Check what you planted before worrying about this one. Red Russian and Scarlet kale are both bred to turn red or purple as it gets colder — that's the variety doing exactly what it's meant to, not a problem. On an ordinary green kale, cold weather alone can trigger the same purple pigment as a short-lived stress response that fades once it warms up again. Treat it as an actual nutrient issue only if the colour sticks to older leaves and the plant has genuinely stopped growing.

Ranked, most likely first

  1. A variety built to do this. Red Russian is blue-green through warmer weather and turns a deep red at the stems and leaf veins as the temperature drops — that's its defining trait. Scarlet is a curled red kale from the same range. If either is what you planted, there's nothing wrong.
  2. A passing cold-weather pigment response, on a green variety. Cold combined with strong light pushes the plant to build up purple pigment, and it becomes visible once the green chlorophyll that normally hides it fades back. It can show up in under a week and clears the same way once the weather lifts.
  3. Phosphorus or potassium stress, made worse by cold. Cold, waterlogged or acidic ground makes it harder for roots to take up phosphorus even when there's enough in the soil, and in the cooler months a similar look can mean a potassium shortfall instead. This is the one actually worth treating — the tell is purpling that settles on older leaves and margins while the plant's growth has stalled, not an even flush of colour on a plant that's otherwise growing fine.

Why a temperate cold snap should ease off faster

Kale goes in on the same schedule everywhere in the country — sow August to October, transplant September to November, harvest November to January, Egmont-sourced — so the calendar isn't what sets this zone apart here. The frost season is: temperate runs April to November, shorter and milder than a colder inland garden gets. That matters because the harmless cold-pigment response is exactly that — a response to cold — so it should fade back faster once a temperate cold snap passes than it would somewhere colder for longer. The flip side is useful too: if purple colouring is still hanging around well after a cold spell has clearly passed here, that's a stronger signal it's the genuine nutrient-stress route rather than the plant just reacting to weather, and worth actually feeding for.

What to do about it

  • Rule out the variety first. Red Russian or Scarlet colouring up as it cools is normal — don't feed or treat for it.
  • On a green kale, give a fresh cold snap a week or two before acting. If it's purely weather-driven, it clears on its own as things warm back up.
  • If it's persisting on older leaves with slowed growth, treat the soil. Avoid waterlogged or compacted ground and check that pH isn't very acidic — both block phosphorus uptake regardless of how much is actually in the soil. A balanced fertiliser that includes available phosphorus and potassium is the direct fix, since either can be behind a purple flush in the cooler months.
  • Feed while the plant's still actively growing, not once it's gone dormant for winter — that's when it can actually respond.

Cultivar notes

Red Russian and Scarlet are the two kale varieties in the Kings Seeds NZ range bred specifically for this red/purple colouring, intensifying as it cools. If you'd rather kale stayed reliably green year-round, the rest of the range — Blue Ridge F1, Cavolo Nero, and Noble — sow on the same autumn, early spring, or late summer windows without the colour swing.

If holes rather than colour are the actual complaint on your kale, that's a different problem with its own causes: see Why Is My Kale Full of Holes in NZ Temperate?

Next season

If a green variety colours up every cold snap and it bothers you, switching to Red Russian or Scarlet turns the same trait into the point of growing it rather than something to second-guess. Otherwise, the only real prevention for the nutrient route is keeping soil well-drained and feeding on a schedule through the cooler months rather than waiting for purpling to show up first.

More on this zone: see the full NZ Temperate Planting Calendar →.