As I write this column (August 6), there are 60 days remaining until October 5, the middle of the date range (October 1 – October 10) for the average first frost in USDA Hardiness Zone 5a. Our Ellsworth garden is in Zone 5a, as are gardens in the Bangor area.
This is sobering information for the gardener who measures success by the number (or poundage) of harvested ripe tomatoes, for if first frost arrives on schedule, tomato flowers formed after the middle of August in Zone 5a are unlikely to bear ripe fruits. Under the best of conditions, it takes about 60 days for tomatoes to go from blossoms to fully ripened fruit. In late summer, as days get shorter and nights get longer, it takes even longer.
If both pollination of the flower and fertilization of the ovules are successful, it takes 40 to 50 days for a tomato fruit to grow to the “mature green” stage. Florida tomato growers routinely pick tomatoes in this condition, load them on trucks and then transport them to large warehouses where they are gassed with ethylene, a fruit ripening hormone that promotes changes in tomato fruit pigment from green to red, pink, yellow, or orange, depending on the variety. The end result are those tasteless tomato facsimiles that consumers demand in order to add a touch of color to winter salads.
The garden-ripened tomato that we love so dearly depends on both temperature and naturally-produced ethylene for ripening. In the garden, the optimum temperature for ripening lies between 68 and 77 degrees (F). The further temperatures stray from optimum, the slower the ripening process will be and, if temperatures are outside of this range too long, the ripening process may halt completely.
Lycopene and carotene, the pigments responsible for tomato color, are not produced when the temperature exceeds 85 degrees. Thus, extended periods of heat can produce fruits that are typically yellow-green to yellow-orange, instead of red.
Understanding this bit of botany related to tomato ripening, it is not surprising that many of us have yet to ripen a tomato, other than Sungold and other cherry tomatoes that seem to play by other rules. Yet there is little the gardener can do except wait and hope. By mid-August, I will be removing any new flowers that form, knowing that they are unlikely to produce even mature green tomatoes that can be ripened indoors. With their removal, the plant can divert more of its energy to ripening fruits already formed.
Heavy fruit set late in the season can work against the gardener. Ripening many fruits at once takes a lot of energy from the plant and delays the entire crop from turning red. If only a few weeks remain until first frost and fruits are not ripening, try removing some of the mature green fruits to ripen what is left on the vine. You can readily identify the mature green tomatoes by their pale green, almost translucent appearance, perhaps with a blush of pink at the blossom end. These removed fruits can be ripened indoors as described below.
(Another diagnostic test for the mature green state is to cut a fruit in half and examine the tissues. If the tissue surrounding the seeds is yellowish and jelly-like or sticky, the fruit is at the mature green state. Any of the remaining fruits that have the same color would like be mature green as well.)
At some point, between late September and mid-October in Zone 5a, when first frost is in the forecast, the non-ripe tomatoes that have reached the mature green stage should be harvested and ripened indoors. Green tomatoes that have not reached maturity should be heaped on the compost pile.
Harvest mature green tomatoes before the first hard frost. If they are hit by a hard frost, use them up as soon as possible for they will not ripen. Recipes for green tomatoes, including several for Fried Green Tomatoes, can be found at http://www.grouprecipes.com/green-tomato.
When harvesting mature green tomatoes, clip the fruit off the plant, leaving a very short stem. Tearing the fruit off the vine will often create wounds that can quickly lead to rot in storage.
Wrap each mature green tomato in newspaper to help prevent bruising that leads to rot. Wrapping also provides the proper atmosphere for accumulation of ethylene. Place the wrapped fruits in single layers on shelving or in shallow boxes. Plan to check them frequently, at least once a week, to remove rotting fruits.
Store the tomatoes at between 55 and 70 degrees, in a location away from sunlight and not too humid. Possible suitable storage areas include garages, cellars, porches, or pantries. Do not try to ripen tomatoes on a windowsill where temperatures are typically too hot.
The length of time needed for ripening will depend on the temperature. Ripening occurs in about 14 days at 70 degrees, 28 days at 55 degrees.
Once the fruits have ripened, store them between 45 and 50 degrees, not in the refrigerator. The optimum relative humidity for storage is between 90 and 95%, but this is difficult to produce in most homes. Try to avoid extremely dry air which will quickly dehydrate the fruits. Canning or freezing the ripened tomatoes may be the best approach.