Why Are My Tomatoes Not Turning Red?

Are your tomato vines covered in plump green fruits that refuse to turn red? While tomatoes are relatively easy to grow, they can be finicky at times. Several factors can slow or stop fruit from ripening, causing tomatoes to linger in the green stage. Extreme temperatures are the most common culprit, but soil nutrition and plant care can also play a role. If your tomatoes are not turning red, don’t fret. Discover the reasons behind this common challenge and easy ways to promote tomatoes to ripen in the garden and kitchen. 

Factors Affecting Tomato Ripening

Growing tomatoes is a slow process. The developing fruits require six to eight weeks to fully mature from the time of flower pollination. Maturation time varies between tomato varieties, of course, but sometimes there is more at play. Understanding how weather conditions, soil nutrition, and plant care impact fruit development will help you troubleshoot problems with ripening.

Temperature

Temperature is among the most limiting factors when it comes to fruit set and ripening. Summer temperatures often exceed the optimal temperature range for ripening tomatoes, which falls between 68 and 77˚F. Autumn temperatures commonly drop below this range. When temperatures stray from the ideal range, the ripening process slows down. 

Excessive heat, in particularly, can stop the ripening process because the pigments responsible for coloration, lycopene and carotene, are not produced when temperatures rise above 85-90˚F. Heat stressed plants will also divert energy away from ripening fruits and into survival, such as growing deeper roots. When temperatures are too hot or too cold for vine ripening, tomatoes can be finished off the vine, as described below. 

Nitrogen Levels in the Soil

When plants fail to perform as expected, gardeners often reach for fertilizer or the watering hose, trying to nurse plants into production. Such nurturing can be detrimental to fruit ripening. Although tomato plants are heavy feeders, too much nitrogen fertilizer encourages leaf growth at the expense of flowers and fruits. A tomato plant’s nitrogen needs vary as the plant grows, requiring more during the early stages of establishment and vegetative growth. Excessive watering can also slow fruit ripening by reducing air movement in the soil.  

Avoid excessive pampering when fruits fail to ripen. Maintain regular irrigation practices to provide consistent moisture and time fertilizer applications according to plant development. Tomatoes benefit from a fertilizer low in nitrogen, high in phosphorus, and medium to high in potassium. Make your first fertilizer application when preparing soil for planting using a complete fertilizer (10-20-10 or similar), then side dress plants once the first tomato is golf-ball sized using a nitrogen-rich fertilizer. Whenever possible, base fertilizer applications on soil tests conducted in the spring or fall prior to planting. 

Fruit Load on the Plant

Tomatoes have been bred to produce ever larger harvests, but sometimes tomato plants can be overloaded with fruits. This is particularly true when plants are trying to manage other stresses, such as high heat or drought. The developing fruits compete with roots, flowers, and foliage for limited resources. The same stressors can also cause plants to lose foliage, which means there is less energy being produced through photosynthesis.  

When fruits are failing to ripen during periods of stress, it can be helpful to thin out some of the fruit. Start with any fruits that are fully grown but stuck in the green stage and let these ripen on the countertop. This will reduce the energy demand on the plant, allowing more resources to reach the remaining tomatoes.  

Sunlight Exposure

While tomato plants require full sun for the best performance, the fruits themselves do not require sun exposure to ripen. In fact, direct sun can cause sunscald on developing fruits and increase temperatures in the plant canopy that delay pigment formation. Sunlight powers flower and fruit development through photosynthesis, which takes place in the leaves. It is the foliage, not the fruits, that need direct sunlight. As such, avoid pruning foliage in an effort to ripen fruits. Keep leaves in place to produce energy and shade the developing fruits. 

Troubleshooting Tips for Ripening Tomatoes

It is natural for gardeners to want to “fix” problems, but when it comes to ripening tomatoes, patience is our best course of action. Tomato fruits require adequate sugars and hormones to ripen, which are not something gardeners provide them. We can, however, hasten the process at times and adopt practices that direct energy toward developing fruits.  

Pruning Techniques

Pruning can benefit fruit development of indeterminate type tomatoes by limiting excessive leaf growth that directs energy away from fruit production. It also reduces the number of fruit-producing branches, which can prevent overloading plants with too much fruit. And when plants do over produce, we can thin fruits, as described above, to limit competition for water and nutrients. Pruning also increases airflow through the plant foliage, which helps fight foliar diseases and keep plants healthier.  

A developing tomato plant produces a main shoot, called a vine, from which suckers (side shoots) and fruit clusters develop in the leaf axils, which is where the leaves meet the stem. Begin pruning when the plant produces its first flower cluster. Follow these steps: 

1. Remove all suckers except the first one below the lowest flower cluster. This is the strongest sucker and can be left as a secondary shoot for fruit production.
2.  Remove suckers by pinching them between your thumbnail and forefinger, cutting them as close to the stem as possible. Pruners can be used on thicker suckers.
3. Remove the bottommost leaves to limit fungal infections and reduce competition for water and nutrients.

It is best to remove suckers when they are still small. Check the main vine and secondary shoot regularly for new suckers and remove these. As fruits in the upper canopy begin to plump, stop pinching suckers from the upper portion of the vine to allow the canopy to develop. This will shade fruits and protect them from sunscald. 

Harvesting and Storage

When weather conditions do not favor ripening fruit on the vine, fully grown green tomatoes can be harvested and ripened indoors. Many gardeners do this in fall before a coming freeze. It is important that tomatoes reach their mature size before harvesting, as immature green tomatoes will never ripen. Look for the first hints of color change to indicate a tomato is ready to harvest. Even tomatoes that refuse to turn red will lighten in color to a paler green, or even take on yellowish hues. 

Store harvested tomatoes indoors at room temperature. Do not refrigerate tomatoes (even fully ripe ones) as the flavor deteriorates and green tomatoes need warmer temperatures to ripen. Remember, the optimal temperature for ripening is between 68 and 77˚F, this applies indoors, too. Green tomatoes may take several days to fully ripen. 

Tomato ripening is powered by ethylene, a hormone produced by plants. Many of the tomatoes sold at grocery stores were harvested while still green and ripened through exposure to ethylene gas. Plants and fruits also produce ethylene naturally. We can use this to our advantage when ripening tomato fruits off the vine by placing a few green tomatoes together in a paper bag. This creates a closed environment for the ethylene being released by the tomatoes to collect and power ripening.

Some fruits release a lot of ethylene as they ripen. Bananas, for example, are ethylene powerhouses, which is why many people store them away from other fruits. You can use the ethylene released by bananas (or apples or kiwi) to hasten tomato ripening by placing them together on the counter or in a fruit bowl. 

Green tomatoes ripened indoors taste almost as good as their vine-ripened counterparts, but not quite. For the best tasting tomatoes, allow fruits to fully ripen on the vine. Follow proper pruning, watering, and fertilizer to maintain healthy, vigorous plants and reduce competition for limited resources. Be patient. Tomatoes take a long time to ripen, but the flavor is worth the wait.