As the temperatures begin to cool and growth begins to tail off as we move into autumn, many gardeners are left with green tomatoes in their polytunnels. It can be a race against time to see how many tomatoes will ripen in time before the cold weather arrives. While tomatoes will ripen to a degree off the vine, they will ripen better when still attached to their plants. The good news is that there are several measures you can take to encourage the tomatoes to ripen more quickly.
First of all, if you tomatoes have reached almost full size, you can encourage them to ripen by cutting back on the amount of water you provide to the plants. Keep the foliage and vines dry to make sure you do not encourage blight and other diseases and what make sure what water you do give them is directed towards the soil below, not the plants themselves.
Another way to encourage fruits to ripen is to get out there with some scissors or secateurs and give the plants a 'haircut'. Cutting off the lower leaves and any dying or diseased leaves on each plant will free up energy that the plants can put into ripening the fruits. Another way to refocus the plants' energy is to remove all the flowers to concentrate on ripening up the fruits that have already formed. The flowers will likely not have time to set fruit that will mature before the first frosts, so their loss will not reduce your yield. Immature fruits may also be too small to grow to maturity late in the season, so these can also be removed to allow the energy to go into maturing the fully grown or almost fully grown tomatoes.
It may seem a bit brutal, but giving tomato plants a slight pull to shift their roots can also spur them into action and encourage fruits to mature. The slight shock will send a signal to the plants that will encourage them to hurry on towards producing viable seeds.
As the nights get colder, you can also prolong the season somewhat and allow fruits to continue to ripen by covering up your tomato plants during the night with some horticultural fleece or similar.
Finally, you can speed up the ripening process by placing bananas beneath the plants. These will give off ethylene, which can speed the process. You may still have to bring some tomatoes inside and ripen them off-vine to ensure you do not get frost damage on the fruits, but taking the measures described above may allow you to get a few more ripe tomatoes from the vines before frost threatens.