Start by checking what you actually planted. Red Russian and Scarlet kale are both bred to turn red or purple as it cools — that's the variety doing exactly what it's meant to, not a sign of trouble. On an ordinary green kale, cold alone can trigger the same purple pigment as a brief stress response that clears once conditions warm back up. Only treat it as a genuine nutrient problem once the colour settles into older leaves and growth has actually stalled.

Ranked, Most Likely First

  1. A variety bred to do this. Red Russian runs blue-green through warmer weather and deepens to red at the stems and leaf veins as temperatures drop; Scarlet is a curled red kale from the same stable. Either one colouring up in the cold is simply doing its job.
  2. A brief cold-triggered pigment response on a green variety. Cold paired with strong light pushes the plant to build up purple pigment that becomes visible once chlorophyll fades back — it can appear within a week and fades the same way once warmth returns.
  3. Phosphorus or potassium stress, worsened by cold. Cold, waterlogged or acidic soil restricts phosphorus uptake at the roots regardless of what's actually present in the ground, and a similar look in cooler months can point to potassium instead. The tell is purpling confined to older leaves and margins alongside growth that's genuinely slowed — not an even flush across an otherwise thriving plant.

Why This Zone Gives the Harmless Explanation the Least Room to Apply

Kale follows the same calendar everywhere in NZ — sow August through October, transplant September through November, harvest November through January — so the timing itself isn't the differentiator. The frost season is: this zone's runs May through September, the shortest of the three by a wide margin, against cool/mountain's March–November and temperate's April–November. That gap matters directly here, because the harmless cold-pigment response only has a genuine cold spell to react to for a fraction of the year this zone's siblings get. A normally green kale turning purple in this zone is comparatively the most likely of anywhere in NZ to be real phosphorus or potassium stress rather than a passing weather response — worth feeding promptly rather than waiting several days to see if it clears on its own the way it might further south.

What to Do About It

  • Check the variety before doing anything else. Red Russian or Scarlet turning red at the stems and veins as it cools is the plant working as bred — no feed, no treatment.
  • On a green kale, still allow a short window before acting, since a genuine cold snap can trigger the same look briefly even here — but don't extend that grace period the way a colder zone reasonably would.
  • If it's settling into older leaves alongside slowed growth, treat the soil. Avoid waterlogged or compacted ground and check the pH isn't very acidic, since both block phosphorus uptake regardless of what's actually in the soil. A balanced fertiliser carrying both available phosphorus and potassium is the direct fix, since either shortfall can produce this look in the cooler months.
  • Feed while the plant is still actively growing, not once it's gone dormant — that's when correction actually has something to work with.

Cultivar Notes

Red Russian and Scarlet are the two kale varieties in this zone's sourcing bred specifically for red/purple colouring that deepens with cold. If a reliably green kale matters more to you than the colour show, Blue Ridge F1, Cavolo Nero, Noble and Koanga Institute's Borecole all sow on the same autumn, early spring or late summer windows without the swing.

Holes, not colour, are a different fault entirely — covered separately in Why Is My Kale Full of Holes in NZ Sub-Tropical?

Next Season

Dislike the colour swing on a green variety? Red Russian or Scarlet makes it the whole point instead of an annual surprise. Otherwise, given how little slack this zone gives the harmless explanation, the real prevention is keeping soil well-drained and feeding on a steady schedule through the cooler months rather than waiting for purple leaves to show up before reacting.

More on this zone: see the full NZ Sub-Tropical Planting Calendar →.