Leaf tip curling sideways, canoeing – reoccuring problem

[Withzerai]

Hi growers,
I need your help with my plants.

I’ve been dealing with this issue for a while now. Sometimes it’s worse, sometimes not as bad, but it’s always there – new growth comes in twisted, the tips fold up like a taco, and the leaf edges curl upward.

On my last run it got so bad I scrapped all the plants, thinking it was a russet mite infestation. I replaced everything I could: new watering hoses, pots, carbon filters, HEPA filters. I bleached the room, heated it to 40 °C (104 F) for two days, then bleached again.

This time I switched from soil to coco so I could be 100% sure what I’m feeding, hoping the problem would disappear. Unfortunately, it’s back again.

Setup:
Sealed grow room with a 2.4 × 2.4 m tent inside. The room works like a lung room with AC and dehumidifiers.
Added purge fans that exchange the air every hour for 15 min.
Inside the tent: scrubber, humidifier, and bottled CO₂.
Four MarsHydro TS3000s – one for each 1.2 × 1.2 quadrant.
Medium & pots:
Clones started in 0.5 L plastic pots.
Transplanted to 11 L fabric pots sitting on pot risers.
70% coco / 30% perlite mix.
After transplant they looked rough (pale green with purple stems) for about a week. Once roots poked out the bottom of the pot (1 week after transplant) they perked up. Currently at the end of week 4 veg.
Watering:
Running RO (my well water is trash at 0.7 EC, i don’t even drink that sh*t). Starting at 0.0 EC.
Add CalMag up to 0.3 EC, then Canna A+B to 1.3 EC total.
pH down to 5.8.
Watering twice daily (0.5 L at lights on, 0.5 L mid-day).
Solution temp ~23 °C.
Runoff is heavy, around 0.8 EC and ~6.5 pH.
Coco never dries out, I keep it saturated. Algae growing on the perlite top layer.
Environment:
CO₂ ~800 ppm
Temps: 27–28 °C day / 23–24 °C night
RH: 68% day / 60% night
Leaf temp usually 2.5–3 °C below air temp
VPD ~1.1 kPa
Lights at 45% intensity, 0.9 m above canopy
The tent extractor ramps up with temp/humidity, minimum speed is set to 40%. Two oscillating fans blow across the lights (not directly at the plants).
I’ve checked for pests for hours with 50× and 1000× scopes – nothing. I add BTI to feeding solution occasionally to prevent fungus gnats.
The canoeing happens even on lower growth, so I thought it wasn’t light stress. But I’ve noticed about an hour before lights out the leaves fold even more. I’ve since dropped intensity to 30% with the lights at the same height.

The previous run that i’ve ditched was in soil with Biotabs fertilizers, I’ve managed to find a photo – as you can see the sideways hooking of the leaves is exavtly the same as i have now.

All input is appreciated, thank you.

[jaymoney]

Greetings,

After reviewing everything, here’s what I believe is happening: the additional calcium and magnesium are causing the issue. Let me explain.

Potassium and calcium should be in a 1:1 ratio. Canna A/B already balances this. If you add more calcium or magnesium, it throws off the balance of water regulation. Calcium is immobile and typically shows up as deformation in new growth — not twisting edges,  Your new growth looks fine.

What you’re seeing is serrated edges curling, which is a sign of potassium imbalance. What else shows up with further potassium deficiency? Yellowing along the leaf margins, further and it will show up as necrosis

calcium and magnesium levels are too high. These are positively charged cations and they compete with potassium for uptake in the root zone.

Too much calcium results in potassium deficiency, which prevents water from reaching the edges of the leaves — they dry out and curl.

My suggestion:

#1  Stop adding calcium and magnesium

#2  Increase your total EC to 1.5 using Canna A/B — and add a micro nutrient that includes boron regular dose

#3  Lower PH to 5.8 ( coco and perlite is a highly aerated low CEC media )(PRO TIP!) more air lower CEC lower ph needed, less air higher CEC higher ph needed

#4  Lower watering to 10% runoff. Heavy runoff strips beneficial microbes (which exist in all plants, regardless of media)

#5 Raise your lights as high as possible and increase the intensity to 15,000 Lux. This will give you more even light distribution.

let me know if you have any questions! there’s always more to learn lol

[Author]

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