Unripened tomatoes growing on a plant in Central Oregon. (Amy Jo Detweiler/Oregon State University)

Q: Why aren’t my tomatoes ripening this summer?

A: You would think with this relentless heat that our vegetable crops would be ripening faster than usual, but actually the opposite is true for some crops, including tomatoes. So, although you may be seeing a lot of leafy growth, you may not be seeing a lot of red tomatoes.

The ideal temperature range for ripening tomatoes is 70 to 75 degrees. Once the temperature exceeds this range (it has been in the nineties consistently this season), the ripening process greatly slows down and may even cease. Photosynthesis (the process that makes food for the plant) is greatly reduced at 94 degrees.

Excessive heat also affects the plant’s ability to produce carotene and lycopene, which are the pigments that make our tomatoes turn the much desired orange or red. Too many hot days means green tomatoes galore. As the heat lets up, the tomatoes will start to ripen more quickly. Additionally, sustained heat can lead to poor pollination and fertilization on tomato plants (and other plants for that matter).

You may see blossoms drop off. Heat can also affect the overall fruit quality, including inadequate gel development inside the fruit and the development of hard, white areas when the fruit is cut open. Overall fruit size may also be smaller due to excessive heat. If you are growing plants in a greenhouse, maintain greenhouse temperatures between 55 to 85 degrees.

You can try to ripen green tomatoes off the vine by following these few simple steps, which can also be followed in late fall before the first hard freeze. Only mature green tomatoes can be ripened off the vine. Mature tomatoes are light green in color with a reddish tinge on the blossom end. The pulp inside of a mature tomato should be jelly-like, not firm. To ripen, wrap them individually in newspaper and place them in a box or paper bag so they are not touching one another in a room at 60 to 70 degrees. Check them every three to four days for rotten ones. They will ripen over a three to four week period.

— Amy Jo Detweiler is an associate professor of horticulture at Oregon State University Extension Service.