Why Won't My Tomatoes Turn Red?

You have a plant loaded with big, full-sized green tomatoes, and they have just... stopped. Days pass, then weeks, and they stubbornly refuse to colour up. It is one of the most frustrating waits in gardening, and the most common reason is the exact opposite of what people assume. It is usually not a lack of sun. In fact, in high summer, too much heat is what jams the ripening process. Here is what is going on.

Heat is the usual culprit

The red colour of a ripe tomato comes mainly from a pigment called lycopene, and lycopene production has a comfort zone. It works best between about 68 and 77 degrees Fahrenheit. Once temperatures climb above roughly 85 to 90 degrees, the plant essentially stops producing the red and yellow ripening pigments altogether. So during a heatwave, your full-grown green tomatoes simply sit there in suspended animation, waiting for cooler weather. This surprises people because they assume more heat means faster ripening, when the reverse is true at the top end.

The fix is patience and, if you can, a little relief. When a heatwave breaks and temperatures settle back into the 70s and low 80s, ripening resumes and the fruit colours up over the following days. In extreme heat, light shade cloth over the plants during the hottest part of the afternoon can actually help them keep ripening.

Too much leaf, not enough focus

A second cause is a plant that is putting all its energy into foliage. Over-feeding with nitrogen produces a jungle of lush leaves and a plant more interested in growing than ripening. Ease off high-nitrogen feed once fruit has set. You do not need to strip leaves to "let sun onto the fruit" — tomatoes ripen by heat and internal hormones, not by direct light on the skin — but a wildly overgrown plant can be gently thinned to redirect its energy.

The ethylene trick for stubborn fruit

Here is the grower's secret for hurrying things along. Tomatoes ripen in response to a natural gas they produce called ethylene, and you can use this to your advantage. A tomato that has reached full size and shows the very first blush of colour, or even just a paler "mature green" stage, will ripen perfectly off the plant. Bring it indoors and set it on the counter, ideally near a banana or apple, which give off ethylene and speed the process. Never refrigerate a ripening tomato — cold permanently halts ripening and ruins the texture.

End of season and green tomatoes

When frost threatens and you still have a plant full of green fruit, do not lose it. Pick all the full-sized green and partly coloured tomatoes and ripen them indoors over the following weeks; many will turn red on a windowsill or wrapped in newspaper in a cool room. The truly immature small ones that will not ripen make excellent fried green tomatoes or chutney. Either way, a green plant at season's end is a harvest waiting to happen, not a loss.