Straight answers about growing

I built a device that reads a grow tent and gives advice, which meant I had to go find out what is actually known about growing cannabis versus what merely gets repeated. The gap turned out to be wide, and in a few places the popular answer is the opposite of the measured one.

These pages are what I found. Each one names its sources so you can check my work. Where the research is thin or absent, I say that instead of filling the space with confident-sounding advice — a practice with no evidence behind it isn’t necessarily wrong, but you deserve to know which kind you’re following.

Environment

Does VPD need leaf temperature, or is air close enough?
Air-derived and leaf-derived VPD are different numbers. On my own tent they disagreed by 0.42 kPa — about the width of a whole target band. And you can’t patch that with a correction factor, because a leaf isn’t always cooler than the air around it. When the plant is struggling it runs hotter.

How much light does cannabis actually need?
The best published study kept raising intensity to 1,800 µmol and never found the yield ceiling. But what the extra light buys is grams, not potency — and light is the one input most growers have never measured.

I flipped to 12/12. Why hasn’t it started flowering?
Count the days before you change anything. Then check the dark half, because the plant is measuring the uninterrupted night — one flash splits a twelve-hour night into two six-hour ones and your timer looks perfectly fine.

Diagnosis

My new growth is pale. Is that a deficiency?
Usually not. There is a test you can do with your eyes, in one glance, that separates normal young tissue from real chlorosis: look at the veins.

My leaves are curling. Is it heat or is it hunger?
Almost everyone treats this as a feeding problem, and the usual response is to flush. The test is not what the leaf looks like — it is where on the plant it is. Folding at the top with flat shaded leaves below is heat. Roots feed the whole plant, so a root problem doesn’t stop at the top layer.

Is it a deficiency or a disease?
Stop matching pictures. Two questions about the pattern do more: where the damage sits — oldest leaves, newest growth, or the ones nearest the lamp — and whether it is still appearing in leaves that opened since you looked.

Am I overwatering or underwatering?
They look identical from outside. Water it and watch for an hour: thirsty perks up, drowned does not — because roots need air to drink, and more water is the thing it already has too much of.

Are those bugs still alive?
Aphids leave their shed skins stuck to the leaf, so a colony that is already gone looks exactly as bad as one at its peak. Touch one before you treat: live squashes wet and green, a skin is dry and chalky.

Measuring

pH or EC — which number is actually telling me something?
They answer different questions and neither covers for the other. And a pen you have not calibrated still produces a confident number — which is how a thread spends a week arguing about a reading that was never real.

Harvest

Can trichome colour tell me when to harvest?
It tells you something, but less than the loupe-checking ritual implies. The field’s own reference text calls the link between trichome colour and chemistry poorly understood.

Should I flush before harvest?
There is no published evidence that it does what it’s said to do. It also isn’t known to hurt. Here is the full table of late-flower practices and how each one actually grades.

After harvest

How do I know my flower is dry enough?
Not by counting days. You go by how much water is left in the bud — 8–13% moisture, water activity under 0.65 — and the field’s own textbook admits that curing has no published science at all.

How should I store flower after the cure?
Growers obsess over air-tight seals. The measured ranking puts light first, temperature second and oxygen a distant third — and the clock that actually runs out is terpenes, not potency.

These pages are about growing, not about the device. If you want the device, it’s at croplock.com.