At the end of April I still have some late green tomatos out on my vines. I want to ripen them before frosts arrive so I took some in recently that had just started to get some colour and put them in a paper bag with a ripening banana. Within a few days the tomatos had ripened. Bananas emit ethylene gas, pears or apples can also be used. I have dark green ones in bags at the moment ripening. Well worth doing for ripening late season tomatos if your autumn sun is rapidly dissappearing but you want to make the most of a long summers crop. I have been enjoying tomatos for 4 months now.
rob