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.

Have a grow question? Ask away at
  1. My plants have stretched almost 8 feet tall. My new growth where the buds will soon form is growing in with purple to black spots and leaves. Is this a fungus this is my first time growing outdoors. The plants as a whole look healthy as I said have grown almost 3 feet in a short period of time. Is this a nutrient problem or a bigger issue

    1. Me too!! I’ve been researching all over the internet for 2 weeks trying to diagnose and treat the issue before it does any further damage to my plants I have one that’s about 6 feet and one that’s about 8 feet I’m growing outdoors in the ground I dug 3×3 areas and filled them with pro mox soil mixed with peat moss I live in southwestern Michigan I’ve been feeding my plants basic miracle grow for the veg stage I just started feeding it new nutrients about 2-3 weeks ago and this problem has only gotten worse since I’m planning on flushing them but I feel like that will take forever considering they’re in the ground I’m lost please any advice would be greatly appreciated!

  2. Lights are too intense or close for the size of plant u have that’s why problem starts up top not bottom like it would with Phosphorus deficiency. Can also be that’s you don’t have enough magnesium for the amount of light. Or can be plain overwatering it kills the root hairs so plant can’t take up enough magnesium to convert light, water, and co2 into sugar and oxygen. Hope this helps. Keep t5 light minimum 30 inches from top of plants. Any other types of lights should be up high to the top of the tent. Good luck use common sense while growing

  3. A few of mine are showing the same thing. Last year I lost most of my crop to it. It always starts around the buds and creeps up the leaves. It’s dark purple and my PH is 6.5-6.7 I’m growing outside. I need to know what it is in order to treat it. I’ve bought a couple kinds of organic 3 way but hasn’t been very effective. I’ve been cutting them off.

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