Ripening End-of-Season Tomatoes: Get Six More Weeks

By late August the calendar makes the call for you. The trusses near the top are still small and green, the light is dropping by a few minutes each evening, and the plant keeps pushing new flowers it has no chance of finishing. So here is the decision that buys you the most fruit: stop the plant now, then ripen what is already on it, on the vine while you can and off the vine once the weather turns. Get the timing right and you can ripen green tomatoes for another six weeks; leave it to chance and you feed a damp spell that brings blight.

Think of it as a two-part job with a deadline running underneath. Part one, stopping, means pinching out the growing tip so the plant ripens the fruit it has set rather than starting fruit it cannot finish. Part two is ripening, on warmth rather than direct sun, with a banana doing the work once you pick. Underneath both runs a deadline: the first warm, wet, humid spell of autumn, because that is when blight arrives and your green fruit needs to be indoors before it does.

When and where to stop the plant

Stop a cordon tomato once it reaches the top of its support, or once it has set seven fruit trusses under cover and four trusses outdoors. To stop it, remove the growing point of the main stem at two leaves above the top truss. Those two leaves stay on to feed the fruit below them. One cut does the whole job, and most people leave it too late.

The numbers come straight from the RHS guide to growing tomatoes: seven trusses in a greenhouse, four outdoors, the tip pinched two leaves above the highest one you mean to keep. The reason a greenhouse plant carries seven and an outdoor plant only four is light and length of season. Under glass the fruit has more warmth and a longer run to colour up, so the plant can support more of it.

Why stopping ripens more, not less

It feels backwards to cut growth off a plant you want more from. The logic runs the other way. Removing the growing tip halts vegetative extension, so the energy that would have gone into new stem and new flowers goes instead into the trusses already set. The plant stops starting fruit it cannot finish before the light fades and puts everything it has into ripening what it carries. A tomato near the top in late August will never colour up on the vine; the leaves and roots feeding it can either chase that lost cause or push the ripe-able fruit over the line.

How warm does ripening actually need?

Ripening runs on warmth in the region of 18 to 24°C, centred near 20°C, and it does not need direct sun at all. Below that band ripening slows, and well outside it for long enough, it can halt completely. Warm room temperature does the work, and bright light is optional once the fruit is picked.

Two American extension services bracket the band. Illinois Extension puts the optimum at 20 to 25°C (68 to 77°F), and South Dakota State University gives 15.5 to 24°C (60 to 75°F) as ideal. Hold onto that band, because it kills a habit. The classic move is to line green tomatoes along a hot windowsill, and it is half right and half wrong. The warmth helps. The blazing direct sun behind glass does not, and on a still afternoon it can push the fruit past the point where ripening works at all.

The windowsill myth, taken apart

Two facts undo the windowsill. First, light does nothing for a picked tomato. South Dakota State University is blunt about it: light will not help after the fruit is harvested, because post-harvest ripening is controlled by temperature alone. Light only raises sugars while the fruit is still on the plant, drawing on leaves it no longer has once you pick. Second, too much heat actively stops the colour. Above roughly 30°C, in the mid-80s Fahrenheit, the plant struggles to make lycopene and carotene, the pigments that turn a tomato red, so it goes yellow or stays stubbornly pale instead. Both extension sources put that cut-off at about 30°C, so it is a firm line rather than a rough guess. A south-facing sill in a warm spell sails past that line by mid-afternoon.

So warmth yes, hot glass no. A kitchen worktop at room temperature out of direct sun ripens fruit more reliably than the brightest windowsill in the house.

Ripening fruit on the vine while you still can

While the weather holds, leave the fruit on the plant and let the vine ripen it for you. After stopping, take off the lowest old leaves to let light and air reach the trusses, and keep watering even and steady. Resist the urge to strip the plant bare for speed; over-aggressive de-leafing upsets its balance and can make the fruit more prone to the very ripening problems you are trying to avoid.

Even watering matters more now than at any point in the season. As fruit ripens it drinks, and a dry spell followed by a flood makes the inside swell faster than the skin can stretch, so it splits just as it turns. One habit keeps a colouring truss whole: steady moisture, and it matters most for plants in pots and growing bags, whose small root volume swings hardest between dry and drenched. If you are picking the routine up after a week away, our note on keeping a greenhouse watered while you are away covers how to hold that level.

When the plant simply will not finish them

There comes a point where the late green fruit will not ripen on the plant whatever you do. As light fades through September the vine no longer has the warmth or the day length to push the last trusses over, and waiting only exposes them to the damp. Pick at that point, and move the job indoors. The fruit you bring in still ripens; the fruit you leave out is gambling on the weather, and by late September the weather rarely pays out.

How to ripen green tomatoes off the vine

Pick the green fruit and put it somewhere warm and dark, and let trapped ethylene do the rest. The simplest version: place the tomatoes in a paper bag, stem-end up, punch a few holes for airflow, and add a ripe banana or a ripe tomato. The ripe fruit gives off ethylene, the gas that triggers ripening, and the bag keeps enough of it around the green fruit to speed things along. A drawer works the same way, which is why the RHS suggests putting unripe tomatoes in a drawer with a banana. Keep the whole arrangement near 20°C and out of direct sun, the same warmth the vine would have given the fruit.

