Commercial grapevine varieties require some degree and duration of low temperature exposure in order to fulfill plant chilling requirements. Adequate chilling during the intercrop period is critical to synchronous budbreak the following growing season. However, in northern production areas, vines can be exposed to very low temperatures, or the rapid occurrence of low temperature extremes, that are outside of their range of adaptation. When temperatures fall below the level of vine cold hardiness, there can be damage to buds, canes, cordons, trunks, or roots, and even death of the vine. For further detail regarding the definition and identification of grapevine terms used here, please see WSU publication EB2018E, entitled “Canopy Management for Pacific Northwest Vineyards.”
Damage or death of buds has a variety of consequences. Dormant grape buds are called compound buds because each bud complex is comprised of three smaller buds, each of which represents a compressed shoot capable of growth (Fig. 1). The primary bud is the largest and contains the most preformed flower clusters. (Clusters on this year’s vine were actually formed last year.) The secondary bud is less developed and may or may not contain flower clusters. The tertiary bud generally only contains a shoot. The secondary and tertiary buds usually only grow out if the primary and secondary buds have been damaged. Unfortunately, the primary bud is generally the first to die when exposed to cold, followed by the secondary and tertiary buds. Therefore, knowing the extent of primary, secondary, and tertiary bud damage is critical in determining pruning strategies and estimating crop potential for the upcoming growing season.
Initial Steps in Assessing Cold Damage in Vineyards
Vineyard assessment of cold damage can be a laborious process and an unnecessary one if cold damage has not occurred. Prior to making assessments, check local weather data and compare to Washington State University’s grape cold hardiness data (available from http://wine.wsu.edu/research-extension) to see if critical temperature thresholds for damage have occurred. (More details are provided from the Cold Hardiness Program at WSU.) If these thresholds have been met, then proceed with damage assessment. Note of caution: Hardiness varies with local temperature conditions and your site may differ slightly from what WSU data indicates.
To properly assess damage, divide the vineyard into zones based on grape varieties and landscape characteristics. Factors which should be used to determine a zone include different grape varieties, changes in elevation, structures inhibiting airflow (windbreaks, roads, and surrounding agriculture), general air drainage, soil variation, and vineyard size. Select zones that are representative of the potential different temperature regions within a vineyard (Fig. 2). Once you have determined how to divide your vineyard, you can begin bud, cane, and trunk assessments as described below. This is one area where random sampling of your vineyard may not be the most appropriate and efficient means to assess cold damage. If you know that a given zone has a history of cold damage, focus assessment efforts there.