To ripen green tomatoes, place the tomatoes two layers deep in a box and keep it covered. Wrapping each individual green tomato in newspaper or wax paper before putting them in the box also assists the ripening process.
Select the green tomatoes with a glossy, green appearance. To increase the ripening process, place a ripe fruit in the box with the green tomatoes. The ethylene gas produced by the ripe fruit will allow the green tomatoes to ripen faster. Remove the fruit if it becomes moldy or discolored, and replace it with another ripe fruit.
Green tomatoes also ripen by sitting in an open area. The tomatoes do not need light to ripen, but indoor environments can affect the process. While low humidity levels will make the tomatoes shrivel, high humidity levels can cause them to become moldy. Find a cool, dry place to ripen the tomatoes.
Another method to ripen green tomatoes is to pull the entire plant out at the root and shake off the excess dirt. Hang the plant upside-down in a cool, dry environment, such as a garage.
Green tomatoes ripen in about two weeks in temperatures between 65 and 70 degrees Fahrenheit. Cooler temperatures do not allow the flavor of the tomatoes to develop as well.