The weather forecasts for the coming week show three days with potential frosts. For Wednesday morning, the National Weather Service is forecasting a low of 29F in Winchester and a low of 30F in Tyro. I’m sure other locations in VA are also at risk of frost.
In the Winchester area, apples are 1/4″ green tip with some cultivars showing pink. Peaches are at first pick to some early blooming cultivars having a few open flowers. The phenology in Central VA is a bit more advanced, with peaches having been in bloom for nearly two weeks. According to the critical temperature charts, we can expect a small amount of damage from these temperatures but not a complete crop failure. So, fingers crossed that it doesn’t get any colder than predicted. However, even if flowers are not killed by the frosts, there is also the risk of russets and frost rings. Temperatures are forecasted to warm up towards the end of the week.
Below are links for the Washington State University fact sheets (as pdfs) on the critical temperatures for flower buds:
Michigan State University has adapted the WSU critical temperature fact sheets into this easy-to-read fact sheet.
MSU has also put together an excellent set of resources about freezes and frosts in fruit crops.
Additionally, work done by Dr. Steve McArtney (NCSU) and others has shown that applications of Promalin (gibberellin 4+7 plus the cytokinin 6-BA) at 25 ppm immediately following a frost event may help increase fruit set and cropping. In essence, the Promalin sends signals to the developing fruit that the seeds are still intact and growing, even though the frost has killed the embryo. The result is a crop of parthenocarpic (seedless) fruit. This “rescue treatment” is not a full proof way of setting a crop, and it is still unclear under what environmental conditions and for which cultivars it works best. If the temperatures drop much below 28F, each grower will have to make a decision as to whether or not it is worth trying to rescue the crop with Promalin.
You can read more about the “rescue treatment” with Promalin in this Good Fruit Grower article.