Yellowing top leaves 2 weeks into flower

Hoping someone can help me with my problem.
This is only my 2nd grow so i'm a newbie
My 2 plants are at 14 days since i flipped to 12/12
The top leaves are turning yellow but the middle of the leaves are still green, any ideas what is causing this?
50/50 Coco/perlite mix
Tap water at 110ppm, canna coco a/b, rhizotonic, cannazym, boost, magne-cal+ coming out to 1200ppm
ph is 5.8
600w hps at 16" in a cooltube
Temps 24c day 20c night


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I'm guessing def by lockout(then flush/fix ur pH) or by lack of feeding, which u will bump u nutes...when was last time u flushed?

Have you moved your lights closer when you flipped to 12/12?

What else did you change apart from timing !?
Cannabis leaves and roots show abnormal and slowed growth when showing a boron defiency. The first signs of a cannabis boron deficiency is abnormal or thick growth tips. New leaves may display chlorosis (yellowing of leaves). Stems may become rough or hollow. The plant may look like it has a calcium deficiency because boron is needed for the plant to properly use calcium. New growth is affected the most, and may look like it's been burnt or scorched.
=> Boron is not absorbed will without enough potassium and nitrogen, or if there isn't enough moisture. You plant may also exhibit signs of a boron deficiency if the pH is too high or low. Flush your system with clean, pH'd water that contains about half the nutrients needed for the plant and watch to make sure that the problem clears up for new growth. Cheers !!

Bublonichronic said:

I'm guessing def by lockout(then flush/fix ur pH) or by lack of feeding, which u will bump u nutes...when was last time u flushed?

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Sorry i forgot to put my ph, it is 5.8 exactly every time i feed, currently every 2 days.
I feed to 20% run off (2litres in 400ml out) which i have checked and it is coming out at 1050ppm 6.0ph so i haven't flushed at all.
Using canna coco so it is the same a/b mix in veg and flower (although it is at a higher ppm for flowering) so i didn't flush when i changed to 12/12.

spliffbuddy said:

Have you moved your lights closer when you flipped to 12/12?

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Yes i did but that was 2 weeks ago and this seems to have happened in the last 3-4 days, i'm sure its not heat related as i have 2 thermometers at canopy level (1 over each plant) to check temps

chairroller said:

What else did you change apart from timing !?

Click to expand...

I upped the feed from 1000ppm to 1100 and then to 1200 over about a week and thats where it is now.

chairroller said:

Cannabis leaves and roots show abnormal and slowed growth when showing a boron defiency. The first signs of a cannabis boron deficiency is abnormal or thick growth tips. New leaves may display chlorosis (yellowing of leaves). Stems may become rough or hollow. The plant may look like it has a calcium deficiency because boron is needed for the plant to properly use calcium. New growth is affected the most, and may look like it's been burnt or scorched.

Click to expand...

I have no abnormal growth and no burnt or scorched looking leaves, in fact the rest of the plant looks absolutely fine and is growing at a reasonable rate.

A couple of people have mentioned mg def, i am adding magne-cal+ to my water but only a small amount (tap is 110ppm and i bring it up to 200ppm with the magne-cal+)
Is it worth upping the dosage to bring it to 300ppm and drop the base nutes slightly as they don't look hungry at 1200ppm (1.7ec)?

reddiamond said:

I upped the feed from 1000ppm to 1100 and then to 1200 over about a week and thats where it is now.


I have no abnormal growth and no burnt or scorched looking leaves, in fact the rest of the plant looks absolutely fine and is growing at a reasonable rate.

Click to expand...

Still not really sure but read this and compare with your case :
Cannabis leaves and roots show abnormal and slowed growth when showing a boron defiency. The first signs of a cannabis boron deficiency is abnormal or thick growth tips. New leaves may display chlorosis (yellowing of leaves). Stems may become rough or hollow. The plant may look like it has a calcium deficiency because boron is needed for the plant to properly use calcium. New growth is affected the most, and may look like it's been burnt or scorched.

What to do :
Boron is not absorbed will without enough potassium and nitrogen, or if there isn't enough moisture. You plant may also exhibit signs of a boron deficiency if the pH is too high or low. Flush your system with clean, pH'd water that contains about half the nutrients needed for the plant and watch to make sure that the problem clears up for new growth.

chairroller said:

Still not really sure but read this and compare with your case :
Cannabis leaves and roots show abnormal and slowed growth when showing a boron defiency.

Click to expand...

I have no abnormal or slowed growth

chairroller said:

The first signs of a cannabis boron deficiency is abnormal or thick growth tips. New leaves may display chlorosis (yellowing of leaves). Stems may become rough or hollow.

Click to expand...

The growth tips are fine and the stems are not rough.

chairroller said:

The plant may look like it has a calcium deficiency because boron is needed for the plant to properly use calcium. New growth is affected the most, and may look like it's been burnt or scorched.

Click to expand...

They do not look burnt or scorched.

Thanks but i'm pretty sure its not boron def.

I'd forget about all the deficiency stuff and move your lights back to about 22 inches. Think this may be the problem. I accept they are not scorched, but may be start of bleaching.

spliffbuddy said:

I'd forget about all the deficiency stuff and move your lights back to about 22 inches. Think this may be the problem. I accept they are not scorched, but may be start of bleaching.

Click to expand...

Thanks for the input, i was going from this chart when i set my lamp height and also put it in a cooltube to keep temps down and make sure they didn't get heat stress. If it is bleaching i'm assuming the colour won't come back when i lift the light but it shouldn't get worse .. is that correct?

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