let the smaller, airier lower buds finish.

On what to spray — deliberately not on this page

Nothing that is safe to apply to a flowering cannabis plant will reverse an infection that is already inside a bud, and what is legal to apply at all varies by where you live. The greenhouse management literature is explicit that in the flowering phase the levers are cultural and environmental — humidity, airflow, canopy density, removing infected material — and that is what this page covers.

What would show that we have this wrong

If you open the bud at the browning leaf and the tissue inside is firm and green and smells like the plant, it is not rot — you are looking at a ripening bract or a bit of leaf damage, and the rest of this page does not apply. And if your overnight trace never leaves the 50s and you still have rot, the moisture is coming from somewhere other than the air — foliar spraying, a wet floor, condensation off a cold wall onto the plant — and that is where to look next.

Where our device fits, stated narrowly

Croplock does not see rot inside a bud, and we would not trust anything that claimed to from a photograph of the outside. Where it fits is the part a hygrometer glance cannot do: it logs humidity and temperature at canopy height every minute, so the question “how long was it above 65 last night?” has an actual answer in the morning, and the trend across the last week of flower is a record rather than a memory. On a tent it controls, it can hold humidity down and keep the fan on through the dark period; it cannot open a bud for you.

Sources

• Punja, Z.K., Collyer, D., Scott, C., Lung, S., Holmes, J. & Sutton, D. (2019). “Pathogens and Molds Affecting Production and Quality of Cannabis sativa L.” Frontiers in Plant Science. DOI 10.3389/fpls.2019.01120 — Botrytis cinerea as the bud-rot pathogen, infection of dense inflorescences, the role of humidity and free water, absence of a curative treatment.
• Buirs, L. & Punja, Z.K. (2024). “Integrated Management of Pathogens and Microbes in Cannabis sativa L. under Greenhouse Conditions.” Plants 13(6):786. DOI 10.3390/plants13060786 — environmental and cultural control as the flowering-phase management.
• Cannabis in Controlled Environment Production (CRC Press), Ch. 9 — the visible sequence of infection and the prevention-over-treatment guidance, restated in our own words.
• The humidity, temperature and dew-point figures in “why the number is not the answer” are the risk model on our own device, given as such. The overnight humidity surge and the chop-early reasoning are first-party observation and judgement, not citations.

More of these at Answers. The device is at croplock.com.

Related: Can trichome colour tell me when to harvest? — the other half of the chop-early decision. From the build: In band, and sick anyway — nine humid days that every reading called fine.