If you’re only a few weeks away from the first frost and still see a fair amount of green tomatoes on the vine, the best way to promote faster ripening is to pinch off (cut) the top of your tomato plant—just remove the tip of the main stem above the topmost blossom.
You can induce stress simply by reducing the amount of water you give them. Do this gradually to allow the plants to adapt, giving less and less water each time over a three-week period.
Another easy way to induce stress is via root pruning, which interrupts the plant’s growth cycle. This special technique hampers the plant’s absorption of water and sends out distress signals, telling it to hurry up and ripen the tomatoes it’s produced.
If a sudden early frost has you bringing in all your unripe tomatoes indoors, you can speed up ripening by placing your tomatoes in brown paper bags with a bit of apple peel inside.
Apples emit high amounts of ethylene, a colorless, odorless, naturally occurring gas that causes the flesh to soften and the sugar content to rise (a process we know as ripening).