Answer: Fruit will fail to set on tomatoes for a number of reasons, most common of which is low nighttime temperatures. Blossom set sprays are plant growth regulators in convenient form. The ingredient that keeps the fertilized part of the flower from aborting is 4-chlorophenoxy acetic acid. There have been a number of studies about the safety of the product, and it passes with flying colors, when used according to label directions. You can try to avoid the problem of lack of fruit set by planting your tomatoes where they'll receive radiated heat at night (against a wall or fence), or by choosing shorter-season tomato varieties; those that will perform well in cool-season areas.
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