Why Won't My Tomatoes Ripen?

by Leslie Finical Halleck

‘Celebrity’ tomato. Photo by Leslie Finical Halleck. Success with Tomatoes This time of year, I get a lot of questions about tomato fruit production ... or lack thereof during the heat. If you were expecting a big June harvest, but didn’t get one, there are a few important factors you need to consider. Tomato fruit development is impacted heavily by temperature, as well as rapid fluctuations in temperature. If temperatures get too cool or hot, or go from cool to very hot too quickly, fruit set can be disrupted. Optimal temperature for tomato fruit set is between 65 and 80 F. Once day temperatures exceed 85 F and night temps exceed 75 F, plants usually go into heat delay and will have poor fruit set, or no fruit set at all. Newer hybrids often have better heat tolerance than heirloom varieties. Let’s say your plants do set fruit, but now they aren’t ripening properly. Usually, a tomato fruit will spend 40 to 50 days in its "immature green" stage. Then it will begin to ripen, and it's called "mature green." The optimum temperature for tomato ripening is 68 to 77 F. The farther temperatures stray from that optimum, be it cooler or warmer, the more the ripening process is disrupted and the colored pigments will fail to develop. When this variation in temperature continues for extended periods of time, it can totally shut down the ripening process. The most common mistake gardeners make with tomato crops in our Texas climate is planting them too late. Plants then don’t have enough time to develop before temperatures got too hot or too cool. Proper timing of tomato planting is crucial to success. We’re lucky in Texas to be able to plant two crops of tomatoes. Your summer harvest will come from plants planted in early spring (mid-February to mid-March, depending on your zone). Fall harvest tomatoes will come from plants planted right now through mid-July. Be sure to follow the spring and summer planting times closely for your area in order to ensure a good tomato yield. Fall tomato transplants should be available now in your local garden center. So get planting!

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