Curling tomato leaves are alarming to look at but usually harmless. The most common cause is "physiological leaf roll" — a stress reaction rather than a disease — and in a cool or mountain garden it's more often wind and cold snaps behind it than heat. You can spot the harmless kind: it begins on the lower, older leaves, which roll upward and inward and turn thick and leathery while staying a normal green. If that's the picture, your plant is fine. The causes worth acting on — hormone weedkiller and tomato-potato psyllid — look distinctly different, and telling them apart is what this comes down to.

Start by asking which leaves are affected

The single most useful question is whether the damage is on old growth or new. If it's the older, lower leaves rolling upward and turning thick and leathery while staying green, that's the harmless physiological roll. If instead the newest growth at the tips is twisted, cupped or fern-like, that points to a hormone weedkiller. And if the curling comes with yellowing or purpling and tiny insects on the leaf undersides, it's tomato-potato psyllid.

The most common reasons — for a cool or mountain garden

  1. Physiological leaf roll (most common, harmless). A stress response — and in this zone the stress is more likely wind exposure or a sharp cold snap than the heat that drives it up north. Tomatoes are wind-sensitive and want shelter, so an exposed mountain bed rolls its leaves readily. The plant is unharmed and needs no treatment.
  2. Hormone-weedkiller damage. The new growth is distorted — twisted, stringy, fern-like — across the whole plant, worst at the tips. It arrives via spray drift or through compost, manure, mulch or grass clippings contaminated with a persistent hormone weedkiller.
  3. Tomato-potato psyllid. A real NZ pest, though generally under less pressure in cooler southern and inland gardens than in the warm north. Look for curling with yellowing or purpling and stunting, and check leaf undersides for tiny winged insects and white sugary specks.
  4. Cold and wind stress. More prominent here than in warmer zones: a wind-blasted or cold-shocked plant rolls its leaves to cope, and eases once sheltered and settled.

What to do now

  • Old leaves, leathery, still green? Leave them be. The plant is working normally — just keep the water even and, more important in this zone, get it out of the wind: a shelter, cloche or warm wall cuts the stress that's most likely at fault here.
  • Twisted, fern-like new growth points at weedkiller. Cut out whatever you introduced lately — a compost, manure, mulch or feed — since any of them can carry it. The distorted growth won't recover, though fresh clean growth can outrun a mild dose. Keep weed-and-feed lawn clippings well away from tomatoes.
  • Yellowing leaves with insects on the undersides mean psyllid: inspect underneath, and take out badly affected plants before they spread it.

What to change next season

  • Shelter is the big lever in this zone — a warm, wind-protected spot (or a cloche/tunnel) cuts the stress roll that cold and wind cause here.
  • Keep watering consistent and mulch to steady soil moisture.
  • Watch what you mulch and feed with, so you're not reintroducing contaminated compost or clippings each year.

For the wider picture of why tomatoes struggle in cool, windy NZ conditions — and what actually works — see the Wellington tomato guide below.

More on this zone: Why Tomatoes Fail in Wellington — and What to Do Instead and Why Are My Tomatoes Not Ripening in NZ Cool/Mountain?, or the full NZ Cool/Mountain Planting Calendar →.