The short answer: purple cannabis gets its color from anthocyanins, the same pigments that color blueberries and red cabbage. Genetics decide whether a plant can make enough of them to show. Cool temperatures can bring the color out. And purple does not mean stronger.
What Turns Cannabis Purple?
Anthocyanins are plant pigments in the flavonoid family.[1] Their color shifts with acidity: they look red in acidic conditions and turn blue as the pH rises.[2] That is why anthocyanin-rich cannabis can range from pink and red to deep violet. This refers to conditions inside the plant’s pigment-containing cells, not the pH of the soil or nutrient solution. Altering root-zone pH to force color can instead disrupt nutrient uptake.
Color often intensifies late in flowering as anthocyanin production changes and declining chlorophyll makes the pigments easier to see.
Is It Genetics or Cold Weather?
Both, but genetics come first.
- Genetics set the limit. A 2024 study of differently colored cannabis varieties found that anthocyanin levels in the leaves tracked the activity of a set of known pigment genes.[3] A 2025 University of Wisconsin thesis linked purple flowers to a region on chromosome 6, and purple stems and leaves to a region on chromosome 1.[4] Genetics set the plant’s capacity for pigmentation. Cold can intensify color in a responsive variety, but it cannot reliably turn a genetically green cultivar into richly purple cannabis.
- Cool temperatures bring it out. In many plants, cold switches on the genes that make anthocyanins.[1] In a 2025 cannabis study, one purple hemp line made the most pigment at steady temperatures of 46 °F and 59 °F (8 °C and 15 °C). It made the least at near-freezing (33 °F) and at 72 °F (22 °C).[5]
Is Purple Cannabis Stronger?
No evidence says so. In the same 2025 study, purple pigment and CBD did not rise together. The plants made the most pigment in the cool range, but they had the highest CBD and the heaviest flowers at the warmest temperature tested, 72 °F. The researchers linked CBD and yield mainly to how far the plants had matured.[5] The study did not measure THC, and it used a single hemp line, so it cannot speak for every variety. But it gives no reason to think color signals potency.
Choose purple cannabis for its appearance—or for the aroma and flavor of a particular cultivar—not because color predicts strength. The only way to know potency is a lab test.
Can You Make a Plant Turn Purple?
Only if its genetics allow it. Cooler nights may deepen the color in a variety that can make anthocyanins, but they will not create purple in one that cannot. And there is a cost: in the 2025 study, the cool conditions that maximized pigment also produced lighter flowers and less CBD than warmer ones.[5] If purple is what you want, start with seeds or clones bred for it.
Can Purple Be a Sign of a Problem?
Yes. Purple can be genetic, but it can also accompany stress. Phosphorus deficiency can turn stems and leaf stalks purple, usually alongside other signs:[6]
- slow growth and dark green leaves
- older leaves turning bronze, with browning between the veins and at the edges
- fewer or smaller flowers, and delayed maturity
Ontario’s official hemp guide warns that purpling is an unreliable sign of phosphorus deficiency, because many other factors can cause it.[6] So purple color alone is not a diagnosis, and growers should not add phosphorus based on color alone. Phosphorus also becomes less available when the root zone drops below pH 6, even when the nutrient is present.[6] A soil or tissue test is the reliable check.
For more on growing healthy plants, see our guide to living soil and our look at companion plants for cannabis.
References
- Xue X, Duan Y, Wang J, Ma F, Li P. Nighttime temperatures and sunlight intensities interact to influence anthocyanin biosynthesis and photooxidative sunburn in “Fuji” apple. Frontiers in Plant Science. 2021;12:694954.
- Khoo HE, Azlan A, Tang ST, Lim SM. Anthocyanidins and anthocyanins: colored pigments as food, pharmaceutical ingredients, and the potential health benefits. Food & Nutrition Research. 2017;61.
- Gagalova KK, Yan Y, Wang S, Matzat T, Castellarin SD, Birol I, Edwards D, Schuetz M. Leaf pigmentation in Cannabis sativa: characterization of anthocyanin biosynthesis in colorful Cannabis varieties. Plant Direct. 2024;8(11):e70016.
- Prillaman C. A study in purple: an investigation into the genetics of anthocyanin accumulation in Cannabis sativa. MS thesis, University of Wisconsin–Madison; 2025.
- Kim SR, Basnet P, Kovaleski AP, et al. Anthocyanin accumulation, inflorescence dry weight and total cannabidiol content have different temperature optima in Cannabis sativa. Journal of Cannabis Research. 2025;7:51.
- Todd J, ed. Nutrient deficiency ID guide for hemp. Ontario Ministry of Agriculture, Food and Rural Affairs; 2023.
Editor’s note: This article was originally published on September 7, 2018, by Asia Mayfield. In October 2026 the editors rewrote it around new cannabis-specific research on genetics, temperature and potency.
Last updated October 2026 · Originally published September 7, 2018.
