Of course, we all hope for a late spring to help avoid the risk of damage from late spring frosts. But when the temperature warms up then turns cold again, it leaves us in a real bind. The warm temperatures push tree development, then we want the warmer conditions for good pollinating weather. When we have open flowers but conditions not favorable for high bee activity (cool or very windy conditions), there is a risk of poor pollination and fruit set. Not only do these cool conditions limit bee activity, they result in poor growth of pollen tubes even if bees do successfully pollinate flowers. Daily temperatures of 60F or above are required for good pollen tube growth, so if there are open flowers and the temperatures have consistently been cooler than 60F, you could be in trouble. There is some evidence that apples tend to set mostly on king flowers when conditions are good and more on lateral flowers in less favorable conditions. On many cultivars, fruit on lateral flowers tend to be smaller than those set on king flowers. So the damage is done in two ways: lower levels of fruit set leading to lower yields, and the fruit that does set tends to be on lateral flowers and smaller.
While there is not a lot that growers can do, there is one thing. In response to a bloom freeze a few years ago, Dr. Steve McArtney and coworkers applied growth regulators in an attempt to increase the number of fruit set parthonocarpically (without seeds). They applied promalin at 25-50 ppm a couple of days after full bloom. In their experiments, this increased fruit set sometimes – it did not work in all their experiments. But when it worked, it more than doubled fruit set compared to the frosted controls. Even with the increased fruit set from the promalin treatment, yields were not restored to normal, non-frosted levels. If you have open flowers in your orchard and conditions have been too cool to promote good fruit set, then an application of promalin may be useful.