The approach of the first frost or the end of the growing season often leaves home gardeners with a harvest of firm, green tomatoes. These fruits will not ripen outdoors once temperatures consistently drop below 50°F. Tomatoes are classified as a climacteric fruit, meaning they continue to mature and soften after being separated from the vine. By understanding the underlying science and controlling the storage environment, you can reliably coax these last-of-the-season fruits into their full, ripe color.
Understanding the Ripening Process
Tomato ripening is an internal, temperature-dependent process driven by ethylene, a gaseous plant hormone. This gas triggers changes within the fruit, including the breakdown of starches into sugars and the development of red pigment. Tomatoes naturally produce ethylene as they reach maturity, allowing them to finish ripening even after being picked.
Temperature dictates the rate of ethylene production and action. Ripening occurs best between 64°F and 70°F. Temperatures exceeding 85°F can halt color development, while consistently cold temperatures below 50°F will permanently stop the ripening enzyme activity, resulting in a bland, mealy texture.
Ripening Methods for Individual Tomatoes
For small batches of individually picked fruit, methods that trap the natural ethylene gas are effective. One common technique uses a paper bag or a shallow cardboard box. The tomatoes should be placed in a single layer, ensuring they do not touch each other to prevent the spread of rot or mold.
Sealing the container loosely, such as by folding the top of the paper bag, concentrates the emitted ethylene gas. You can accelerate the process by adding an ethylene-boosting fruit, such as a ripe apple or banana, to the container. These fruits release a higher concentration of the gas, signaling the green tomatoes to speed up maturation. The fruit should be checked every few days, and any that have softened or begun to show color should be removed.
Ripening Tomatoes Still Attached to the Vine
If you are facing an imminent hard frost, you can use methods that keep the fruit attached to plant material for a slower, more sustained ripening. One option is to harvest entire clusters of tomatoes, keeping them attached to a section of the stem. Leaving the small piece of stem (calyx) attached to the fruit helps reduce moisture loss and slows the potential entry point for decay organisms.
Another technique for a large volume is to harvest the entire plant by cutting it at the base of the stem or pulling it out by the roots. The whole plant can then be hung upside down in a cool, sheltered indoor location, such as a garage or basement. Nutrients and moisture remaining in the plant are thought to be drawn into the fruit, allowing for a more gradual, extended ripening period over several weeks.
Storage Conditions and Troubleshooting
To ensure success, maintain consistent storage conditions throughout the ripening period. The ideal temperature range of 60–70°F is best paired with a high humidity level, typically between 85% and 95% relative humidity, to prevent the fruit from shriveling. Ripening picked tomatoes does not require sunlight, so a dark location like a closet or cellar works well, as the color change is chemically driven, not photosynthetic.
Immature green tomatoes, which are dull, matte green and small for the variety, lack the internal maturity to produce sufficient ethylene. Only “mature green” tomatoes, which are full-sized, firm, and often have a slight sheen, will successfully transition to a ripe state.
Inspecting the stored fruit every two to three days is important. Any tomatoes that show signs of damage, mold, or softness should be immediately removed to prevent the contamination of the rest of your harvest.