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Ripening That Huge Crop of Green Garden Tomatoes

By Carl Wilson, Colorado State University Cooperative Extension Agent, Horticulture

Here are some answers for gardeners asking whether the large number of tomatoes set this summer will ripen before frost and what they can do to speed the process.

Though October 7 is the average first fall killing frost date for Denver, cold weather and even killing frosts can come weeks earlier in late September. Standard sized tomatoes require 40 to 50 days after blossom set to reach maximum green size. Newly setting blossoms, small and very green fruit won’t mature in the remaining growing season and are best pruned off.

New, vigorous shoots also may be clipped back. Don’t prune off an excessive amount of fully formed leaves as these supply nutrients to fruit. Pruning directs plant resources to fruit that has a chance of maturing.

When the fruit set is heavy, it can work against gardeners. Ripening numerous fruit takes a lot of energy from the leaves and tends to delay the whole crop turning red. If there are only a few weeks before frost and fruit is not ripening, try removing some of the mature green fruit to ripen what’s left on the vine.

Cooler September temperatures help fruit to ripen because the red tomato pigments, lycopene and carotene, are not produced above 85 degrees F nor lycopene below 50 degrees F.

As late September approaches, gardeners often try to extend the life of their plants by covering with cloth or plastic. Covering plants works well for nearly red tomatoes, but not as well for mature green ones. Though foliage may sometimes be saved, research shows that chilling injury on green fruit occurs at temperatures of 50 degrees and decay losses rise markedly on fruit exposed to 40 degrees F. Red ones well on their way to ripening tolerate colder temperatures.

Before frost hits and plants go down, pick and bring fruit indoors to ripen. Extended exposure to cool temperatures interferes with ripening and flavor development. Clip fruit with a very short stem piece left on but one that’s not long enough to punch holes in other tomatoes. Stems ripped out of fruit will open them to decay.

Eliminate green fruit, as research shows it’s more likely to spoil than ripen and never develops the flavor consumers want anyway. Mature green fruit will develop good flavor. Mature green tomatoes are well sized and have turned light green to white. If cut open, seeds are encased in gel and no empty cavity space is present.

In addition to mature green, sort and store fruit by these groups as they will ripen at similar speeds. Fruit may be "turning" with a tinge of pink color showing, "pink" with 30 to 60 percent color showing, "light red" with 60 to 90 percent color present, and others "fully red" but not soft.

Store mature green tomatoes at 55 to 70 degrees F. Once fruit is fully ripe, it can be stored at 45 to 50 degrees F with a relative humidity of 90 – 95%. Recommended refrigerator operating temperatures of 40 degrees are certainly too cool to ripen mature green tomatoes and are colder than desired for ripe ones. Ripening enzymes are destroyed by cold temperatures whether in the garden or in a refrigerator.

Ripen tomatoes in well-ventilated, open cardboard boxes at room temperature checking them every few days to eliminate those that may have spoiled. Mature green tomatoes will ripen in 14 days at 70 degrees F and 28 days at 55 degrees F.

Gardeners have marveled at the size of tomato plants and number of tomatoes set knowing this doesn’t always happen in Colorado’s variable climate. Proper handling of fruit this fall will allow gardeners to realize the most from a bumper crop.

For color of fruit examples and description of Tomato Ripening Stages, see

Tomato Ripening Stages

Ways to use green tomatoes:

Tomato pickles, Tomato relish, Green tomato mince meat, Stewed tomatoes, Green tomato pie, Green tomato marmalade.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010