Two routes are worth knowing, and both come from sound sources.

• The bag or drawer: green tomatoes with a ripe banana in a paper bag or a closed drawer, kept at room warmth out of the sun. The RHS backs the drawer-and-banana method directly.
• Under cloches: lift whole outdoor plants with unripe fruit still attached and lay them on straw under cloches to ripen, which the RHS also lists for an end-of-season crop you would rather not pick truss by truss.

How long it takes

Time depends on how green the fruit was when you picked. Fruit already showing a blush, what growers call mature-green, can colour up in as little as 1 to 5 days in a paper bag, according to Illinois Extension. Hard, fully green fruit takes longer, closer to the 1 to 2 weeks people usually quote. So check the bag every couple of days, pull anything that has turned, and give the rest more time rather than a single hard deadline.

The blight race, and why you pick before the damp

Pick your green fruit before the first warm, wet, humid spell of autumn, because that is the weather that brings late blight. Blight spreads in warm, wet conditions and needs several hours of surface wetness to take hold, and once it is in the plant it moves through fruit fast. In the UK, outbreaks can start from June onwards, usually earliest in the South West, so the risk is live across the whole back half of the season.

Behind the vague "warm and damp" sits a measurable trigger. The Hutton Criteria, the modern UK standard refined from the older Smith Period, flag a blight risk after two consecutive days with a minimum temperature of 10°C and at least 6 hours of relative humidity at or above 90 per cent. Those criteria were written for potato blight, yet the same pathogen and the same warm-damp trigger drive the disease in tomatoes, so they read as a fair early-warning for both. When a forecast lines up two muggy nights like that, treat it as your signal to get the fruit in.

Why before, not after

The order is the whole point. Pick green fruit from a healthy plant and it ripens indoors as described above. Pick it from a plant the blight has already reached and a large share of it rots in the bag a few days later, because the infection travels in the fruit you cannot see. The RHS profile of tomato blight makes the same point and suggests that once plants are affected, using the unripe fruit straight away in chutney or sauce often beats trying to ripen it. Our fuller guide to recognising and slowing tomato blight covers spotting it early, which is what gives you the day or two of warning you need to clear the plants first.

What honestly will not colour up

Some fruit is too far behind to save, and it helps to know which before you fill three drawers with disappointment. Tiny, hard, pinhead-green fruit set on the last trusses in late August rarely holds enough maturity to ripen even with ethylene and warmth; it stays green and goes soft. What rewards the effort: the larger green fruit and the blushing mature-green, the ones that were close.

So sort as you pick. The full-sized green and the ones with the faintest colour change go in the bag with the banana. The pinheads and the half-formed go to the kitchen for green tomato chutney, where being green is the point rather than the problem. If your plants struggled to set those upper trusses at all in a cool, dull summer, that is a separate story, and our note on fruit failing to set under glass covers why the flowers came to nothing.

The six-week plan, in order

Run it as a sequence and the six weeks look after themselves. In late August, stop every cordon plant two leaves above its top truss, seven trusses under glass and four outside. Through September, leave the fruit on the vine to ripen at warmth near 20°C, keep the watering even so nothing splits, and take only the lowest old leaves. Watch the forecast for two muggy nights running, the Hutton signal of 10°C and 6 hours at 90 per cent humidity, and clear the green fruit indoors before it lands. Finish them in a paper bag with a banana, somewhere warm and out of the sun, checking every couple of days. The pinheads that will never turn go straight into a pan of chutney, and nothing the plant set is wasted.

Four ways to ripen them, compared

Each method below is a way to keep mature green fruit moving towards red once the warmth runs low. Choose by how many days you have before the first frost and how much fruit is still hanging. All four rely on the same engine, the ripening hormone ethylene, which the fruit makes faster in the warm and barely makes at all in the cold.

Method	How it works	Roughly how quick	Best for
Leave on the vine	The plant keeps feeding the fruit and the longer it stays on the plant the better the flavour. Move outdoor plants to the warmest, most sheltered wall while you can.	Days to a couple of weeks while the weather stays warm; it stalls as nights cool.	Mild spells before the first frost, when fruit is already turning.
Pick and bring indoors	The RHS advice is to pick the fruits and place them somewhere warm and dark to ripen, a drawer or a cupboard rather than a bright sill.	About 2 weeks at 18–21°C; nearer 3–4 weeks at 10–15°C.	A frost forecast and fruit you want to colour up evenly.
Paper bag or box with a banana or apple	A ripe banana or apple gives off ethylene; a closed bag or lidded box traps it around the tomatoes and helps them ripen.	Quicker than leaving the fruit loose in the open at the same warmth.	A small batch you want ready quickly.
Lift and hang the whole plant	Pull the plant, shake off the soil and hang it upside down somewhere frost-free; the fruit carries on colouring as the plant slowly dies back. The RHS also suggests laying lifted plants on straw under cloches.	A couple of weeks in the warm, longer as the temperature drops.	A plant still loaded with fruit when frost is days away.

The hard line to remember is temperature. Below roughly 10°C ripening slows right down and fruit held that cold colours poorly rather than turning a clean red. Keep your chosen method somewhere that stays in the 18–21°C range, dark over bright, and check every few days for the first fruit to turn so you can use it before the rest catch up.