Frost is coming, and it is likely to be soon. The average date of the first killing fall frost in our area is Oct. 15, but earlier for cooler spots. Before that happens, and perhaps even a little earlier, gardeners should start picking mature green tomatoes. As we discussed last week, some types of mature fruit will ripen after picking. Tomatoes are one of them.
Our tendency as gardeners is to leave tomatoes on the vine until frost threatens, and then pick everything that has some color. Some gardeners cover their vines with clear plastic to make a sort of greenhouse to keep plants warmer during the day and protect them at night.
However, covering plants with plastic or blankets is not going to help because fruit is injured by night temperatures below 50 degrees. Exposure to repeated chilly nights will damage fruit, resulting in more loss from decay. Once night temperatures drop below 40 degrees, the damage is even greater.
The good news is that mature green tomatoes will ripen well off the vine and still provide the wonderful flavor of homegrown vine-ripened tomatoes. There is no need to pick smaller green tomatoes that have no chance of ripening. Telling the difference is tricky, but not hard. Generally, the fruit should be at least three-quarters the mature size expected for the variety. They will have turned from a bright green to a lighter green or whitish color. They do not need to have started turning red or the expected mature color yet.
Once the fruit are harvested, take them indoors and prepare them for ripening. This is done by first washing them with cool, clean water and allowing them to air dry. If any are cracked or split, they are more likely to rot before ripening, so throw them out or use them in a recipe.
After discarding the damaged tomatoes, sort those that are left. The ones that have developed a tinge of color will ripen first. Put these in one group, and then sort by greenness. The next step for many gardeners varies depending on how many tomatoes they have and the capacity for storage. I recommend placing them in single layers in covered cardboard boxes. (Leave a little space between tomatoes.) Some folks wrap each fruit with newspaper and place them in a box, but this is tedious and makes it difficult to check for ripe or rotten fruit.
Tomatoes will produce ethylene gas as they ripen. Exposure to ethylene gas from another source will speed up the process. If you are in a hurry for ripe tomatoes, place some of your green ones in a closed bag with ripe bananas or tomatoes, because they produce ethylene gas.
Our mothers and grandmothers placed their green tomatoes on the windowsill, thinking that exposure to light was needed for ripening. Actually, they need the warmth, not the light. Tomatoes will take about two weeks to ripen when kept around 65 to 70 degrees. Cooler temperatures, but above 50 degrees, will result in slower ripening.
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.