Why is it purple? Genetics, cold, phosphorus, or something worse?

By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed 2026-10-14

The short answer

• Judge the colour under white light, not the grow lamp. Under a blue-and-red lamp everything is magenta, including a healthy green seedling. Carry the plant out or switch to a white light before deciding anything is purple.
• Even and symmetrical, on a plant growing at full speed, is genetics. Purple stems, petioles, leaf undersides or flowers that appeared early and stayed uniform are a cultivar trait, and cool nights turn it up. Nothing to fix.
• Purple that arrived with cool nights or a stronger lamp is the plant’s sunscreen. Anthocyanins are switched on by cold, bright light and drought. If growth is normal and the leaves are green, watch it; do not feed it.
• “Purple stems means phosphorus” is the weakest rule in the hobby. When phosphorus was withheld from cannabis in a controlled study, the leaves went dark green with dead tips and edges — the purple-stem picture did not appear. Purple stems on their own are not a phosphorus reading; the oldest leaves and the runoff pH are.
• Purple with distortion at the growing tip is the one that matters. Tiny, crumpled, clustered leaves; shortened internodes; a bud site that has turned into a tuft — on an outdoor plant, that pattern is a pathogen question, not a pigment question, and the pathogen confirmed in cannabis is spread by leafhoppers and has no cure.

IG-016One pigment, four reasons

Judge the colour under white light first. Then look at where it sits, whether it is even, and what the growing tip is doing.

Under white light, it is still purple

You seeEven and symmetrical, on a plant growing at full speed

It isGenetics

SoA cultivar trait, turned up by cool nights. Nothing to fix.

You seeArrived with cool nights, a stronger lamp or a dry spell; leaves green, growth normal

It isThe plant's sunscreen

SoAnthocyanin switched on by cold, bright light or drought. Watch it; do not feed it.

You seePurple stems, and that is all

It isNot a phosphorus reading

SoWithholding phosphorus produced dark green leaves with dead tips, not purple stems. Read the oldest leaves and the runoff pH instead.

You seeTiny, crumpled, clustered leaves; shortened internodes; a bud site turned into a tuft

It isA pathogen question

SoOutdoors in leafhopper country: isolate, take no cuttings, get a lab sample. No cure.

Every number here is from this page's text and the sources it lists.

One pigment, four reasons

The purple in cannabis is anthocyanin — in the tissue actually analysed, mostly one compound, cyanidin-3-rutinoside — and it shows up in stems, petioles, leaves and flowers. The plant makes it for its own reasons, and the reasons overlap. That is the whole difficulty: a purple petiole on a purple cultivar, a purple petiole on a cold plant and a purple petiole on a sick plant contain the same molecule and photograph the same way.

• Genetics. Some accessions are purple from the seedling up, uniformly, on every plant of the line, at every stage. The colour is symmetrical — both sides of the stem, every petiole, the whole flower — and the plant is otherwise vigorous. Growers select for it. It is the one cause where the answer is “enjoy it.”
• Temperature. In a controlled study on a purple day-neutral cultivar grown between 0.5 and 22 °C, anthocyanin peaked at constant 8 and 15 °C and fell off at both 0.5 and 22 °C — a temperature response with its own optimum, separate from the optima for yield and cannabinoid content. So cool nights in the last weeks of an outdoor season, or a tent whose night temperature slipped, will bring the colour out in a plant that carries the trait, and the flowers can go purple with no other change at all.
• Light and drought. Across the plant kingdom anthocyanins are induced by strong visible light, ultraviolet, cold and water stress, and the pigment in leaves and stems is understood as protection. A plant moved under a much stronger lamp, or one that dried out hard, can purple its stems within days. Again: green leaves and normal growth mean it is coping, not failing.
• Phosphorus. The rule every guide repeats comes from other crops. In cannabis, the controlled single-element study that the field’s reference handbook draws on produced dark green leaves with necrotic tips and edges when phosphorus was withheld, and did not produce the purple stem the guides show. Phosphorus shortage in a real grow is usually a root-zone pH problem, and it reads on the oldest leaves first. Purple stems are at best a weak hint that something else is wrong; they are not the diagnosis.
• Disease. A phytoplasma — a wall-less bacterium that lives in the plant’s sap and is moved between plants by leafhoppers — has been confirmed in cannabis in China, in Nevada and in Arizona hemp. The published symptom list is leaf curling, mottling, chlorosis, witches’ broom, stunting and node shortening: the growing points proliferate into clusters of small, distorted leaves on shortened internodes, with “green islands” between the veins on the upper surface. The reports describe yellowing and distortion rather than purple; a plant under that kind of attack also makes stress pigment, so a purple-black, crumpled tip is consistent with it, but the distortion is the sign, not the colour. It is an outdoor and greenhouse problem where leafhoppers live. There is no treatment; the infected plant is removed so it stops being a reservoir, and it is never used for cuttings.

