Should You Worry About Purple Or Red Cannabis Stems?

Red or purple stems in cannabis can be a sign of stress, nutrient deficiency, pathogens, or simply genetics. Keep reading to learn when to be concerned about red stems, and when not to.

Red or purple stems caused by genetics or light

You might be alarmed the first time you spot red or purple stems on your cannabis plants. But you don’t always need to be. There are countless strains of cannabis on the planet, and some of them are capable of developing incredible pigmentation.

Purple strains with very dark foliage, for example, often develop purple stems too, especially if you grow them in slightly cooler temperatures. As long as your plants look otherwise healthy, there’s usually no reason to worry.

Other than genetics, another major cause behind discoloured stems is strong light exposure. If you use training methods like LST or defoliation, you may notice the exposed stems of your plants turning red, pink, or purple over time. This is completely normal and shouldn’t be cause for concern as long as your lights are sufficiently distanced and your plants look otherwise healthy.

Red or purple stems caused by stress, nutrient deficiency, or the environment

Unfortunately, red or purple stems in cannabis can also be a sign of stress. When this is the case, your plants will usually experience some other symptoms that can help you narrow down the root cause of the problem.

1. Pathogens

Some fungi, such as Fusarium or Botrytis, can affect the colour of your plants' stems and foliage. Fusarium affects seedlings, attacking their stems and eventually causing them to topple over or "damp off". It can sometimes make stems appear dark brown, red, or slightly purple. Botrytis, on the other hand, affects larger plants and forms a characteristic brown line along the affected stems and branches, robbing them of nutrients and causing them to die.

Both Fusarium and Botrytis can spread from one plant to another and should be taken care of as soon as possible. Check out our previous posts on Fusarium and cannabis moulds for more information on how to deal with these deadly pathogens.

2. Nutrient deficiency

Nutrient deficiencies cause a wide range of symptoms, including yellowing/drying new and old foliage, and discoloured stems and leaves. In particular, some of the early signs of a phosphorous deficiency include red and purple stems, followed by brown, dried out leaves. Magnesium deficiencies, on the other hand, tend to cause petioles (the stalks attaching a leaf to a stem) to turn red.

If you’re worried your plants might be dealing with a nutrient deficiency, check out this post for pictures, descriptions, and easy-to-follow steps on how to identify and cure every cannabis nutrient deficiency.

Remember, nutrient deficiencies don’t just cause discoloured stems; they also cause signs of stunted growth, damaged foliage, and more. Make sure to identify the type of deficiency affecting your plants using our guide and rectify it as soon as possible.

3. A note on pH

Getting the pH of your soil and nutrient solution right is essential for healthy plants. Unfortunately, it’s also something a lot of growers struggle with.

Cannabis likes slightly acidic soil (we recommend keeping it at 6.5 for best results). If your soil is either too acidic or too alkaline, your plants can lose healthy foliage and develop nutrient deficiencies as they struggle to uptake nutrients from your fertilisers. If your plants have red or purple stems and other signs of a nutrient issue, make sure to check your pH levels.

For a clear picture of the pH of your soil, we recommend investing in pH and conductivity testers. These tools provide accurate readings of your soil pH as well as the electrical conductivity of your nutrients, meaning you’ll be able to see just how well your plants are absorbing their fertiliser.

4. Temperature fluctuations

Abrupt changes in temperature can often cause changes in pigmentation in cannabis plants. Cool nighttime temperatures in particular can lead your plants to develop dark red or purple foliage and stems. This is even more common in purple strains; in fact, growers often expose purple strains to cooler nighttime temperatures on purpose to really highlight the purple gene.

If your plants turn red or purple after a particularly cold night, pay close attention to them over the following days. If they continue to grow normally, then there’s nothing to worry about. If, however, you notice slowed growth or other signs of stress, bring up the temperature in your grow room (or consider moving your plants indoors if you’re outdoors).

5. Stress

Cannabis plants react to stress in myriad ways. Sometimes, discoloured stems can be a sign of transplant shock, heat stress, overwatering, or even a bug infestation. That’s why, if you spot purple or red stems, it’s important you pay close attention to identify the root cause of the discolouration.

Below, you’ll find a checklist of potential stressors that might be causing your plants to develop red or purple stems:

🌱 Root shock 

Transplanting comes as a big shock to the root system. If your plants develop red or purple stems after being transplanted, a good dose of TLC should help them recover quickly.

🕷 Pests or plagues

Some cannabis strains are more prone to pests and plagues than others. If your plants have discoloured stems and also suffer from damaged foliage, stunted growth, and signs of mildew, gnats, or spiders, you’ll need to act quickly. Check out this post on common cannabis ailments for more info on how to spot and treat common cannabis pests.

🌡Temperature or humidity issues

If the temperature or humidity levels in your grow room feel off, that may be part of your problem. Adjust temperature/humidity if you suspect they could be stressing your plant.

💡 Light/heat stress

While cannabis loves warm weather and plenty of sun, too much heat or light can damage it. If your cannabis plants develop burnt or yellow foliage, bleaching, or curled leaves a few days after you first notice their discoloured stems, they might be dealing with light or heat stress. Be quick to address these issues, as they can have a devastating effect on your plants and greatly reduce the size and quality of your yield.

Getting to the bottom of red and purple cannabis stems

As we saw earlier, red or purple cannabis stems aren’t necessarily a cause for concern. If your plants suddenly develop discoloured stems, remember to monitor them closely and look out for other symptoms that the discolouration isn’t caused by genetics or light exposure. Also, visit the growing section of our blog for more detailed articles on how to grow cannabis, deal with nutrient deficiencies, pests, heat stress, and more.