Straight answers about growing

By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed 2026-09-18

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

Where should the temperature and humidity probe go?
Canopy height, in the shade of a pole, out of the fan’s blast. In the lamp’s beam ours read 2.6 °C high; a powered board read 2.2 °C high from its own electronics. Every number downstream inherits the error.

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. And “heat trumps VPD” mixes up two dials: VPD is made of temperature, and temperature sets how fast the plant can work, so pick the temperature first and set the leaf VPD with humidity.

What is my VPD? A calculator that does not guess your leaf temperature
Air temperature and humidity in, VPD out, judged against the stage band, with the humidity range that puts your air inside it. Type in a leaf temperature you measured and it shows the leaf number beside the air one; leave it blank and it says so instead of inventing an offset. Same formula as the device. Every reading has its own link, so you can paste yours into a thread.

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.

Starts

Why won’t my clones root, and why is my seedling stalling?
One controlled trial on cuttings: the hormone doubled rooting and trimming the leaf tips hurt. A fungus that wants your dome. And a rooted seedling needs the opposite air from a cutting — I held mine too humid for nine days.

Vegetative

Should I top, train, or defoliate — and when?
One concrete topping procedure, one number for the stretch (25 % to 600 %, so measure yours), one rule for de-leafing (not in the first three weeks of flower), and the reference text’s own caveat that pruning cannabis is not well studied.

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.

Is it cal-mag?
Probably not from the photo alone. Calcium, magnesium and potassium look alike on a leaf — the field’s textbook says so, and our own blind read of expert-labelled calcium photos agreed 5 times and disagreed 40. Read the runoff pH first: all three lock out together.

Is it a virus?
A mottled leaf is what magnesium, iron, pH lockout, transplant shock and some cultivars all look like — and what a virus looks like. The mosaic virus confirmed in cannabis rides an outdoor leafhopper; the indoor viroid mostly shows as stunting and is confirmed by a lab, not a look. Mark the newest leaf, correct the pH, and read the answer in ten days.

White spots on the leaves: mildew, bugs, or residue?
Three things get called white spots and a thread names all three by the fifth reply. Wipe one with a dry finger: powder off is mildew, a fungus that lives on the surface; smear off is spray or hard-water residue; nothing off and the mark is in the leaf is a feeding scar — thrips along the veins with black specks, or a mite stipple. Septoria is brown and starts at the bottom.

Will my buds still get bigger, and do bloom boosters help?
Probably yes, they are still growing: in a harvest-timing study flower weight kept rising to week eleven. What builds it is light at the canopy and time. Four controlled trials raised phosphorus above what the plants needed and the flowers got no heavier; potassium did nothing for yield either. Photograph one bud twice a week, measure the light, and check the runoff if a plant stalls after a booster.

Is my plant turning hermie? Pollen sacs, nanners, seeds, and how to check
Four things get called a hermie and only two make pollen. A pollen sac is a smooth ball on a stalk with no hair; a female pre-flower is a teardrop with a white hair, and a browned hair is just an old one; a nanner is a yellow anther poking out of a mature bud. Pluck one and open it — that settles it in a minute, and it is the one test every thread skips.

Will a streetlight or a neighbour’s flood light stop my plant flowering?
A photoperiod plant counts the uninterrupted night, and it counts in far dimmer light than you would think. Moonlight does nothing; a light you can read by is enough to hold a plant in growth, pull a flowering one back, and stress it toward pollen. Autoflowers do not care. Cover it at dusk, every night, and count the weeks left before frost.

Why is it purple? Genetics, cold, phosphorus, or something worse?
One pigment, four reasons, and the colour will not say which. Look at it under white light, not the lamp. Even and symmetrical on a plant growing at full speed is genetics, and cool nights turn it up. The purple-stems-means-phosphorus rule did not hold when phosphorus was actually withheld from cannabis. Purple with a crumpled, clustered growing tip outdoors is a leafhopper-borne pathogen, and that one gets removed.

Can I spray a flowering plant? What changes after the flip
Before buds, the question is what kills the problem; after, it is what stays on the flower you smoke and what the water does to a dense bud. Residue does not reliably burn off, no pesticide is federally registered for cannabis, and the best mildew trial scored leaves, not buds. Fix night humidity and airflow first.

Mites, thrips, aphids or gnats — which one do I have?
You will rarely see the bug. The damage and where it sits narrows it to one or two — webbing, twisted tips, metallic patches, sticky leaves — then a hand lens confirms. And the root aphid that sends hydro growers to the wrong place.

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

Is this the fade, or a deficiency?
Yellow leaves in late flower can be the plant finishing or the plant starving, and the colour does not say which. A fade is slow, bottom-up and even while the buds keep swelling; a deficiency is fast, patterned or at the top while they stall. Mark a leaf and count in four days.

Bud rot: how humid, for how long, and do I chop early?
Every board answers with a humidity number; the question is hours. The damp ones are overnight, the inspection is opening a bud, and chopping early is a trade priced against the trichomes. Outdoors, rain puts water inside the bud, and a bud rotting from one spot is worth opening for a caterpillar.

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.

It’s getting cold at night. Should I harvest before the frost?
A run of cold nights is not a deadline. Cannabis has been flowered to harvest at a constant 0.5 °C in a published study — cold slows a plant rather than ending it, so a cold snap moves your harvest further away, not closer. What takes outdoor crops in October is the wetness hours, not the forecast low.

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.