Is it wind burn or is it nutrient burn?
Usually neither, and one of the two can be ruled out with a number you probably already have.
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
The short answer
Two tests, and between them they settle most of these.
One: does the damage track the air? Wind damage lands where the air lands. If the leaves in front of the fan are chewed up and the ones a foot to the side are clean, that is the fan. If the damage is spread across leaves the fan never reaches, it is not.
Two: what does your substrate or run-off actually read? A feeding problem is a salt problem, and salts show up on an EC meter. If your run-off comes back low, you do not have a salt excess, and you can stop arguing about it. That is not an opinion about your plant; it is arithmetic about your soil.
When both tests come back negative — the damage is not where the air is, and the substrate is not loaded — there is a third answer that gets offered far less often, and it is the subject of most of this page.
Test one: does it track the air?
This is the easy one and it takes a minute.
Wind damage is mechanical and local. A leaf held in a steady stream of moving air dries faster at the edges than it can replace the water, and the tissue that gives out first is the thinnest and the furthest from the veins. It appears on the leaves in the path of the air and it stops where the air stops.
So stand where the fan is and follow it. If the affected leaves are the ones it hits, and the sheltered side of the same plant is clean, you have your answer and the fix is obvious: point it at the wall, drop it a speed, or put something between it and the plant so the air spreads before it arrives.
If the pattern does not line up with the airflow, wind is not the explanation, regardless of how much the leaf looks like a picture on a wind-burn page. A fan moving a fraction of a tent cannot produce damage everywhere in it.
Test two: the number that eliminates one of them outright
Feeding damage is salt damage. Too much fertilizer in the root zone pulls water back out of the roots and scorches tissue starting at the tips, because the tips are the end of the line.
The thing about a salt problem is that it is measurable, and it is measurable cheaply. Collect what runs out of the bottom of the pot and read its EC. If you have a substrate probe, read that.
Here is why this matters more than it sounds. A salt excess cannot exist in a substrate that reads near-empty. If the number comes back low, over-feeding is off the table — not unlikely, not probably-not, but excluded. You have removed one of your two candidates without diagnosing anything, and you can spend the rest of your effort on the one that is left.
It is worth saying what “low” is doing here, because a number on its own is not a verdict. What matters is the direction and the gap: a substrate that has been fed on schedule reads meaningfully above plain water, and one that has been watered daily with plain water in between feeds may not. There is a fuller answer on what EC is actually telling you, and on the trap of reading what goes in rather than what comes out, over here.
One more reason to run this test before the other: a daily watering habit quietly leaches a pre-charged soil. Growers who water a little every day and feed twice a week are often feeding far less often than the bag intends, and the substrate drifts down rather than up. The leaves can still look dark and healthy while that happens, which is exactly why the meter beats the eye.
The third answer, which is usually the real one
If the damage is confined to the tips and the growing points, and the rest of the leaf is a good color, and neither test above landed — look at how much the plant is transpiring, not at what it is being fed.
Calcium is the reason. Calcium moves through a plant in the water it is drinking and essentially only in that water. It does not get redistributed afterwards the way nitrogen or magnesium do. Wherever the transpiration stream goes, calcium goes; wherever it slows down, calcium stops arriving. The tissue that suffers first is the newest and least-transpiring — the growing tips.
The cleanest isolation of this we could find is old, and it is not cannabis, and both of those are worth knowing before you lean on it. Barta and Tibbitts (1986) wrapped the young inner leaves of lettuce plants in sheaths to stop them transpiring, and left everything else alone — the plants stayed in a full nutrient solution with calcium freely available the entire time.
- After four days, 53% of the enclosed inner leaves had tipburn. On the untouched control plants, under 1% did.
- Calcium in the enclosed leaves measured 0.63 mg/g dry weight, against 1.48 mg/g in inner leaves that were left to transpire.
- The outer leaves, which were transpiring normally the whole time, measured 9.9 mg/g — more than fifteen times the enclosed ones, on the same plants, from the same solution.
- Magnesium barely moved: 2.25 mg/g enclosed against 2.34 mg/g not enclosed. So this is not the plant failing to take things up in general. It is calcium specifically, and it is a transport problem.
That is the mechanism with the feeding held constant. The calcium was always there. It simply did not arrive.
Two honest limits on it. It is lettuce, and a lettuce head enclosing its own young leaves is not the same shape of problem as a cannabis plant in a humid tent. And the transpiration was suppressed by a physical sheath, which is a much blunter instrument than high humidity. We read the abstract and the reported figures, not the full paper, and we are saying so. What it establishes is the direction: suppress transpiration and you get tipburn from a root zone that is not short of anything.
In a tent, the two things that suppress transpiration are high humidity and still air, and they compound. Humidity in the seventies leaves very little gradient for a leaf to evaporate into. Still air is worse than it sounds, because a leaf sheds water into a thin skin of air sitting on its own surface; if nothing moves that skin along, it saturates and the leaf stops. The canopy chapter of Cannabis in Controlled Environment Production puts figures on how fast air movement falls off going into a canopy — citing Thom (1971), air moving at 2.5 m/s above a crop drops to roughly 0.9 m/s at the canopy top and about 0.25 m/s partway inside it — and gives a useful in-canopy target of roughly 0.25–0.5 m/s, citing Nelson (2011). We have not read either original; we are citing them as that chapter does.
