What Makes Weed Turn Purple?

Updated August 21, 2022

Anyone who has spent at least some time looking at cannabis strains, either in person or online, will likely notice that they come in a few different colors. While most are green, some cannabis strains can also be purple.

Given this dramatic difference in color, it's natural to wonder why certain strains are purple. This comes down to several factors, including the plant’s biology, genetics, along with the climate they're grown in. The quick answer is that some strains turn purple due to anthocyanin, but there is much more to this.

Understanding Anthocyanin

Anthocyanin is the key differentiator between cannabis strains that turn purple and those that don't. This water-soluble vacuolar pigment forms within the final few weeks of a cannabis plant’s flowering stages. The levels of chlorophyll must decrease for the levels of anthocyanin to be higher enough and for the color changes to take place. Anthocyanins are one of the flavonoids found in cannabis.

It's an Antioxidant

While anthocyanin is responsible for the purple color in some cannabis plants, that isn't the only role that it plays. Anthocyanin also has evolutionary benefits for plants and health benefits for people. In fact, anthocyanin is an antioxidant, which means it can boost overall immune system and bodily health in humans.

They Are in Numerous Plants

There are actually 400 different types of water-soluble anthocyanin, and cannabis is far from the only plant to contain it. It is also commonly found in citrus plants, including blood oranges. In fact, this is the component of blood oranges that give the fruit its unique color compared to its citrus relatives. Anthocyanins are also found in eggplants, blueberries, concord grapes, violets, and red cabbage. It's also these anthocyanins that cause leaves to change their colors in the fall.

With that in mind, let's look at the other factors that determine whether a cannabis plant turns purple.

How Anthocyanins Benefit Plants

Since anthocyanin is present in so many different plants, it must have an evolutionary purpose, right? It actually has two main purposes.

One of these is increasing the ability to attract pollinators to the plant, including butterflies, bees, and hummingbirds. Anthocyanins can also convince some harmful insects that the plant is unhealthy or dangerous, discouraging potentially harmful insects from visiting or destroying it.

Additionally, anthocyanins protect the photosynthetic tissue in plants from extreme light conditions.

The Grow Climate

The climate where the cannabis plant grows has a very strong impact on whether the anthocyanin in the plant causes it to turn purple. Specifically, the climate must change as the growing stages of the plant do.

Less Daylight

As the growing cycle comes to an end, the concentrations of anthocyanin increase as daylight exposure decreases.

Cooler Temperatures

Cooler temperatures also play a role in increasing anthocyanin production, especially when combined with cool lighting.

The Cool Must Come at the End

The caveat here is that these limits to daylight and lower temperatures must come at the end of the growing cycle. During the early development of cannabis, it needs to have a warmer climate. For best results, these changes should be applied just a few weeks before harvest occurs.

Example Temperatures

To provide a frame of reference, consider that the growing temperature that allows for the best results when growing cannabis will be from 22°C to 25°C (71.6°F to 77°F ). That being said, cannabis plants can still photosynthesize and grow at temperatures of 15°C (59°F). Even so, you should try to keep the temperature in your grow room above 20°C (68°F).

You can also drop the temperature in the grow room when it is night. Only do this when the plants are in the flowering stage. You could ideally drop the temperature to about 10 degrees Celsius (50 degrees Fahrenheit), just overnight. This should maximize your chances of getting the purple you want from the cannabis plant without harming it. You would want to reduce the temperature nice and slow, as this helps prevent shocking the plants.

If you do choose to drop the temperature this low at night, pay extra attention to your plants. Remember that these lower temperatures will be stressful, so you may have to deal with a color that is not purple if you want your plant to be healthy. You should always prioritize the health of the plant over the color.

PH Levels

PH levels also play a role in anthocyanin production, which leads to the production of purple. Research conducted in 2006 and published in the Pakistan Journal of Nutrition discovered that higher pH levels tend to kill off anthocyanins.

As such, those who want to encourage the purple coloring on cannabis plants should opt for pH levels that are either neutral or a bit acidic.

Creating the Right Environment Artificially

Assuming you grow your cannabis indoors, then you can make the appropriate climatic changes to encourage the production of anthocyanin and the change of color to purple. All you have to do is ensure you control the lighting properly, taking care to make the appropriate changes a few weeks before harvest.

At this point, consider opting for blue lighting. That color light has a lower temperature, which will encourage anthocyanin production.

Genetics Matter

Not all cannabis plants will automatically turn purple if they grow in the right conditions, such as lighting and climate. Some genetic factors are also incredibly important.

Essentially, the strain must be able to produce high anthocyanin levels. You can tell whether this is the case for a given strain by doing some quick research online or looking at photos to tell its color.

Look at Parent Strains

When creating a hybrid or looking at information about a hybrid strain, having a parent plant that has high anthocyanin levels increases the chances of the child strain having the same.

Some examples of purple strains that are incredibly popular include Blueberry, Granddaddy Purple, Purple Urkle, Purple Trainwreck, Purple Kush, Blackberry, Blueberry Kush, and Purple Haze.

Other Colors Occur, as Well

In addition to the potential for purple cannabis buds, the presence of anthocyanin can also cause red or blue hues to appear in the plant. This typically occurs via the same process as the plant turning purple. It simply does not occur to the same extent, resulting in the plant not turning completely purple.

There are also various shades of the purple color. The intensity of the purple depends on the anthocyanin levels, but all purples tend to be visually appealing.

