First check what you actually planted. Red Russian and Scarlet kale are both bred to deepen to red or purple as the cold sets in — that's the variety working as intended, not a fault. On an ordinary green kale, cold on its own can still trigger the same purple pigment as a passing stress response, and in this zone it's worth giving that more time to clear than you might further north before assuming anything's actually wrong.

Ranked, most likely first

  1. A variety bred to colour this way. Red Russian stays blue-green through the warm months and turns a deep red at the stems and leaf veins once temperatures drop — that's simply what the variety does. Scarlet is a curled red kale from the same range. Rule this out before looking for a problem.
  2. A cold-triggered pigment response on a green variety. Cold combined with strong light pushes the plant to build up purple pigment, which becomes visible once chlorophyll — the green that normally hides it — drops back. It can appear within a week and fades again once conditions ease.
  3. Phosphorus or potassium stress, made worse by the cold. Cold, waterlogged or acidic ground blocks roots from taking up phosphorus even when it's present in the soil, and a similar look in the cooler months can point to potassium instead. This is the one actually worth treating — look for purpling that's settled on older leaves and margins alongside growth that's genuinely stalled, not an even flush across a plant that's otherwise doing fine.

Why this zone needs more patience before treating it

Kale's calendar doesn't shift by zone — sow August to October, transplant September to November, harvest November to January, Egmont-sourced — so timing isn't the differentiator here. What is: this zone's frost season runs March to November, longer and colder than further north. Both the harmless cold-driven colour change and the genuine cold-soil route to nutrient trouble are more drawn out wherever cold itself lasts longer, so purpling that would have faded within a week or two in a milder garden can reasonably hang on for longer here without meaning anything's gone wrong. The practical upshot: hold off reaching for fertiliser here longer than you might elsewhere, and let a stretch of genuinely cold weather run its course before treating the colour as a deficiency — a temperate garden's shorter, milder cold season means the same colouring should clear faster there (see Why Is My Kale Turning Purple in NZ Temperate?).

What to do about it

  • Check the variety before anything else. Red Russian or Scarlet turning red or purple as it cools is doing exactly what it's supposed to.
  • On a green kale, wait out a cold stretch first. Since this zone's cold runs longer, give it more time than you'd think before treating purely weather-driven colouring as a problem.
  • If it's stuck on older leaves and growth has slowed, feed for it. Keep the ground from staying waterlogged or compacted, and check pH isn't too acidic — both get in the way of phosphorus uptake regardless of what's actually in the soil. A balanced fertiliser carrying both phosphorus and potassium covers either deficiency, since the two can look alike in the cooler months.
  • Feed while there's still active growth, rather than once the plant's settled into winter dormancy — that's when correction actually takes.

Cultivar notes

Red Russian and Scarlet are the two kale varieties in the Kings Seeds NZ range built for this red/purple colouring, and it only intensifies as the cold deepens — a good fit for a zone that holds onto cold longer than most. If you'd rather kale stayed reliably green, Blue Ridge F1, Cavolo Nero, and Noble all sow on the same autumn, early spring, or late summer windows without the colour swing.

Holes rather than colour change is a different problem with its own causes: see Why Is My Kale Full of Holes in NZ Cool/Mountain?

Next season

If a green kale colours up with every cold snap here and it bothers you, Red Russian or Scarlet turns the same trait into the reason to grow it rather than something to second-guess each winter. For the nutrient route, keep soil well-drained through the wet months and feed on a schedule rather than waiting for the leaves to turn before acting — by the time colour shows, the plant's already been short on something for a while.

More on this zone: see the full NZ Cool/Mountain Planting Calendar →.