In the world of cannabis cultivation, few things capture the eye like a canopy of deep purple flowers. For decades, these vibrant hues have been a hallmark of “exotic” or “top-shelf” genetics, often associated with unique aromas and sought-after effects. But what actually makes cannabis turn purple? Is it just a genetic lottery, or can growers influence this colourful transformation?

The answer, like much in horticulture, is a fascinating blend of genetics and environment. The purple colour isn’t a gimmick; it’s the visual expression of a specific class of compounds called anthocyanins. Understanding these molecules and the factors that trigger their production can empower you to cultivate visually stunning cannabis with purpose.

Let’s dive into the science behind the purple, how to encourage its expression, and what it truly means for the quality of your harvest.

What Are Anthocyanins, and Why Do Plants Make Them?

Anthocyanins are a type of water-soluble pigment belonging to the flavonoid family. They are the same compounds responsible for the rich red, blue, and purple colours found in many other plants, such as blueberries, raspberries, and black rice.

In plants, anthocyanins serve a crucial purpose. They are primarily protective compounds, produced in response to environmental stressors. Their functions include:

Photoprotection: Shielding the plant’s sensitive tissues from damage caused by excess UV or visible light.
Temperature Regulation: Helping the plant cope with cold temperatures.
Attracting Pollinators: Creating vibrant colours in flowers to attract bees and other pollinators.
Antioxidant Power: Functioning as powerful antioxidants that neutralise harmful free radicals, protecting the plant’s cells from damage.

This last point is particularly exciting. The presence of these antioxidants is what makes so many purple fruits and vegetables “superfoods.” Remarkably, recent research by Bassolino et al. (2023) has not only identified the primary anthocyanin in cannabis as cyanidin-3-rutinoside (keracyanin) but also found that its concentration in some purple cannabis tissues can be significantly higher than in well-known antioxidant-rich berries. This positions purple cannabis not just as aesthetically pleasing, but as a potentially valuable source of these beneficial compounds.

The Two Pillars of Purple: Genetics and Environment

For a cannabis plant to turn purple, two conditions must be met: it must have the genetic potential to do so, and it must be exposed to the right environmental triggers to express that potential.

1. Genetics: The Blueprint for Colour

This is the most critical factor, and it cannot be overstated: a cannabis plant that does not carry the genes for anthocyanin production will never turn purple, no matter what you do. The ability to produce these pigments is written into the plant’s DNA.

Strain Selection is Key: If your goal is to grow purple cannabis, you must start with genetics known for this trait. Strains like Granddaddy Purple, Purple Kush, Purple Urkle, and many modern hybrids have been specifically bred over generations to reliably produce anthocyanins.
The Science Behind the Trait: Researchers are actively identifying the specific genes and transcription factors (with names like MYB, bHLH, and TTG1) that act as molecular switches, turning on the anthocyanin production pathway in cannabis. This genetic understanding confirms that the purple phenotype is a heritable, predictable trait when the right variety is chosen.

2. Environment: The Trigger for Expression

Once you have the right genetics, you can use environmental cues to encourage the plant to express its purple potential. The most effective and well-documented trigger for anthocyanin production in cannabis is cold temperature.

How Cold Works: As the UW-Madison research poster demonstrates, cooler temperatures, particularly during the late flowering stage, signal to the plant that winter is approaching. In response, the plant may slow its chlorophyll production (the green pigment responsible for photosynthesis) and increase its production of protective anthocyanin pigments. This change is often most pronounced in the final 2-3 weeks of flowering. As the green chlorophyll fades, the underlying purple anthocyanins become strikingly visible.
Practical Application: To encourage purple colouration, growers can introduce a significant temperature drop between the “lights on” and “lights off” periods. Dropping nighttime temperatures to a range of 10-15°C can often trigger a strong colour change in genetically predisposed strains.
A Word of Caution: This must be done carefully. Do not shock your plants with sudden, drastic temperature drops, and never let them freeze.- I am sure some growers are thinking, ‘but this is how I go about it with no issues.’ By all means, to each their own. Freezing temperatures will damage plant cells, harm trichomes, reduce yield, and negatively impact overall quality. The goal is to simulate cool autumn nights, not a hard winter frost. Remember that this cold stress can also slightly slow down the final swelling of the buds, so that it can be a trade-off between maximising colour and maximising bulk.

While other stressors like specific nutrient deficiencies (e.g., phosphorus) or pH fluctuations can sometimes induce colour changes, these are signs of poor plant health. Manipulating temperature is the safest and most effective environmental tool for promoting anthocyanin expression in healthy plants.

Busting a Common Myth: Purple Does Not Equal Potent

A persistent belief in cannabis culture is that purple weed is inherently stronger or more potent. This is a myth.

The biochemical pathways that produce cannabinoids (like THC and CBD) are completely separate from the pathway that produces anthocyanins. A plant’s genetic potential for high THC content has no direct link to its genetic potential for purple colouration. You can have a vibrant purple plant with moderate THC, and a green plant with exceptionally high THC.

Likewise, the “grape” or “berry” aromas often associated with purple strains are not caused by the anthocyanins themselves (which are largely odourless and tasteless). These aromas come from the plant’s terpene profile, a distinct set of aromatic compounds. While breeders have often selected for strains that have both purple colours and berry-like terpenes, these are two separate traits.

The Grower’s Takeaway: A Holistic Approach to Colour

If you want to cultivate visually stunning purple cannabis, the path is clear:

Start with Genetics: Your most important decision is choosing a strain with a proven history of purple phenotypes.
Use Temperature as a Tool: In the final weeks of flowering, introduce cooler nighttime temperatures gradually and carefully to encourage anthocyanin expression.
Manage Expectations: Understand that you are cultivating for aesthetics and a higher antioxidant content, not necessarily higher THC potency.
Prioritise Overall Plant Health: Never sacrifice the fundamental health of your plant for colour. A robust, healthy green plant will always produce a better overall harvest than a stunted, overly stressed purple one.

Cultivating cannabis with deep, rich colours is a rewarding experience that blends the science of genetics and environmental control with the art of horticulture. By understanding the role of anthocyanins and the factors that influence them, you can intentionally guide your plants to express their full, colourful potential.

Anthocyanins: The Science and Art of Purple Cannabis