Ripening and color development in tomatoes is governed primarily by two factors: temperature and the presence of a naturally occurring hormone called “ethylene.”

The optimum temperature range for ripening mature green tomatoes is 68–77 F. The further temperatures stray from the optimum, the slower the ripening process will be. And, when temperatures are outside the optimum range for extended periods, conditions may become so stressful that the ripening process virtually halts.

At the same time, tomatoes do not produce lycopene and carotene, the pigments responsible for ripe tomato color, when temperatures are above 85 F. So, extended periods of extreme heat cause tomatoes to stop ripening. The resulting fruits often appear yellowish green to yellowish orange.

There’s not much the gardener can do but wait out the weather. As temperatures become more favorable, the ripening process should get back on track, assuming other stresses do not take their toll!

Tomatoes that have reached at least the mature green stage can be ripened off the vine. Look for a color change to at least a lighter green — and a little bit of blush is even better. Those that are still immature green will never ripen.

Store mature green to slightly blushed fruits at 60-65 deg. F, or warmer if faster ripening is desired. Ripe fruits can be stored cooler, as low as 45 deg. F. The typical home refrigerator is too chilly for storing tomatoes. Instead, pack fruits in shallow layers and keep in a well-aerated location where temperatures can be maintained and progress monitored.