Grow Q&A: Stems and Leaves Turning Purple

Dear Dan,
I recently moved to a state with medical marijuana, (gotta love it!) and I’ve been trying like hell to become a producer of high quality medicine. I have recently harvested my second crop and it was full of very sticky buds, however there is still lots of room for improvement.
I think my biggest problem is purple. I have a recurring problem where first the leaf stems turn purple and then I get purple stripes on the main stems. On one plant, all of the stems turned dark purple so I threw it away. However most have stripes here and there, grow well and recently still produced very smokeable sticky buds.
What the hell am I doing wrong?  Am I feeding them too much? I flush and monitor my PPM’s and pH. I grow in Pro-Mix and keep my pH around 6.5. I keep my EC 1.6 during veg, and flush every 3-4 weeks. I use T-5’s for veg and I grow in a basement.
I just started a new grow (with fresh different seeds) and within two weeks, I have signs of these purple stripes again on my main stems. Is this purple a disease or fungus or is this just from improper care? – Purple Hazed in Michigan

Dear Hazed,
I believe what you have is a phosphorus deficiency. When plants lack phosphorus, purpling or reddening of the leaves and stems can occur. Signs usually begin in lower leaves and branches and slowly work their way up the plant.

Bear in mind that some plants naturally turn red or purple due to their genetics but if you keep having this problem with many different strains, it’s probably not due to genes. Overwatering and cold temperatures can also exacerbate the problem so make sure those aren’t an issue for you before trying to add more nutes.

Phosphorus is the “P” in NPK (Nitrogen, Phosphorus, Potassium) meaning it’s a major nutrient needed by the plant to thrive. Most fertilizers made for the flowering stage will contain plenty of P and sometimes growers will mistake a pH imbalance for a deficiency, although it sounds like you check pH often so in your case I think it’s truly a lack of Phosphorus.

In order to treat this deficiency, you need to add nutrients with phosphorus. Organic forms of these include worm castings, bat guano, bone and blood meal and crab shells among others. If using bottled solution, use something where the NPK shows high levels of P. Most flowering or blooming nutrients meant for cannabis contain plenty of P.  You should see the problem clear up within a week or so with new growth looking lush and green.

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