Stems of cannabis plants turning purple, how do I fix this?

by Nebula Haze

Question: Help! My cannabis plants are young (growing in soil indoors with sunlight and my leaves are medium sized) yet my main stems are turning purple from the top downwards. Have you seen this before?

Answer: When a cannabis plant or seedling has a purple stem without any other signs of problems, it is often the result of genetics. However, sometimes you’ll see red stems (pictured below) which are often caused by light exposure (kind of like a “tan”), nutrient deficiencies, various types of stress, poor environment, or pH problems.

Purple stem on cannabis plant - is this a nutrient problem, how do I fix it?

Wait a week or two before making any alterations if your plant otherwise appears healthy and is growing fast. 

If there are no other symptoms and the rest of the plant is growing fast and healthy, red stems usually aren’t a big deal. As long as you’re continuing to take good care of your plant and responding quickly to any other problems, you should be okay.

These red stems were caused by transplant shock and overall stress, but the clone just needed some TLC and time to grow out of it.

Example of a clone with red stems

Truly purple stems are often caused by genetics

Example of healthy purple stems caused by genetics - this strain of cannabis just grows this way normally

If you believe there is a nutrient problem with your cannabis plant, for example, if your plant is also growing slowly or the leaves are discolored, then the plant may be suffering from…

  • Incorrect pH – The pH being too high or too low at the roots is the most common reason to see a nutrient deficiency)
  • Magnesium deficiency can cause purple stems. A good cannabis-friendly nutrient system has a significant amount of magnesium, but if you’re using very soft water, or RO water, you may need to add a Cal-Mag to your nutrient regimen.
  • Temperature – Heat or especially cold can trigger red stems in some places. Big temperature fluctuations can also do it even if the temperature is in a good range.
  • Humidity – Very high or low humidity can stress plants (especially young seedlings)
  • Other types of stress such as light burn, transplant shock, or overwatering can trigger red stems
  • Sometimes you’ll never know why and plants just grow out of it 🙂



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