Why the photo cannot settle it

Three ways. The lamp: most indoor photos of “purpling” are taken under a magenta light that makes the medium, the pot and the leaves all the same colour, and a phone will not correct for it. The pigment: the same anthocyanin, at the same intensity, appears in the healthy and the stressed plant. And the frame: the thing that separates pigment from pathology is the growing tip — is it growing normally, or is it a tuft of tiny twisted leaves — and a photo of a purple stem rarely includes it. A reply that names phosphorus, cold and genetics in the same thread is not wrong; it is answering a question the photo cannot ask.

How to check this yourself

• White light first. If the purple disappears under daylight or a white lamp, it was the grow light.
• Even or patchy? Symmetrical colour on every petiole and both sides of the stem is genetics or temperature. One plant of several on the same feed, or one side of one plant, is the one to look at harder.
• Is it growing? Measure the height or count the nodes today and in four days. A purple plant adding nodes at the usual rate has a pigment, not a problem.
• Look at the growing tip with a hand lens. Normal new leaves, only coloured: pigment. Tiny, crumpled, clustered leaves on a shortened stem, a bud site turned into a tuft: stop, isolate the plant, take no cuttings, and get a sample to an extension office or plant lab — especially outdoors in leafhopper country.
• Check the night temperature for the last week. If nights have been in the low teens Celsius, or the tent’s night low dropped, that is the likeliest switch.
• If you suspect phosphorus, read the oldest leaves and the runoff pH, not the stem. Dark green lower leaves with dead tips and edges, and a runoff pH out of range on a calibrated pen, is the pattern. Purple stems with green, healthy lower leaves is not.
• A purple seedling with a purple hypocotyl and cotyledon edges, growing, is the commonest and least worrying version of all. Give it a week.

What would show that we have this wrong

If the purple is spreading into leaves that opened this week, the plant has stopped adding nodes, the night temperature has not moved, the runoff pH is squarely in range and the plant is indoors with no leafhoppers — then none of the four boxes above fits cleanly, and a photo cannot go further; that plant gets a lab sample. And if the whole line has been purple since the cotyledons and every plant of it is growing at the same rate as its green neighbours, stop reading this page.

Where our device fits, stated narrowly

Croplock does not identify why a plant is purple from a photo, and under a blue-and-red lamp its camera is as colour-blind as yours — it does not compare colour across dates for exactly that reason. What it holds is the one cause you can check without looking at the plant: the tent’s night temperatures, with dates, so “did the nights drop last week?” is a lookup rather than a guess. And it keeps dated pictures of the growing tip, so “is it still adding nodes?” is a comparison rather than a memory.

Sources

• Bassolino, L., Fulvio, F., Pastore, C., Pasini, F. et al. (2023), When Cannabis sativa L. Turns Purple: Biosynthesis and Accumulation of Anthocyanins, Antioxidants 12(7):1393. DOI 10.3390/antiox12071393 — cyanidin-3-rutinoside as the major pigment; purple pigmentation in stem, petioles, leaves and inflorescences; environmental cues eliciting synthesis.
• Kim, S.R., Basnet, P., Kovaleski, A.P. & Ellison, S.L. (2025), Anthocyanin accumulation, inflorescence dry weight and total cannabidiol content have different temperature optima in Cannabis sativa, Journal of Cannabis Research. DOI 10.1186/s42238-025-00311-w — the 0.5–22 °C range, the peaks at 8 and 15 °C, the separate optima.
• Chalker-Scott, L. (1999), Environmental Significance of Anthocyanins in Plant Stress Responses, Photochemistry and Photobiology. DOI 10.1111/j.1751-1097.1999.tb01944.x — induction by visible and UVB radiation, cold and water stress; the protective reading.
• Cockson, P., Landis, H., Smith, T., Hicks, K. & Whipker, B.E. (2019), Characterization of Nutrient Disorders of Cannabis sativa, Applied Sciences 9(20):4432. DOI 10.3390/app9204432 — the single-element deficiency study; and Cannabis in Controlled Environment Production (CRC Press), Ch. 5 §5.3, DOI 10.1201/9781003150442 — the phosphorus presentation as restated on our cal-mag page.
• Feng, Kyotani, Dubrovsky & Fabritius (2019), First Report of ‘Candidatus Phytoplasma trifolii’ Associated with a Witches’ Broom Disease in Cannabis sativa in Nevada, U.S.A., Plant Disease. DOI 10.1094/PDIS-01-19-0098-PDN — the symptom list and the “green islands.”
• Hu (2021), First Report of ‘Candidatus Phytoplasma trifolii’ Related Strain Associated with Yellowing and Witches’ Broom of Industrial Hemp in Arizona, Plant Disease. DOI 10.1094/PDIS-04-21-0847-PDN. Zhao et al. (2007), the China report, Plant Disease. DOI 10.1094/PDIS-91-2-0227C.
• The white-light check, the four-day node count, the “even or patchy” rule and the leafhopper-country framing are first-party practice, not citations; the note that phytoplasma-infected tissue also makes stress pigment is our reading of the two literatures side by side, not a published finding about cannabis.

More of these at Answers. The device is at croplock.com.

Related: Is it cal-mag? · Is it a virus? · Is it a deficiency or a disease? · pH or EC — which number is actually telling me something? · Will a light leak stop it flowering?