3 Reasons Your Tomatoes Aren't Turning Red—and 4 Ways to Help Them Ripen

Why aren't your tomatoes turning red? The answer may be simpler than you think.

If your tomatoes aren't turning red as quickly as you think they should, or aren't turning red at all, a simple mistake may be the reason. Tomatoes need the right temperature, pruning, and harvesting process to reach peak color; properly balanced fertilizer, correct amounts of sunlight, and exposure to ethylene gas are also important factors. Ahead, we spoke with an expert to help de-mystify this head-scratcher—and provide tips for getting your tomatoes to ripen and turn red so you can enjoy sweet, juicy tomatoes in your recipes.

Jessica Mercer works with Plant Addicts, an online plant retailer that sells bushes, trees, annuals, perennials, houseplants, and edible plants.

What Keeps Tomatoes From Turning Red

Your tomatoes may not be turning red because of things like air temperature, whether you've picked them just a tad too early, or even not pruning your tomato plants correctly. All of this can contribute to fruits not ripening properly.

Temperature

Temperatures that are too warm—or too cold—are the most common reason your tomatoes aren't ripening, says Jessica Mercer of Plant Addicts. The ideal ripening range is between 66 and 78 degrees Fahrenheit, though you have a little leeway outside those guidelines. "Temps above 85 degrees Fahrenheit inhibit the production of lycopene and carotene, which are the pigments that cause tomatoes to turn from green to their ripe color," she says. "Colder temperatures—below 60 degrees Fahrenheit—also are not conducive to fruits ripening."

Picking Too Early

While tomatoes picked at their full size will continue to ripen off the vine, those picked too early will be stunted. "Immature tomatoes will not ripen off the vine and will stay small and green after they are picked," says Mercer. "You can tell when a tomato is mature because it will stop growing and develop a blush or reddish color on the bottom of the fruit."

As a general rule, tomatoes are usually picked between July and September, with peak harvest time in August. However, some varieties ripen in July, and others can be harvested in the fall.

Insufficient Pruning

Not all types of tomato plants need heavy pruning, but some pruning can help. "Pruning suckers early from an indeterminate tomato plant can redirect the plant's energy into producing more productive vines. Determinate varieties that fruit all at once typically do not need much pruning," says Mercer.

However, stopping the growth of new vines as the plant's production comes to a close allows the current crop to receive more energy. "Some gardeners pinch the ends of the plants at the end of the growing season to arrest further growth of the vines," says Mercer. "The process is called 'topping', and it can help the plant focus on ripening the fruits that are present instead of putting on new growth."

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Tips for Helping Your Tomatoes Ripen

Now that you know why your tomatoes aren't turning red, consider these tips to help them ripen.

Expose Them to Ethylene

The production of lycopene and carotene, the compounds that give tomatoes their color, is triggered by a hormone called ethylene, which tomatoes produce in abundance while ripening, says Mercer. "Tomatoes that grow closely together in an enclosed space like a greenhouse can help each other ripen by giving off ethylene gas," she says. "Other mature fruits like apples and bananas also emit ethylene, so if you want to ripen mature tomatoes off the vine, store them in a bag with an apple."

Shelter Them From Sunscald

Light-loving tomato plants need six to eight hours of full sun per day to develop and produce fruit, but more sun doesn't mean they will be ready sooner. "Too much direct sunlight can scorch fruits and will not make them ripen more quickly," says Mercer. This scorching, called sunscald, shows up as a white, dried-out spot on the tomato and can become an entryway for fungus and disease.

Focus on Fertilizer With Phosphorus

Though feeding your plants with a balanced fertilizer when they're just starting can give them a healthy start, says Mercer, "too much nitrogen can impede fruit production and should be avoided—switch to a feed with higher phosphorus (P) when flowering begins. Tomato plants need phosphorus for fruit production and ripening."

Know Your Variety

While studying up on your tomato variety won't help the fruit ripen any faster, it gives you a better idea of exactly how long is too long for ripening. It can also help you watch for the right signs.

"Each tomato variety has a specific harvest time and ripe fruit color: Some of the larger tomato varieties can take more than 85 days from planting to reach their harvest time, whereas some cherry tomatoes are ready in just 45 days," says Mercer. "Also, not all tomatoes are meant to turn red when ripe; there are purple, pink, yellow, striped, yellow, and black varieties, so read the plant label or tag to find the specific harvest time and fruit color for your tomato plants."