Why Are My Sweet Potato Leaves Turning Bronze or Purple?

Sweet potato leaves that develop unusual bronze, purple or reddish tints — rather than their normal deep green — are showing a stress response. The discolouration is caused by the accumulation of anthocyanin pigments in the leaf tissue, which happens when normal chlorophyll production is disrupted by cold temperatures, phosphorus deficiency, or potassium shortage. Note that many ornamental sweet potato varieties are bred specifically for purple or bronze foliage — if you are growing an ornamental type, coloured leaves are normal and expected.

Cold stress — the most common cause in spring

When sweet potato slips are planted out in cool conditions (soil temperature below 18°C), or if temperatures drop significantly in the weeks after planting, leaves commonly develop a strong purple or bronze flush. This is a stress response to cold, not a disease or deficiency. As soil and air temperatures warm up and the plant establishes, the new growth comes in green and the older purpled leaves may green up or be replaced. Do not feed aggressively to correct cold-induced purpling — just wait for warmer weather and ensure the plant is growing in suitable conditions.

Phosphorus deficiency

Purple colouration in sweet potato can also result from phosphorus deficiency, which impairs normal chlorophyll production and allows anthocyanins to dominate. Phosphorus deficiency is most common in cold soils (cold soil reduces phosphorus availability even when the element is present), very alkaline soils, and soils that have never received phosphorus-containing fertiliser. A balanced vegetable fertiliser applied at planting, or a watered-in liquid feed high in phosphorus (the P component), usually resolves the symptoms within two to three weeks in warm weather.

Potassium deficiency

Potassium shortage in sweet potato shows as a reddish-bronze tint in the leaf margins and tips, often with a scorched appearance at the leaf edges. Potassium is especially important for tuber quality and size — plants that are potassium-deficient may produce relatively lush foliage but small, poorly developed tubers. Apply a potassium-rich fertiliser (such as sulphate of potash) as a watered-in soil drench. Avoid excess nitrogen at the same time as this worsens the imbalance.

Normal leaf colour variation

Young sweet potato leaves often emerge with a reddish or purplish flush before greening up as they expand — this is entirely normal and not a sign of any problem. Similarly, leaves on the sun-exposed side of the canopy may have a slightly redder tint than shaded interior leaves. Only persistent, widespread purpling that does not green up in warm conditions warrants investigation.