It takes about 40-60 days of good weather for tomatoes to go from flowering to ripe. Ripening tomatoes are a joy for gardeners as the tomato colors change, acidity increases, starches convert to sugars, and intense flavors develop.
However, sometimes the ripening goes awry. Troubled tomatoes may develop unevenly with green stripes, streaks, blotches, and yellow or green shoulders, or they may even remain green.
Below, we’ll go over how tomatoes ripen and why your tomatoes may not be turning red, not ripening on top, or not ripening evenly.
How Tomatoes Ripen
Ethylene and carotenoids are essential components of the tomato ripening process.
As the tomato fruit matures, ethylene, a plant hormone, builds up before triggering a cascade of chemical processes we equate with ripening.
Ripening helps with seed dispersal, as animals and humans are attracted to the ripened fruit color and taste.
Red tomatoes are high in the carotenoid lycopene, which gives them their red color. Yellow tomatoes are high in the carotenoid lutein, which is associated with the color yellow.
Fruits such as tomatoes, bananas, and apples are considered climacteric fruit, meaning they will continue to produce ethylene and lycopene and ripen even after being picked off the vine.
This makes dealing with green tomatoes quite easy, as they can be ripened off the vine or cooked green.
However, if you’d like to find out why your tomatoes are still green, keep reading below.
Reasons Why Your Tomatoes Are Not Ripening (Turning Red)
Temperatures Too Hot
The most often cited reason why your tomatoes are not turning red is due to unseasonably warm weather.
The optimum temperature for ripening tomatoes is between 70-75°C (21-24°C).
The more the mercury pushes past 85°F (29°C), the less ethylene and lycopene (chemicals that affect ripening and color) are produced by your tomato plants.
Fortunately, high temperatures do not cause fruits to stay permanently unripened. Once temperatures start to drop, your fruit will ripen naturally. 
When temperatures are hot, provide some shading or harvest mature green tomatoes early and store them in a paper bag or cupboard together to encourage quick and even ripening.
Temperatures Too Cold
Similarly, temperatures below 54°F (12°C) will also inhibit ethylene and lycopene levels in tomatoes, causing discoloration and tomatoes to stay unripe.
Tomatoes that are subject to cold chills, however, may permanently lose their flavoring, nutritional content, and coloration.
Avoid chilling tomatoes to preserve their flavor and color. Store tomatoes in a fridge for 1-3 days only if tomatoes are overripe and cannot be eaten immediately.
Nutrient Imbalance (N, P, K, Mg)
Excessive use of nitrogen fertilizers will cause tomato plants to produce abundant foliage and set very few tomatoes. It may also lead to a condition called blossom drop.
The effects of nitrogen on ripening, however, are unclear, with some studies showing the use of lime fertilizers may increase the proportion of unevenly colored fruit in some varieties. 
High levels of magnesium and phosphorus, combined with low levels of potassium, however, will cause tomatoes to develop waxy, blotchy, uneven ripening, and yellow shoulders.
Although tomatoes need a lot of water during the early growth stages, their water needs should be reduced during the second half.
Overwatering may cause root damage, which inhibits nutrient uptake, affecting fruit setting and ripening. It also increases oxidative stress, leading to reduced calcium levels and a condition called tomato rot. Which causes discoloration and spoiled fruit.
Overwatering also causes the fruit to grow faster than the skin, leading to cracked fruits. It may also lead to bland fruit, as the flavors are diluted by the extra water.
Surprisingly, underwatering your tomato plants will increase the sugar, acid, soluble solids, and lycopene in your tomatoes. Potentially leading to better-tasting and more colorful tomatoes.
There are hundreds of tomato viruses that may affect the appearance and size of ripening tomato fruits. Some viruses will also cause tomatoes to not turn red.
Tobacco mosaic virus and tomato mosaic virus will cause fruits to develop a bronzed, patchy appearance.
Tomato spotted wilt virus and Pepino mosaic virus will cause tomato fruits to ripen unevenly with pale patches or marbling.
Most viruses are incurable, so it’s best to destroy affected plants and fruits. Control disease-spreading insects such as aphids, and practice good sanitation and crop rotation.
Silverleaf White Flies
Silverleaf white flies feed on tomato plants and then inject toxins into the plant. Causing irregular ripening of tomato fruits. Mature green fruit will appear normal until they begin to ripen.
Symptoms during ripening include green or yellow striping on the outside fruit which fails to ripen. The pattern also extends inside the fruit, causing parts of the fruit to remain green and unripe.
Control with insecticide or natural predators.
When tomatoes are planted too closely together, they may start competing with each other for nutrients and sunlight. Although sunlight does not directly affect fruit ripening, it is essential for plant growth.
Lack of sunlight and overcrowded plants also invite pests that spread viral and fungal diseases. That may lead to fruit discoloration and damage.
Picked the Wrong Variety
Most hybrid tomatoes grow uniformly red; however, some heirloom tomatoes have naturally green shoulders that ripen later than the base of the fruit.
Other heirloom tomatoes turn yellow, orange, bi-color, black, or even stay green when ripened. Research what ripened fruit looks like for the variety chosen.
Frequently Asked Questions
Do tomatoes ripen faster on or off the vine?
Once tomatoes reach the breaker stage, it does not matter if they ripen on or off the vine. They can be harvested at this stage without losing flavor, quality, or nutrition. Tomatoes at the breaker stage have reached their full size and are sealed off from the main vines. Ripening on the vine may be slower in extreme temperatures. Which inhibits the ripening chemicals lycopene and ethylene. 
Will tomatoes ripen if picked green?
Yes, tomatoes (like bananas, apples, and avocados) are considered climacteric fruit. Meaning they will continue to produce ethylene and ripen even after being picked off the vine. However, it is better that you pick tomatoes after the breaker stage. This is when the vines are sealed off from the fruit and no further nutrients are transferred from the vine.
Does pruning tomatoes help ripen?
Yes, removing suckers from tomato plants will help the plant redirect energy toward developing fruit. Removing the growing tip of each main stem will also stop new flowering and fruit development. Allowing the plants to direct energy and nutrients to fruit development. Trimming roots will also stress the plant, triggering the existing fruits to ripen faster in order to survive. Pruning excess fruits also redistributes energy into the remaining fruit.
 Biggs, M. S., Woodson, W. R., & Handa, A. K. (1988). Biochemical basis of high-temperature inhibition of ethylene biosynthesis in ripening tomato fruits. Physiologia Plantarum, 72(3), 572–578. https://doi.org/10.1111/j.1399-3054.1988.tb09167.x.
 Winsor, G. W., & Long, M. I. E. (1967). The Effects of Nitrogen, Phosphorus, Potassium Magnesium and Lime in Factorial Combination on Ripening Disorders of Glasshouse Tomatoes. Journal of Horticultural Science, 42(4), 391–402. https://doi.org/10.1080/00221589.1967. 11514223.
 Author unknown, (nd). Harvesting and Ripening Tomatoes. Kansas State Research and Extension. https://www.johnson.k-state.edu/lawn-garden/agent-articles/vegetables/harvest- ripen-tomatoes.html.