The Influence of pH

The variations between purple, blue, and red typically occur due to differences in the pH level that the plant gets exposed to. Acidic environments tend to give you pink or red buds, while purple tends to occur in environments with a neutral pH, blue occurs with a higher pH, and yellow occurs in alkaline conditions. However, those distinctions all assume that the plant in question contains high anthocyanins.

What About Gold and Yellow?

If you are curious about the other colors of cannabis later in their growth cycle, specifically the yellow and earthy gold, these are due to carotenoids. As with plants that turn purple, cannabis strains that turn gold or yellow will do so near the end of the growth cycle.

Theories to Avoid

There are also some other theories floating around of how to get your cannabis strains to develop anthocyanins or turn purple, but the above are the only ones that are proven to be true. Some of the others can be downright harmful to your plants.

Oxygen Starvation

One popular theory you are likely to come across is that depriving the cannabis plants of oxygen will help them turn purple. You should not try this method. Limiting your plants’ access to oxygen, nitrogen, or carbon dioxide will not get it to change color. All that will happen is damage to your plants and a potentially ruined harvest.

Freezing

There are also rumors that you can freeze your cannabis to turn it purple, and this is not only untrue but also not good for the plant. This suggestion typically involves either using ice water flushes or flash freezing the cannabis.

Doing this will likely change the color of the plant, but not in a healthy way. Instead, this method is likely to do more harm than good. Suddenly exposing your cannabis plant to an extremely cold temperature will cause stress. If this does not kill your plants outright, it would likely at least lower the THC content, which you probably do not want.

If you insist on trying this method, proceed with caution and have low expectations. To minimize the damage, freeze the buds gradually and only do this near the end of your plants’ flowering cycle. Otherwise, you may prevent them from flowering healthily.

Food Coloring

Another method that some people claim will work is dying the cannabis purple using food coloring. Yes, it may work, but there is not really much point in going with this option. There would be no real benefit, and the food coloring may harm your plant. Not to mention that it would be hard to get a uniform color across the buds.

How Cannabis Turns Color

Based on all the information above, you know that anthocyanins play a major role in whether cannabis strains turn purple, but chlorophyll is equally important. Even if anthocyanins are present normally, the presence of chlorophyll with its dominant pigmentation typically makes it hard to see them.

The cold weather mentioned above takes care of this by converting the chlorophyll into sugar. The plant uses sugar as energy to produce seeds, cannabinoids, and flowers. At the same time, the reduction in chlorophyll means that it is no longer overpowering the anthocyanin, letting the anthocyanin and its colors show through.

Are There Benefits to Choosing Strains That Turn Purple?

The follow-up question is whether you should choose a cannabis strain that turns purple. This depends on your personal preference. There are some benefits, but not enough to make it crucial you choose a purple strain or even to strongly recommend it.

Health Benefits

As mentioned, there are some mild health benefits associated with purple cannabis strains. Specifically, the anthocyanin is an antioxidant. Therefore, when you ingest it, you may receive some of the health benefits.

Visual Appeal

There is also the benefit of the visual appeal. This will not matter to many people who smoke marijuana, but if you want the beautiful purple color instead of the typical green of cannabis, then go ahead and choose a purple strain.

It Does Not Affect Potency

Keep in mind that whether a plant turns purple or remains green will not impact its potency. This is a somewhat common myth and an important point. It means that you should not choose a purple cannabis strain just because you expect it to be more potent.

Potential Benefits of Anthocyanins

In addition to its status as an antioxidant, some people also believe that anthocyanins may have health benefits for people. Some of the benefits that people claim anthocyanins provide include antioxidant effects; anti-inflammatory effects; protecting the liver and heart; boosting vision; and even preventing diabetes, obesity, and certain types of diabetes. However, research supporting this is limited.

Additionally, for you to get any benefits from anthocyanins, you would have to ingest them, which you are unlikely to do by smoking cannabis. Instead, you would have to consume cannabis as edibles. Even more importantly, you would need to consume a lot of them because the digestive tract absorbs most of the anthocyanins, meaning your body would not even be able to put them to use — assuming the hypothetical benefits are true.

Choose Based on Other Factors Instead

Instead of choosing your cannabis strain based on whether it turns purple, look at other factors that are more important. Think about why you want to smoke marijuana, including whether it is for recreational use or a specific medical condition. Then, look for a strain that does a good job in producing those results, which will depend on factors like terpenes and genetics. You will also want to consider how much experience you have using cannabis and use that information to choose your ideal THC content.

When Purple Cannabis Plants Are Bad

Purple buds are not problematic, but if other parts of your cannabis plant are turning purple, you need to be careful. For example, one of the common symptoms of phosphorus deficiency is purpling or reddening in the leaves and stems. This will typically begin with the lower branches and stems before working up the plant.

Remember that your cannabis plant needs phosphorus to grow healthily. As such, you should check your soil concentrations right away if you notice any indications of a phosphorus deficiency. If the levels of phosphorus are low, you will want to begin supplementing them right away. You can maintain the proper levels of this key nutrient with crab shells, blood meal and bone, bat guano, worm castings, and certain other types of organic matter. Or, you can opt for a synthetic fertilizer that is ideal for the flowering stages.

The Take-Away

Some cannabis strains turn purple because they have higher levels of anthocyanin or can produce it. This comes from specific growing conditions in the final few weeks of cannabis growth, including cooler temperatures and more limited exposure to daylight. Anthocyanin contains antioxidants, but other than this and the beautiful coloring, there are no significant reasons to choose purple cannabis strains over green ones.

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