Dear Tom –
The tomato plants in my cold frame are dropping blossoms.
What is causing that?
— Perplexed in Barnardsville
Dear Perplexed –
I suspect either heat in the evening or, more likely, the fruit to foliage ratio on your tomato plants.
Tomatoes need relatively cool evening temperatures (in the sixties to low seventies) to set fruit. Our unusually warm summer plus the added warming of a coldframe may have affected your fruit set for a few weeks in July.
A more likely cause which we see each year is the number of fruit on your plant. A fellow grower once told me that Trust tomatoes prefer no more than 21 fruit on the vine at any one time. If you have seven “hands” in production at once, then that means only three fruit per cluster. We also notice that the fruit on the first few hands are usually very large, placing more demands on the plant’s ability to produce sugars and to ripen fruit. One way to think of it is that the tomato’s main goal is to produce seed for the next generation so it wants to be sure that it can ripen the fruit that it does set. When the fruit load exceeds some number, it responds by dropping blossoms until you harvest some of the fruit.
Growers who need a steady production usually remove immature green fruit to keep the foliage in balance with the amount of ripening fruit. Another approach is plant rotations of tomatoes so that the peak in production of the second rotation fills the gap created by the aborted blossoms.
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Author: Tom Elmore
Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.