Why do marijuana plants change color?
The colors of marijuana plants can vary depending on the type of crop you’re growing and the climate in which they are cultivated. In many cases, experienced growers may wake up to find that, at some point in their plants’ life cycle, certain strains have changed color—from green to purple, blue, or even black. This change can sometimes be attributed to the plant’s genetics, such as in the famous Purple Haze or Blueberry strains. For example, certain phenotypes may turn black in parts during the last few weeks of flowering, particularly in indoor setups with temperatures between 18°C and 24°C. While Blueberry strains tend to develop a bluish hue, Purple Haze often shifts from lilac to dark purple. In contrast, some Panama and Colombian strains may even display a hint of pink.
The role of anthocyanins in color change
The color change in cannabis plants is generally due to the presence of anthocyanin, a water-soluble pigment produced by the plant. This pigment can appear either due to the plant’s genetic makeup or in response to environmental factors like a potassium deficiency or low temperatures. However, it’s important to note that plants may sometimes exhibit color changes due to excess nitrogen. In these cases, you might mistake this for a genetic trait, but it’s crucial to use the correct substrate and fertilizers. Nitrogen excess doesn’t just darken the leaves; it can also cause the stems and trunk to turn purple. When color change is genetic, it often affects the buds too, creating strikingly colorful flowers. Keep in mind that this genetic color change is inevitable and cannot be reversed.

Nutrient deficiencies and color changes
If the color change isn’t linked to genetics, it could be a sign of a phosphorus deficiency, which typically shows up during the flowering phase. While the plant may remain a healthy green during the growth period, by the fourth week of flowering, leaves may begin turning purple, lilac, or even slightly red. If you mistakenly attribute this change to temperature or other environmental factors, you might overlook an essential nutrient issue, ultimately affecting your final yield. In such cases, the color shift is often accompanied by yellowish welts on the leaves, signaling a lack of potassium. To address this, a few waterings with a fertilizer rich in potassium should help improve the plant’s health. However, in some instances, the problem may stem from excess salts in the soil, requiring you to flush the plant’s roots and then reintroduce nutrients. Additionally, an incorrect pH level can also trigger color changes, even if the plant’s nutrient levels are balanced. To fix this, you’ll need to flush the roots and adjust the pH, ensuring that all future waterings are within the correct pH range. Once corrected, you’ll know the issue is resolved when the plant’s color stops worsening, and new leaves grow in green and healthy. This problem typically arises with overly acidic pH levels, while basic pH levels tend to cause yellowing.
The impact of cold temperatures
Cold temperatures, particularly around 10°C, can also cause some strains to turn purple or bluish. It’s essential to remember that cannabis plants generally thrive in temperatures between 20°C and 30°C. Indica strains, however, are better equipped to handle colder climates compared to sativa strains. If you adjust the temperature to optimal levels, the plant will usually return to its original color. This phenomenon, known as Ruby, is similar to what happens with blood oranges, which need cooler temperatures to turn red. The pigments involved in this process arise from changes in DNA in specific sections of the plant, known as retrotransposons.

The science behind anthocyanins
As previously mentioned, the color changes are largely due to the presence of anthocyanins. These are flavonoids that occur naturally in plants and can be found in various parts, including the leaves, stems, branches, flowers, and roots. Depending on the strain and growing conditions, anthocyanins may become more prominent, especially in response to an acidic pH, resulting in deep purple hues. Their primary role is to protect the plant from UV rays and pathogens, serving as a defense mechanism against predators by changing the plant’s appearance.
The benefits of anthocyanins
Interestingly, anthocyanins possess several beneficial properties, including anti-inflammatory effects, pain relief, and neuroprotection. Research suggests a connection between anthocyanins and the CB1 and CB2 receptors in the body. Furthermore, studies have shown that a diet rich in anthocyanins can improve overall health, benefiting the cardiovascular system and helping to prevent obesity. However, in the case of cannabis, anthocyanins do not have a direct impact when smoked. Instead, they might be more effective when consumed in other forms, such as in teas or infusions.