John Strang, April Satanek, and Tom Shearin
University of Kentucky Dept. of Horticulture, Zina Merkin
University of Kentucky, Dept. of Landscape Architecture.Tom Priddy
University of Kentucky, Agricultural Meteorologist
Dr. Tony Wolf, Viticulturist from Virginia Polytechnic Institute and State University has developed site selection criteria for vinifera grapes. This information is based on some of his work.
Grape tissue freezing studies and field results show that -8F is the point at which significant injury is sustained by vinifera grapes. By examining historical weather records for a site it is possible to calculate the number of years that -8F or lower was reached for a particular site. If this data is matched with economic data the following can be used to determine whether a site is acceptable for growing vinifera grapes.
-8F one year out of 10 is profitable
-8F two years out of 10 is a break even situation
-8F three years out of 10 results in an economic loss
The following Kentucky maps were developed using National Weather Service records from Kentucky and surrounding states. The average number of years with -8F or lower over a 10-year period was calculated using the temperature data for as many years as there were records. Temperature records used to calculate these numbers ranged from as little as 12 years to 106 years.
If a year sustained -8F or lower more than once a winter this was still recorded as one incidence. In other words if there were multiple numbers of events during a winter this did not add to our average. A site that received multiple injurious events in a winter would be considered colder and less desirable than a site where -8F or lower was reached only once. Thus, the numbers on these maps are conservative in this respect. Additionally these maps are for low temperature winter injury and do not take into account crop losses from late spring frost injury.
The first map lists the average number of years where -8F or lower was reached per 10-year period and the number of years of records used to calculate this number for each weather station. The second map (color version, black and white version) contains isolines drawn around mostly half year increments for the average number of years that -8F or lower was experienced for a site over a 10-year period.
Please keep in mind that we have no control over the elevation at which these temperatures were recorded. There can be substantial differences in temperature based on the relative elevation of a site in a certain area. Lower sites tend to be colder and higher sites tend to be warmer. Rivers and large bodies of water can substantially warm sites.
American French hybrid grapes and American grapes are considerably hardier than Vinifera grapes and are recommended for colder Kentucky sites.
In selecting grape varieties growers should also consult with the USDA Plant Hardiness Zone Map to see where Kentucky fits into the big picture. Variety and cultural recommendations made in other states may differ considerably from our recommendations because of climatic differences. For example, most of the grape growing areas in Virginia are in the 7a and 7b hardiness zones, while most of Kentucky lies in the 6a and 6b hardiness zones.