LED lights too close?

I have been fighting a battle here - and I think I finally might have gotten to the bottom of it. Need input from you guys!!

I have 4 plants in a 3x3 tent. All of them grew nicely throughout veg ; but about 4 weeks into flower, the white widow started showing signs of deficiencies. Small, dotted rust spots on the top fan leaves were the first sign, so I started looking at immobile nutrient deficiencies.

That research sent me down a pretty complicated path - to the point where I thought that it was a magnesium surplus locking out phosphorous.

I flushed the plant and tested the runoff - 6.4pH and about 700ppm. I use Happy Frog and my own tap water. The tap water comes out at about 230 - 250ppm ; so I figured the ppm isn’t all that out of whack and pH was almost dead on where it needs to be - so maybe nutrients aren’t the issue?

The rust spots had spread pretty quickly throughout the top of the widow’s canopy at this point. And, worse yet, 2 other plants were starting to show the rust spots. I tested the run off on those 2 plants and got pretty much the same results as the widow. To make it even more odd, I tested the run off of the 4th plant that had no rust spots and was just cruising through fantastically, and found that it also tested at the same levels as the other 3. So I was even more sure whatever the issue is does not seem to be water, pH, or ppm related.

I started digging deeper into my environment and came across an article on LED lighting and the distance required from the top of the plant to prevent leaf bleaching/burning. I use 2 1000w full spectrum LED panels in my flower tent. Which could very possibly be overkill for the space I am using, but using just one of them didn’t seem to cover the entire area.

I measured the distance between the light and the top of the canopy and it was just under 10 inches. The manufacturer recommendation is >18in. So, I am turning to the forum here on ILGM.

Could the distance between the light and the tops of the plants be the issue? If so, there isn’t much I can do about it since the tent itself is only 72in tall and the plants are around 55in. Any suggestions?

Still not sure why it is only effecting White Widow, Gold Leaf, and Chronic Widow - while the Cali Dream is perfectly fine. They are the same height for the most part.



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How old are your plants
In flowwryyou shpuld be feeding in the 1500 ppm range
Ph seem ok

They are in the middle of week 5 of flower.

I usually feed around 1100ppm at this point, using the General Organics BioThrive line. That comes out to about 800ppm nutrients plus the +/- 250ppm of the tap water.

I did back the feed down for the last 2 weeks, when the rust spots first started - cut down to about 700ppm total.

I am going to kick the feed back up to normal next week.

Do you think the 1100ppm needs upped to closer to 1500?

Looks like calcium deficiency. I’m not familiar with calcium content on your line, could be plenty and just from running lower ppm.

Where you’re at in grow I would look for something nitrogen free if you decide to supplement.

I am pretty sure I had this same problem and fixed it by increasing my feeding. I am almost positive I narrowed it down to a phosphorous issue.


Your lights are too close BTW… 18-20” air gap is recommended

Me to. It was phosphorus and feeding a high dose of micro nutes cured it

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