Why do my cannabis plants have red or purple stems?

In the world of cannabis, there’s a common curiosity amongst growers as to why the stems of the plants turn red or purple. This phenomenon, which can often be worrying, is either a simple natural occurrence or actually a sign of stress, so understanding its cause is essential to obtaining healthy and productive plants. Here we will explore the various factors that can cause the appearance of red or purple stems in marijuana plants.

While cannabis plants are usually lush green, they can also exhibit other beautiful colours, ranging from red or purple to pink or even black. In many cases, this isn’t a cause for concern. Nevertheless, the colour of the stems and petioles (the small stems that connect the leaves to the main stem of the plant) might be something worth investigating, especially in strains that are not genetically predisposed to developing these colours.

In this comprehensive guide, we will delve into the various reasons behind the development of these hues on the stems, offering practical solutions and helping you to determine when it’s really time to worry.

What causes red or purple cannabis stems?

First of all, it’s important to acknowledge that genetics plays a crucial role in the appearance of cannabis plants. And certain strains are known for developing red or violet hues on their stems, leaves and buds. These stems may be perceived as ‘red’, although they lean more towards purple rather than ‘bright red’.

This is a natural feature; so, if you’re growing a strain that’s known for these characteristics, you can breathe easier knowing this is just part of its unique genetic makeup. For instance, varieties like Purple Kush, Gelato-K, or Swiss Dream Rosé Auto CBD are renowned for their vibrant colours, which can extend all the way to their stems.

Also, as cannabis plants ripen and focus their energy on producing buds, it is normal for them to develop reddish or violet hues on their stems. In this case, this is a sign of healthy growth and not a cause for concern, as changes in the expression of anthocyanins and other pigments lead to the appearance of red or purple stems. Anthocyanins are a type of flavonoid pigment which is responsible for the red, purple and blue colours that appear in a great variety of flowers, fruits and, in this case, in the stems of cannabis plants.

There are several reasons why the stems can turn purple or red, beyond the genetic expression or natural maturity of the plants, so this could also be a symptom of an underlying problem. To differentiate between them, the stems that are naturally red usually have a uniform colour all over the plant. On the contrary, problematic red stems may show discoloration in patches or be accompanied by other symptoms like withered or yellowish leaves.

Next, we’ll explore the causes and solutions to this type of red or purple stems in cannabis plants.

1- Light exposure

If a stem is fully exposed to intense direct light, it will often turn red, pink or purple over time. If this is the cause, you’ll probably notice that any part of the stem that is in the shade will remain green. This is actually a plant defence mechanism to protect the stems from excessive light (the same as for tanning in humans). This type of red colouring is especially common with grow lights that produce a lot of ultraviolet light, such as LEC lamps.

In addition, with the rise of training techniques that expose the stems to maximum lighting, such as SCROG, LST or defoliation, red stems caused by direct light are more common than ever. So you probably have nothing to worry about in this case, as long as you don’t notice any other symptoms and the plants are growing fast and healthy.

• Solution – Since too much light intensity can cause red stems and other stress symptoms, it is always a good idea to keep the grow lights at the recommended distance at each stage of development. LED lamps are especially effective for placing closer to the canopy, but if for whatever reason you can’t move them further away, just dim them a little.

2- Temperature fluctuations

Temperature changes can cause parts of the cannabis plant to acquire colours other than green, especially in genetics that are already predisposed to express those shades. In fact, some growers deliberately expose their plants to cooler temperatures at night to highlight these colourful phenos.

Changes of about 5 to 10 degrees centigrade (between day and night) can cause the specimens to develop these shades. In general, this is nothing to worry about, but if it’s accompanied by slow or stunted development, it may be too cold for the plants to grow properly.

• Solution – Fixing this is rather simple: Keep temperatures constant between day and night. Or, if you’d rather lower the night temperature a little, make sure the change is not too dramatic. Keep in mind that cannabis plants start to have problems at temperatures below 15°C, and really start to come to a halt at 10°C.

