This is the classic cool-climate tomato problem, and it's mostly about temperature. Tomatoes ripen best around 20–25°C, and below about 12°C at night the process slows right down. In a cool or mountain garden with a short warm season and cold nights, fruit that set late simply runs out of warm weather and sits green on the vine as autumn closes in. The good news: you don't have to leave it there hoping. Ripening needs warmth, not sunlight — so the reliable fix is to pick mature green fruit and finish it indoors.

Why it happens here

  1. Cool nights and the end of the season — the dominant cause. Below roughly 12°C at night, ripening stalls. Your harvest window runs December to March, and the first autumn frost can arrive in March, so late-set trusses often don't get the warm run they need. This is a season-length problem more than a plant-health one.
  2. Variety too late for the season. A long-maturing variety may never ripen its main crop before the cold arrives. It takes roughly 6–8 weeks from flower to ripe fruit — in a short season, that maths matters, and late trusses may simply run out of time.
  3. Too much foliage or too much nitrogen. A dense leafy plant shades its own fruit and, if it's been fed a lot of nitrogen, keeps making leaves instead of ripening what it has.

What to do now

  • Pick mature green fruit and ripen it indoors. This is the standard cool-zone move and it genuinely works: keep the fruit at room temperature, in the dark — a drawer or a paper bag, not a sunny windowsill — and it colours up in a few days. Do a whole-truss harvest ahead of the first autumn frost rather than losing fruit to the cold.
  • Thin the canopy. Remove some leaves around the trusses to let what warmth there is reach the fruit, and stop feeding high-nitrogen fertiliser — switch to a potassium-richer feed.
  • Don't chase sun onto the fruit. A green tomato on the bench ripens about as well as one on a cooling vine, so late in the season the bench is the safer bet.

What to change next season

  • Treat early / short-season varieties as essential, not optional in this zone — this is the single biggest lever. Choose varieties bred to ripen quickly, and get them in on time so the main crop sets while there's still warm season ahead of it.
  • Plant into the warmest, most sheltered spot you have, ideally against a warm wall or under a cloche or tunnel to lift both day and night temperatures.
  • Go easy on nitrogen once fruit has set.

The wider reasons tomatoes struggle in cool, windy NZ gardens — and the honest case for which varieties and sites actually work — are covered in the Wellington tomato guide below.

More on this zone: Why Tomatoes Fail in Wellington — and What to Do Instead and Why Are My Tomato Leaves Curling in NZ Cool/Mountain?, or the full NZ Cool/Mountain Planting Calendar →.