Green tomatoes not ripening: heat, cold, and what to do

Tomatoes that sit full-size and green are usually waiting on temperature. Red pigment will not form when it is too hot or too cool, and late in the season the plant may simply be out of time. Work out which of the three you have, then either shade and wait, or pick the fruit and finish it indoors.

For how to read a tomato that is already coloring, see when container vegetables are ripe. What follows is for the fruit that does not get there.

Which problem do you have?

What you see	Likely cause	What to do
Full-size green fruit, or yellowish green to yellowish orange fruit, during a hot spell	Heat above about 85 F	Shade the plant, wait for cooler days
Full-size green fruit in autumn, nights in the 50s F	Cool weather	Pick before frost and ripen indoors
Lots of small, hard green fruit near the end of the season	Not enough time left	Prune the smallest, ripen the largest indoors
Frost in the forecast	End of season	Harvest full-size fruit now

Heat: the red pigment switches off

Purdue Extension explains that tomatoes do not produce lycopene and carotene, the pigments behind ripe color, when temperatures are above 85 F. The fruit that results often looks yellowish green to yellowish orange. Colorado State Extension gives the same 85 F figure. South Dakota State Extension says temperatures above 86 F significantly reduce the red and yellow pigments, and depending on the variety the fruit may turn yellow or stay partly green. Purdue gives 68-77 F as the best range for ripening.

The plant is not sick, so there is nothing to treat. Purdue’s advice is blunt: there is not much a gardener can do but wait out the weather. You can still take the edge off a pot on a balcony. A pot in full afternoon sun on a balcony carries extra heat. Shade the pot and the fruit through the worst hours; the heatwave guide for container plants covers how. Varieties differ too: South Dakota State points to cultivar differences in resistance to both heat and cold.

Cold: ripening slows, then stops

South Dakota State Extension says nights below 54 F slow ripening, and that red color will not develop below 50 F. Colorado State Extension adds that extended exposure to cool temperatures interferes with ripening and flavor development. A cool, bright September can leave a plant covered in fruit that stays green outdoors. Fruit that has already started to blush is the best candidate to hold on the plant under a cover; Colorado State says covering works well for nearly red tomatoes, but not as well for mature green ones. If a cold night is forecast, the cold snap guide lists what protects a pot, and your first frost date tells you how long you have.

End of season: too late to finish on the plant

Colorado State Extension points out that ripening a lot of fruit takes a great deal of energy from the leaves and tends to delay the whole crop. It recommends pruning off small, very green fruit that will not mature in the remaining season. That sends what the plant has left into the larger fruit.

If flowers are still dropping rather than setting, that is a separate problem: see tomato flowers falling off.

How to ripen green tomatoes indoors

Pick before frost. Colorado State Extension says to pick and bring fruit indoors before frost hits and the plants go down. Which fruit is worth picking? Purdue Extension says tomatoes that have reached at least the mature green stage, full size, can be ripened off the vine. Look for a color change to at least a lighter green; a little blush is even better. Colorado State says mature green fruit will develop good flavor, though South Dakota State notes it will not quite match fruit ripened outdoors. Iowa State Extension says to remove all mature, greenish white fruit just before the first frost.

Then:

1. Wrap or space them. Iowa State wraps each tomato in newspaper. Purdue packs fruit in shallow layers in a well-aerated place. Colorado State uses open, well-ventilated cardboard boxes at room temperature.
2. Keep them mild. Purdue gives 60-65 F for mature green to slightly blushed fruit, or warmer if you want faster ripening. Iowa State also uses 60 to 65 F in a dark place such as a basement. Colorado State gives 55 to 70 F for ripening tomatoes, and says mature green tomatoes will ripen in 14 days at 70 F.
3. Skip the sunny window. South Dakota State Extension says light is not necessary for ripening and that a sunny window shortens shelf life.
4. Do not use the refrigerator. Purdue says the typical home fridge is too chilly for tomatoes.
5. Check often. Iowa State says to inspect frequently and discard damaged or decaying fruit; South Dakota State adds that rotting fruit contaminates the rest.

Iowa State also describes a second route: pull up the whole plant and hang it upside down in a cool, dark place.

Sources

• Purdue Extension: Tomatoes not ripening?
• Colorado State University Extension, PlantTalk Colorado: How to ripen green tomatoes
• Iowa State University Extension: Can I ripen green tomatoes indoors before frost?
• South Dakota State University Extension: How to handle those green tomatoes
• South Dakota State University Extension: Will heat make my tomatoes ripen faster?