3- Nutrient deficiency

Nutrient deficiency, either because of inadequate feeding or nutrient lockout, can cause stems to discolour and often turn red. In this case, this is a clear indication of a problem. To confirm it, you also need to look at the leaves: if they’re turning yellow or look wilted, you may need to take steps to stop the health of the plants from quickly plummeting.

• Solution – If you think your plants are suffering from a deficiency caused by a lack of nutrition, then the solution is quite simple: just give them more food. Magnesium and phosphorus seem to be specifically related to the reddening of the stems, so starting with these elements might be a good way to go. Nitrogen and potassium can also have an influence, even if it is to a lesser extent. Don’t be too enthusiastic, though: more doesn’t always mean better when it comes to fertilising cannabis. It is often better to add it little by little and see how your plants react.

4- pH levels

When pH levels are too far off in either direction, the plant’s ability to absorb food through its roots can be reduced or even become completely disrupted. This will lead to nutrient lockout and to the same symptoms mentioned previously, including red and purple stems. You may also spot other symptoms of deficiency, such as a yellowish colour, brown leaf edges, curly leaves, burned tips, etc.

• Solution – You need to make sure that your substrate or irrigation water is at the optimal pH level. With a pH of 6.5, you’ll see positive results in no time. We suggest using tools to test conductivity and pH to see if your plant is getting its nutrients correctly. Use a bottle of ‘pH Down’ to lower the pH, and a ‘pH Up’ solution to raise it: these liquid supplements can be used to adjust the pH simply by adding them directly to the water.

5- Plant stress

Cannabis plants react to stress in an unpredictable way, and all sorts of stressors can cause the stems of a plant to turn red or purple. Excess water, insufficient irrigation, pest infestations… The list goes on and on. These issues can have direct or indirect effects that cause discoloration of the stem and leaves. But how can you determine the specific cause and solution?

• Solution – Most forms of stress also involve other signs and symptoms. For instance, if you add excessive amounts of water, the lower leaves will fade first; whereas if you water too little, they will fade evenly and become crunchy throughout the plant. If you spot small creatures either on or around the plants and the stems are fading, then the culprit may be a pest (for example, grasshoppers). Root shock can also occur when transplanting cannabis. If you notice plants with purple stems after transplanting, take great care to help them recover quickly.

6- Pathogens

Parasitic fungi like Fusarium or Pythium can cause the stems of cannabis plants to turn red or purple. Far from being a desirable aesthetic feature, this can be extremely problematic, since this coloration is caused by necrosis of the vascular system… which certainly isn’t a good thing! This symptom is one of the last to become visible, preceded by spots on the leaves, which may also appear curved and wilted.

• Solution – Sorry to be the bearers of bad news, but this type of fungi doesn’t have a simple cure. Once they’ve settled in, there is little you can do about it, other than getting rid of the affected plants. However, there are certain measures you can take to prevent these parasites from taking over. A plant with a healthy immune system will be much better prepared to fight a root fungal infection than a weak plant.

7- Additional environmental factors

Other environmental factors, such as air quality and mechanical stress, can also affect the health and colour of the stems of marijuana plants. An environment with adequate CO2 levels can promote vigorous growth, whilst a lack of CO2 can stress the plants. Mechanical stress, such as that caused by strong winds or rough handling, can damage the plants and result in the production of anthocyanin.

• Solution – If you grow indoors, use fans to ensure proper air circulation, but try to avoid excessively strong aeration. Outdoors, consider using protection against strong winds. You also need to treat your plants gently during pruning or transplanting to avoid unnecessary damage.

Should you be worried about red or purple stems?

Spotting stems with this colour on your cannabis plants for the first time can be scary, but now that you know the causes and how to treat them effectively, you will quickly realise that it is not such a big deal. As you advance in the world of cannabis growing, you’ll learn that observing and understanding your plants’ responses to different stimuli is as essential for the health of your plants as the other tasks you perform.

With your knowledge and experience, you’ll enjoy fruitful harvests for a very long time. And if you’re looking for seeds that grow into red- or purple-stemmed plants, then just take a look at our catalogue and choose the one that best suits your aesthetic taste. They can certainly give your grow the colour boost it deserves!