Below are a few different methods for ripening end-of-season green tomatoes. Keep in mind that they will never taste as good as vine-ripened tomatoes, but a homegrown tomato is going to beat a store-bought winter tomato any day. There are a number of ways to do this, some more involved than others.
NOTE: Ripening them on a sunny windowsill is not the correct way to do it. They need warmth, not light to ripen. The light shining on the fruit will actually cause the skins of the tomatoes to become tough.
Option 1- Put unripe tomatoes on a shelf and cover them with sheets of newspaper. Every few days check under the newspaper and remove ripe fruits or any that have begun to rot. The newspaper covering helps trap a natural ethylene gas that tomatoes give off, which hastens ripening. Some people wrap each tomato individually, but this causes a lot of work when you want to check for ripe tomatoes: You have to open each one!
Option 2- You can also place tomatoes in a paper bag with an apple or banana. The fruits give off ethylene gas, which helps to speed the tomatoes’ ripening process.
Option 3- Wash and dry thoroughly. Put the green tomatoes into shallow cartons packed with straw, leaves, or shredded paper. Keep in a cool place where the temperature ranges between 55 and 70 degrees. Check the tomatoes every few days and remove them as they ripen.
Option 4- Place clean, dry tomatoes one layer deep in a shallow box. Space them out, so no tomato is touching another. 2 or 3 inches between tomatoes works well.
Store the box of green tomatoes in a cool (50 – 65 F), dry area. An unheated basement, insulated garage, or enclosed porch would work very well. If the temperature is on the cooler end, say 50 – 60, ripening will be slower, and you may have some into January. Temperatures in the 60’s will cause much more rapid ripening. High humidity typically causes more rot.
Check the tomatoes at least every week. Every other day is better. Remove any that are 50% or more red, and let them finish ripening on your kitchen counter. Check the tomatoes for signs of rot. Any rotting tomatoes should be removed. Once a tomato starts to rot, it will spread quickly.
The tomatoes should slowly ripen over a period of 3 weeks to 3 months!