The practical version: one clip fan oscillating across part of a tent leaves most of the canopy in still air, and the leaves in the still part are the ones that stop transpiring. Which is the quiet joke in this whole question — the grower suspects the fan of causing the damage, and the fan is usually not doing enough.
And misting the leaves makes it worse rather than better, which is counterintuitive when they look scorched. Wet leaf surfaces in already-humid air suppress the evaporation that moves the calcium. If you are misting to treat tip damage, that is worth pausing on its own.
Why your VPD readout may be flattering you
Most controllers show a VPD number, and almost all of them work it out from the air temperature and humidity the box itself is sitting in. That is a fact about your tent. What decides whether a leaf transpires is the gradient at the leaf surface, and the leaf is not the same temperature as the air.
The best measurements we know of on that gap are Nelson & Bugbee (2015, PLOS ONE), who compared leaf and air temperature under sunlight, greenhouse light, HPS and LED. Two findings matter here, and the second is the one that gets repeated wrongly:
- Plants that were not water stressed held their leaves within about 2 °C of air temperature under all four light sources, and swapping HPS for LED moved it by only around 1.3 °C. The lamp is a small term.
- Under water stress with high light and low airflow, leaves ran hotter than the air — by around 8 °C under LED. The gap is not a fixed offset in one direction. It is small when the plant is fine and large, and reversed, when it is not.
You will see it claimed in a lot of grow guides that leaves run a couple of degrees cooler than air under LED, full stop, and that you can therefore correct your VPD with a fixed subtraction. That is not what the measurements show, and the direction it gets wrong is the one that matters: the moment a plant is actually in trouble, the offset points the other way. This is why we do not publish a correction factor, and why a spot reading with a twenty-dollar infrared thermometer is a snapshot of one leaf at one moment rather than an answer.
How to check this yourself
None of this needs equipment you do not have.
- Walk the airflow. Find what the fan actually reaches. Compare a leaf in the stream with one out of it, on the same plant, at the same height.
- Read what comes out of the pot. Not what you mixed going in. If it is low, stop considering over-feeding.
- Find the still part of the tent. A strip of light tissue or thread taped up at canopy height will show you in seconds where air is moving and where it is not. The corners are usually dead.
- Stop misting for a week if you are doing it, and change one thing at a time so you can tell what did it.
- Then judge by the new growth, not the damaged leaves. A burnt tip does not heal. The leaves that open after you change something are the only honest readout you have.
What would show that we have this wrong
If the new growth comes in with clean tips over the following week, whatever caused this is already behind you and nothing on this page needs acting on.
If your run-off EC comes back high, then it was a feeding problem after all, the second test did its job, and you should be flushing rather than reading about calcium transport.
If the damage is spreading into leaves that were fine yesterday, and it is spreading rather than sitting still, that is a different question — a moving problem is usually not an environmental one, and the pattern test is the better place to start.
And if the leaves are curling or folding rather than burning at the tips, this is the wrong page. Curl is a different question with a different test.
Where our device fits, stated narrowly
The awkward part of everything above is that the two things which decide it — how hot the leaf itself is, and whether air is moving where the plant is — are the two things a box on the tent wall cannot tell you. It reports the room, confidently, while the leaf a foot away is in a different set of conditions.
Croplock reads leaf-surface temperature from an infrared sensor in the canopy and steers on that reading rather than on a VPD worked out from air alone. When it cannot get a leaf reading it trusts, it falls back to air and records that it fell back, rather than inventing a leaf temperature it could not see.
That is a measurement, not a diagnosis. It does not tell you why your tips are burnt. It tells you whether the surface those tips are on is transpiring into the gradient you think it is, which is the part you otherwise have to guess. Every test above is the version you can run today with nothing but a meter and a piece of thread, and they get you most of the way.
Sources
- Barta, D.J. & Tibbitts, T.W. (1986). Effects of artificial enclosure of young lettuce leaves on tipburn incidence and leaf calcium concentration. Journal of the American Society for Horticultural Science 111(3): 413–416. PMID 11539770. Abstract and reported figures read; full text not read. Lettuce, not cannabis.
- Nelson, J.A. & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLOS ONE 10(10): e0138930.
- Matzneller, P., Gutierrez, G. & Caplan, D. “Canopy Management.” Ch. 7 in Cannabis in Controlled Environment Production (CRC Press). DOI 10.1201/9781003150442.
- Thom (1971) and Nelson (2011), on air-movement gradients into a canopy and on target air speed — both cited as they appear in the chapter above, not read in the original.
More of these at Answers. The device is at croplock.com.
Also read: pH or EC, which number is telling me something? · Curling leaves: heat or hunger? · Is it cal-mag? · Does VPD need leaf temperature?