Tomatoes colour best at around 20–25°C, and ripening stalls at both ends of that range. Below about 12°C at night it slows right down. Above about 30°C the plant stops making the pigments that turn fruit red. In most of New Zealand the problem is the cold end: cool nights, or a season that runs out before late fruit can finish.
The fix that works almost everywhere is simple. Ripening runs on warmth, not light, so mature green fruit picked and kept indoors will colour up on its own.
Quick Diagnosis
| What you see | Likely cause | What to do |
|---|---|---|
| Full-sized fruit sitting green as autumn cools | Cool nights, end of season | Pick and ripen indoors |
| Lots of leaf, few ripening trusses, plant well fed | Too much foliage or nitrogen | Thin leaves, switch feed |
| Fruit set late and still small or hard | Late set, or a variety too slow for the season | Wait if there's warmth left; pick before frost |
| Green or patchy fruit with yellow shoulders in a heatwave | Heat above ~30°C | Afternoon shade, pick and finish indoors |
These causes come from general horticulture and extension services, not NZ-specific trials.
1. Too Cold, or the Season Is Ending
This is the most common cause in cooler NZ gardens. Once nights drop below about 12°C, colour change slows to a crawl, and trusses that set late in summer can sit green for weeks as autumn closes in. The plant is fine; it has run out of warm weather.
2. Too Much Leaf or Too Much Nitrogen
A dense, leafy plant shades its own fruit, and a high-nitrogen feed keeps it putting energy into leaves rather than finishing the tomatoes it already carries. It's the most fixable cause.
3. Late Set, or a Variety Too Slow for the Season
Tomatoes take roughly 6–8 weeks from flower to ripe fruit. Trusses that set late need that time, and a long-maturing variety may never ripen its main crop before the cold arrives.
4. Too Hot
In a heatwave above about 30°C, the plant stops producing lycopene and carotene, so fruit stays green or ripens unevenly with yellow or green shoulders. This is more a warm-north or heatwave problem than a general NZ one. It needs nearly the opposite response to a cold stall.
By Region: What's Most Likely Where You Garden
The causes are the same everywhere. What changes is which one tops the list. Timings below come from our planting data. The regional emphasis is general guidance, not measured data.
Northland, Auckland and coastal Taranaki (sub-tropical). Plants go out earliest here, with transplanting from August to October, and the harvest runs from October right through to May. The more likely fault is the hot end: fruit stalling green or getting yellow shoulders in the hottest weeks of summer. Check for heat before reaching for cold-climate advice.
Most of the country (temperate). Transplant from October to December, after the last frost in November, and pick from December to April. Tomatoes usually ripen without fuss here. When they don't, look first at an over-leafy, over-fed plant, then at late-set trusses or a passing cool snap.
Central Otago, Queenstown and the Central Plateau (cool/mountain). Transplanting runs October to December here too, but the harvest closes earlier, from December to March, with the first frost arriving in March. This is where green tomatoes are the classic problem. Late fruit often won't colour outdoors, so plan to pick whole trusses of mature green fruit before that first frost.
What to Do Now
- Pick mature green fruit and ripen it indoors. Keep it at room temperature in the dark, in a drawer or paper bag rather than on a sunny windowsill. It colours up in a few days.
- Don't chase sun onto the fruit. Late in the season, the bench ripens fruit about as well as a cooling vine.
- Thin the canopy. Remove some leaves around the trusses to let warmth reach the fruit.
- Change the feed. Stop high-nitrogen fertiliser and switch to one richer in potassium.
- In a heatwave, give afternoon shade rather than more sun, and finish stalled fruit indoors.
What to Change Next Season
- Choose for your season length. In a cool garden, early or short-season varieties are essential, not optional.
- Plant on time so the main crop sets while there's still warm weather ahead of it.
- Use your warmest spot. In cold gardens, a sheltered wall, cloche or tunnel lifts both day and night temperatures.
- Go easy on nitrogen once fruit has set.
- In the hot north, pick a site with some afternoon relief rather than maximum sun.
Leaves curling as well? See Why Are My Tomato Leaves Curling? NZ Causes and Fixes. Dealing with blight or psyllid? See Early and Late Blight in NZ Tomatoes or Tomato-Potato Psyllid in NZ. For planting times in your zone, see the planting calendar for the sub-tropical north, most of the country or the cool/mountain south and inland.