HO-88 Kentucky Viticultural Regions and Suggested Cultivars S. Kaan Kurtural and Patsy E. Wilson, Department of Horticulture, University of Kentucky; Imed E. Dami, Department of Horticulture and Crop Science, The Ohio State University rapes grown in Kentucky are sub- usually more harmful to grapevines than Even in established fruit growing areas, ject to environmental stresses that steady cool temperatures. temperatures occasionally reach critical reduceG crop yield and quality, and injure Mesoclimate is the climate of the vine- levels and cause significant damage. The and kill grapevines. Damaging critical yard site affected by its local topography. moderate hardiness of grapes increases winter temperatures, late spring frosts, The topography of a given site, including the likelihood for damage since they are short growing seasons, and extreme the absolute elevation, slope, aspect, and the most cold-sensitive of the temperate summer temperatures all occur with soils, will greatly affect the suitability of fruit crops. regularity in regions of Kentucky. How- a proposed site. Mesoclimate is much Freezing injury, or winterkill, oc- ever, despite the challenging climate, smaller in area than macroclimate. curs as a result of permanent parts of certain species and cultivars of grapes Microclimate is the environment the grapevine being damaged by sub- are grown commercially in Kentucky. within and around the canopy of the freezing temperatures. This is different The aim of this bulletin is to describe the grapevine. It is described by the sunlight from spring freeze damage that kills macroclimatic features affecting grape exposure, air temperature, wind speed, emerged shoots and flower buds. Thus, production that should be evaluated in and wetness of leaves and clusters. winterkill is much more costly, as entire the site selection process and to shorten plants can be destroyed, not just the the trial and error process of finding the Components of crop. Common injuries include winter best cultivar and climate match. sunscald, frost-splitting of trunks, death Macroclimate of dormant buds, stem blackening, and Climate Many prospective vineyardists will death of tissue in twigs, branches, and have a narrowly defined interest in vine- trunks. However, the injuries listed do Climate is defined as the prevailing yard site selection. Some regions in the weather of a geographic region. Prospec- not occur indiscriminately; many factors world have had hundreds and thousands of plant hardiness and health determine tive vineyardists must consider three of years to define, develop, and under- categories of climate: macroclimate, the probability and extent of such inju- stand their macroclimatic regions, but ries. Levels of damage from minimum mesoclimate, and microclimate. newer regions such as Kentucky typically Macroclimate is the climate of a large temperature exposure have been linked face a trial and error process of finding to tissue type, level of plant dormancy region measuring many square miles. the best cultivar and macroclimate For example, the lower Midwest region and season, mid-winter temperature fluc- match. The macroclimate is analyzed as tuation, plant size, wood maturity, and is characterized by a continental climate follows for viticultural purposes. where temperatures fluctuate on a day-to- cultivar hardiness. Hardiness is a product day basis. The macroclimate in Kentucky of not only the lowest temperatures that a Dormant Season plant can withstand but also how well the is characterized as humid and conti- Critical Low Temperature Frequency nental with severe winter temperatures plant acclimates to the winter conditions and warm summer temperatures. The Low temperature injury is the factor of an area. conditions in these climates are excellent determining the distribution of grape- The protection of cultivated plants for the growth of annual row crops. Most vines on earth. Many fruit species were against winter injury may present prob- rainfall occurs in the summer months. either bred for specific fruit quality factors lems not found in natural environments. However, in some years rainfall is sparse, or have been moved from the climate in Grapes have been subjected to severe resulting in drought. The fluctuation of which they evolved. Thus, many domestic winters and late spring frosts for thou- daily temperatures during midwinter is forms are not completely adapted to the sands of years, but many cultural prac- environment in which they are cultivated. tices exist that can augment the ability of fruit species to survive outside of their The frequency of critical temperature exposed to -15˚F may sustain 50 percent indigenous range. In general, the hardi- occurrence in a given region is the basis primary-bud injury, and possibly cane, ness of the major temperate fruit crops, for identifying minimum temperature cordon, or trunk injury. This threshold is from hardiest to most sensitive, is best hardiness zones. Advective freezes affect representative of moderately cold-hardy summarized as follows: the occurrence of minimum winter tem- hybrids (e.g. “Chambourcin”, “Chardonel”, peratures and therefore the commercial “Traminette,” and “Vidal blanc”) that are most hardy success of vineyards in Kentucky. predominantly grown in Kentucky. apple pear Radiative freeze events usually occur However, setting a critical threshold at plum during calm, clear weather as the ground -15˚F would not exclude injury at warmer cherry cools—by infrared radiation to space af- temperatures since cold hardiness varies peach/grape ter sunset. As the ground heat dissipates with cultivars, acclimation, and season. least hardy into the atmosphere, the ground becomes Figure 1 depicts the occurrence of -15˚F cooler and begins to cool the air directly between 1974 and 2005 in Kentucky. This means that most apple and pear cul- above it. Because the earth and air are The occurrence of -15˚F happens with tivars can withstand lower temperatures naturally cooler at higher elevations, varying regularity in Kentucky. Regions than can the peach or grape, and they they cool more quickly. Cold air is much that experience -15˚F less than 5 percent possess superior acclimation processes. denser than warm air and will actually of the time in a 30-year period are suit- However, great variation occurs within begin to flow in a viscous manner, from able for the most tender hybrid cultivars. and among each fruit crop; native and high to low areas, when radiative condi- Kentucky counties Pike, Martin, Floyd, hybrid cultivars are naturally hardier tions prevail. The flowing cold air “fills” and Letcher (eastern); Daviess, Breck- than introduced ones. As an example, lower lying areas, displacing warmer air enridge, Butler, Warren, McCracken, cold hardiness of grapevines can be clas- upwards, thus creating a temperature Caldwell, Graves, Lyon (western); Shelby, sified as follows: inversion, where temperature increases Jefferson, Spencer, Lincoln, Casey, Taylor, most hardy with altitude. Whitley, and Bell experience -15˚F events V. riparia The frequency of specified critical less than 5 percent of the time in a 30-year V. labrusca temperatures can be predicted for a period (Figure 1). Regions that experience hybrids (interspecic crosses) proposed vineyard site on the basis of -15˚F, 5-10 percent of the time in a 30-year V. vinifera historical temperature data, the proposed period are suitable for the most tender V. rotundifolia ˚ least hardy site’s elevation, and latitude. The thresh- cultivars. Regions that experience -15 F, old at which 50 percent of the primary 10-15 percent of the time are suitable for The greater hardiness of the hybrid grape grape buds are killed (economic failure moderately cold-hardy hybrid cultivars. cultivars is the main reason they became of the crop) at maximum hardiness level The majority of the inner Bluegrass and established in the eastern United States is presented in Table 1. Hybrid cultivars the Bluegrass regions fall within this over vinifera. designation. Regions that experience ˚ A prerequisite for understanding -15 F more than 20 percent of the time minimum temperature occurrence is an Table 1. Temperature required to kill 50 in a 30-year period include Campbell understanding of the two main types of percent of the primary buds in commercially and Pendleton counties. In these coun- important wine grape cultivars in Kentucky ties only cold-hardy cultivars are recom- freezes: advective and radiative. Advec- during January (Bluegrass region). tive freezes involve the movement of an Relative mended for commercial viticulture. entire frontal system of cold air across the Coldhardiness Lethal of Cultivar Cultivar temp. (ºF) Winter Severity Index (WSI) landscape. These polar-derived cold air Hardy Nortona -22 Another limiting factor for grape masses tend to be turbulent and rapidly Chancellor -18 production is winter severity. In very moving, allowing little or no temperature GR7M -18 stratification near the ground. They are Seyval blanc -18 continental areas, such as central Eu- also termed “top-down” freezes because Vignoles -18 rope, Asia, or North America, midwinter the standard atmospheric lapse rate, or Moderately Chambourcin -15 temperatures can be cold enough to decreasing temperature with increasing hardy Chardonel -12 kill or seriously damage the grapevines. altitude, usually holds true. Both types of Traminette -12 Damage and death can occur in French- freezes can produce critical temperatures Vidal blanc -12 American and native grapevines if the Tender Cabernet -5 winter temperature falls below 5˚F. The at any time; however, radiative freezes Sauvignonb usually happen in spring and fall; advec- a Although the Norton grapevine is hardy, it WSI is the mean temperature of the cold- tive freezes are most prevalent in winter. should only be planted in regions with greater est month (January) in any given 30-year than 181 frost-free-days and greater than 4000 ˚ growing degree days to ensure proper ripening. period. If the WSI is <5 F then the winters b The commercially available clone 337 on are extremely cold (Figure 2). If the WSI is C3309 rootstock was used in tests. 2 between 5˚F and 14˚F, the winters are very Figure 1.