A UW Wisconsin Extension State specialist recently put out a bulletin about our week or so stretch of unusually warm weather.   They have received many phone calls and emails asking how the extraordinary weather will affect fruit production this year.  The late spring frost of last year is still alive in our memories, and many of us are wondering if the warm weather in mid-February this year might cause early bud break and leave us susceptible once again to a disastrous spring frost event. 
Woody perennial fruit crops grown in our climate need to experience a certain amount of chilling temperatures (between 32 and 45 degrees) to be able to break out of dormancy (complete rest) and resume growth in the spring.  This requirement varies in length among the many different species and cultivars.  For example, apple trees require between 800-1700 chilling hours to complete their dormancy.  Peach trees require between 700-1000 chilling hours to complete rest.  Grapevines are thought to have a much lower chilling requirement, but the study is not complete on cold hardy grapes.
At this point, we have had around 900-1100 hours of chilling in the majority of the fruit growing areas in the state.  This means that some cultivars with low chilling requirements might have completed dormancy.  However, most cultivars grown in Wisconsin are in the higher range of chilling requirements and are not ready to “break bud”.
Reaching the chilling requirement might happen fast from now on, if temperatures stay in the 32-45 degree range.  After the required chilling hours have been experienced, plants need warmer temperatures (over 50 degrees F) to break bud and resume growth. 
This unusual warm weather does have a significant effect on the ability of dormant buds to withstand freezing.  In the middle of the winter, when temperatures are consistently below freezing, plants achieve their maximum level of cold hardiness and become resistant to freezing stress.  However, the plants will lose cold hardiness if the weather stays warmer.  So, with the warm weather we have been experiencing, buds of fruit trees and grapevines are less hardy than they were in mid-January. This means that if the temperatures were to abruptly drop into the single digits, we would experience significant damage in dormant buds.  At the moment it doesn’t look like we will experience any more of the deep winter cold temperatures.  Even though buds have lost some degree of cold hardiness it is doubtful we will have temperatures in the single digits any time soon.
Most fruit crops in Wisconsin still require more chilling hours (between 32-45 degrees F) before they will leave dormancy.  Even when they do reach their chilling requirement, temperatures would have to return to levels about 50 degrees F.  Hopefully the weather will stay cool, holding those buds dormant for longer, and giving us time to prepare for the new growing season.