Purple patches on some leaves

[Custom45]

Hi,
2 out of 4 plants have recently had a few top fan leaves turn purple in places, this only happens on upper leaves and the purple starts at the tip end and works inward. Grown this plant before and didn't have this.

-Can't remember name of the strain, but it's sativa I believe
-In week 4 flower, this started 2 weeks ago
-20l fabric pots
-Hand feeding from top with a little run off each day
-Coco/pebble mix (60/40)
-300w led lights currently at 100%, 15" from canopy
-Feeding shogun nutrients, using tap water with a little calmag added
-Using co2, was at 1100ppm in week 2, 1200 week 3 and 1300 now in week 4
-Temps around 28-30⁰c, 82-86⁰f, day and around 18-20⁰c, 64-68⁰f night
-Humidity trying to keep around 55-60 day and a little less at night
-Diffuseair fans mixing air plus a few floor fans blowing just below canopy

Think i was a bit too hasty with light strength and heat after turning on the co2, caused a bit of light and heat stress in late veg, early flower, but I don't think that's related to purple leaves?

Thanks!

 

[Zill]

How many weeks before you harvest?

 

[Custom45]

How many weeks before you harvest?

It's an 8 week strain, this started around week 2 of flower and I harvest in about 4 weeks.

 

[Zill]

One month. Purpling of leaves is usually an indication of phosphate deficiency. Lacking phosphate the plant has difficulty building things with the fixed carbon so it stores the carbon in molecules that are colorful, pigmented. Dense form of carbon.

One month out, the plant is beginning to shut down. Not sure if a shot of phosphate will help. But, a shot of phosphate won’t kill the plant either.

 

[Custom45]

I was just reading about the phosphorus deficiency and how that may be a cause.

Previously I always used Growth Technology pH down, which is phosphorus acid based, at the end of my last grow and for this entire grow I've switched to Canna pH down, which is citric based. I'm wondering if the Grow Tech pH down was putting phosphorus into the plant that it needed and switching has now left it deficient?

Added to fact I'm using co2 on this grow which I've read can cause plants to require more nutrients, I'm wondering if this is the issue. I'll switch back the phosphorous pH down and look at getting some phosphorous into them as quick as possible.

One thing I didn't mention, I was a bit disappointed with the growth stretch in the first 2 weeks of flower, they only made about 2/3 the height I expected them to, possibly related to the above.

 

[Zill]

You’ll be fine. One other point to consider is the genetics of the strain you're growing. Some of those cultivars are naturally intensely pigmented.

 

[Custom45]

I grew this strain last time and didn't get any purple leaves, so think its something I've done.

I think the pH down is a likely culprit as, come to think of it, I've had rusty spots appear here and there that looks like ph issues. Ive read citric acid can breakdown quicker and cause pH in medium to rise. I try to feed in range of pH 5.6-6.1 but my run off is always nearer 7.

 

[Zill]

That’s good to know, you have experience with this cultivar.

Cannabis can be infected with a rust fungus. Do the red spores wipe off with a white napkin? Diagnostic.

Citric acid lowers pH unless it’s formulated as a salt. Your pH isn’t too far off. But at high pH, approaching 7.0 phosphate becomes limiting.

What was the pH profile of your previous grow?

 

[Custom45]

No, doesn't wipe off, it's the leaves themselves that have turned purple.

I did read that it doesn't hold it stable for as long as phosphorus acid based pH downs do, I was wondering if there was something in the Coco and that causing the pH to rise and slow the uptake of phosphorous.

I fed the same pH range in the last grow.

 

[Zill]

Hey custom,

I grow outside in my vegetable garden. That coco medium is tricky. There is very little nutrient or pH buffer capacity in coco.

I found this article on coco. This light help,

How to Grow Cannabis with Coco Coir | Grow Weed Easy

Ever wondered how to use coco coir blocks to make a cannabis-friendly rich potting mix? This tutorial breaks it down...

www.growweedeasy.com