Why Are My Kale Leaves Turning Purple?

Kale leaves developing a distinct purple tinge — on the undersides, around the edges, or spreading across the whole leaf — can look alarming if you planted what you thought was a green variety. In most cases, purple colouring in kale is a completely natural response to cold temperatures, and in some varieties it is a deliberate characteristic. However, it can also indicate phosphorus deficiency, which is worth ruling out if the purple is severe and accompanied by poor growth.

Cold-induced anthocyanin production

The purple colour in kale (and many other plants) is caused by anthocyanins — pigments that form a natural response to cold stress. As temperatures drop in autumn and winter, kale ramps up anthocyanin production as a protective mechanism. This is harmless and very common — you may notice green kale that was entirely green in summer developing purple-red tones on the older leaves, the undersides, and the stem as the weather cools. The purple fades back to green as temperatures rise in spring. The leaves are perfectly safe and often sweeter than ever during this cold-induced purple phase.

Purple varieties are bred for this colour

Several kale varieties are specifically bred for ornamental and culinary use with deeply purple or red leaves: Redbor, Red Russian, Scarlet Kale and various purple curly types maintain their colour year-round, though it intensifies in cold. If you purchased a purple variety, the colour is exactly what was intended. Red Russian kale in particular is well known for its attractive red-veined, grey-green leaves that deepen to red-purple in cool weather.

Phosphorus deficiency

If the purple colouring appeared on young plants in warm conditions — not cold — and is accompanied by slow, stunted growth, small leaves, and a generally poor-performing plant, phosphorus deficiency may be the cause. Phosphorus is essential for root development and energy transfer; a shortage shows as purple colouring (again from anthocyanins, produced when the plant cannot process sugars normally) plus poor growth. Phosphorus deficiency is most common in cold, wet soils where the element becomes less available, in very acid soils, or in containers using old, depleted compost. A balanced fertiliser or a specific phosphorus-rich feed (such as bone meal worked into the soil) corrects it within a few weeks.

How to tell the difference

If the purple appeared in cold weather and the plant is growing well otherwise, it is cold-induced and normal. If the purple appeared in warm conditions alongside slow growth and small leaves, check whether phosphorus deficiency is the cause — apply a balanced feed and see whether growth improves. Purple kale is perfectly edible in all cases; the colour is a cosmetic variation that does not affect food safety.