The transition from a firm, green tomato to a soft, red fruit is a complex biological process heavily reliant on environmental conditions. Gardeners often wait for this transition, which can seem longer than the growth period leading up to it. Understanding the science behind the color change and applying specific techniques can help ensure a successful and flavorful harvest.
The Biological Mechanism of Ripening
Tomato ripening is a genetically programmed process triggered by the gaseous plant hormone ethylene. As a climacteric fruit, the tomato produces a surge of ethylene once it reaches a mature green stage, signaling the start of the final ripening phase. This hormone initiates changes, notably the breakdown of chlorophyll, which causes the green color to disappear.
Simultaneously, the fruit synthesizes carotenoids, the pigments responsible for red, orange, and yellow hues. The most prominent is lycopene, which gives red tomatoes their characteristic color, and its production is strictly temperature-dependent. The ideal temperature range for efficient lycopene synthesis is between 68°F and 77°F (20°C to 25°C).
Temperatures outside this optimal window can slow or halt the ripening process. When temperatures consistently rise above 85°F (29°C), lycopene production is significantly inhibited. The fruit may still soften and turn yellowish-orange due to carotene production, but it will not achieve a deep red color until temperatures cool down.
Encouraging Ripening While Still on the Plant
To encourage existing fruit to ripen on the vine, gardeners can employ cultural practices that redirect the plant’s energy. Strategic pruning is one effective method, focusing resources away from new growth and toward developed fruit. This includes removing new side shoots, known as suckers, which compete for the plant’s energy and nutrients.
Late in the season, about a month before the expected first frost, “topping” the plant can be beneficial. This involves pinching off the growing tip of the main stem above the highest fruit cluster, preventing new flowers from setting fruit that will not mature. Removing some lower, older leaves also improves air circulation and allows more filtered light to reach the fruit, aiding ripening.
Another technique is to slightly reduce water intake once fruits reach full size and are in the mature green stage. This mild stress signals the plant that its growing season may be ending, prompting it to ripen the fruit. However, this must be done carefully, as severe drought can cause issues like blossom end rot or fruit cracking.
Post-Harvest Ripening Techniques
When frost looms or high summer heat prevents color development, mature green tomatoes can be successfully ripened indoors. A tomato is “mature green” when it has reached full size and the skin develops a slight sheen or pale green break, indicating the internal ripening mechanism is active. Fruit picked at this stage will continue to ripen, though the flavor may not fully match vine-ripened tomatoes.
The most common indoor method utilizes the ethylene gas naturally produced by the fruit. Placing tomatoes in a brown paper bag or a cardboard box confines the gas, concentrating it around the fruit to accelerate ripening. To speed up the process, a high-ethylene-producing fruit, such as a ripe banana or apple, can be added to the container.
The ideal indoor temperature is the same as the optimal on-vine range, between 68°F and 77°F. Check the tomatoes every few days and remove any showing signs of decay, as one spoiled fruit can affect others in the enclosed space. Avoid storing tomatoes in direct sunlight, which can cause overheating and lead to a mealy texture.
Troubleshooting Why Tomatoes Remain Green
When tomatoes stall in the green phase despite reaching full size, the cause is often an environmental stressor. The most frequent reason is heat stress, where sustained temperatures above 85°F halt the enzyme activity required to create lycopene. The fruit will not turn red until the weather cools, though temporary shade cloth can help reduce the microclimate temperature.
Another common problem is an imbalance of nutrients, especially an excess of nitrogen fertilizer. High nitrogen levels encourage vegetative growth and lush foliage at the expense of fruit maturation. Shifting to a fertilizer with less nitrogen and more phosphorus and potassium, or stopping fertilization, can redirect the plant’s energy back to ripening.
Potassium deficiency is linked to uneven ripening, sometimes causing yellow shoulder, where the top of the fruit stays pale or green. Finally, any significant plant stress, such as disease, persistent pest damage, or inconsistent watering, causes the plant to prioritize survival over fruit ripening. Addressing these underlying health issues is necessary for color development to proceed.