Still have green tomatoes? Gardening experts use this simple trick to ripen them indoors

The gardening season is coming to an end, but your tomato plants may still be holding on to plenty of green fruit. If cooler weather has stopped them from changing colour, you might be tempted to bring them inside and line them up on the sunniest windowsill.

But that isn't necessarily the best place for them. There is a simple way to encourage green tomatoes to ripen indoors, and it involves something you probably already have in your fruit bowl. First, though, you need to know which tomatoes are worth saving.

Not every green tomato will ripen

Before picking everything from the plant, take a closer look at the fruit. According to Colorado State University, tomatoes that have reached their mature green stage can continue ripening after they have been picked.

Look for tomatoes that are nearly full size, firm and glossy, rather than small, hard fruits with a dull green surface. A pale green or whitish, star-shaped marking at the blossom end (opposite the stem) is another sign that a tomato may be mature enough to ripen indoors.

Fruit that has already started turning pink or red is also a good candidate. However, very small, immature tomatoes are less likely to ripen successfully. Leave out any fruit with bruises, damaged skin or signs of rot, as these can spoil before they have a chance to turn red.

Why a ripe banana can help

Here's where the simple trick comes in. Tomatoes naturally produce a gas called ethylene, a plant hormone that helps trigger ripening. A ripe banana releases ethylene too, which means keeping one near your green tomatoes can encourage them to change colour more quickly.

Colorado State University’s gardening advice recommends adding a ripe banana or a red tomato to the container to speed up the process.

You don't need any special equipment. Put a few healthy green tomatoes in a paper bag with a ripe banana, then fold the top loosely closed. Alternatively, place them in a cardboard box with the banana nearby.

The idea is to keep some of the ethylene around the fruit while allowing enough air to circulate. Avoid sealing the tomatoes in an airtight container, where excess moisture can encourage mould.

Warmth matters more than sunlight

You might assume tomatoes need a sunny windowsill to turn red, but direct sunlight isn't necessary once they've been picked. In fact, Colorado State University’s PlantTalk guidance explains that tomatoes can ripen without light.

Temperature is more important. Aim for a comfortably warm indoor spot, around 18–21°C (65–70°F), away from direct sunlight and cold draughts. At this temperature, mature green tomatoes may take roughly two weeks to ripen. Cooler conditions can slow the process considerably.

Don't put them in the fridge while they're still green, either. Cold temperatures can interfere with ripening and affect the eventual flavour.

Check your tomatoes regularly

Once your tomatoes are tucked away, check them every day or two. Remove any fruit that has turned red and is ready to eat, along with anything that has become soft, mouldy or damaged. Tomatoes won't all ripen at the same speed, so regular checks help prevent one spoiled fruit from affecting the rest.

If you have a large harvest, spread the tomatoes in one or two layers in a cardboard box instead of crowding them into a bag. Good air circulation helps reduce moisture build-up.

And if some tomatoes remain stubbornly green, don't despair. Mature fruit may still ripen with a little patience, but immature ones are better used in recipes such as green tomato chutney or fried green tomatoes.

With a ripe banana, a warm spot and a little attention, you may be able to enjoy the last of your garden tomatoes long after the growing season has ended.