I have unfortunately received many recent calls about frost issues in vineyards.
For the third time, I am going to post the extension pub that is currently in editorial phase before it will be officially published through UGA Extension. That bulletin should cover what I’ll say here in brief.
Cultivar selection: it is no surprise that I have heard several reports of frost in Merlot and Chardonnay (see picture below), two “eager” cultivars in terms off bud break. Choose cultivars wisely based on your site’s annual frost risk. Bordeaux reds such as Cabernet Sauvignon and Petit Verdot are relatively late-breaking, as are popular hybrid cultivars such as Traminette, Vidal blanc, and Chambourcin. Choose planting site wisely. Plant early bud breaking cultivars on convex over concave landforms. Better yet, plant ALL cultivars on convex land if you have the luxury of doing so.
Spray materials: Spraying vines with materials claimed to delay bud break or alter the temperature at which frost injury occurs is a hot topic. Research on these products shows questionable results. There are also negative side effects of attenuated crop yield and fruit maturity associated with some of these materials. While I know farmer’s need to be reactive to weather and take action accordingly, these spray materials may not be a great idea for a sustainable solution to frost. Just not enough evidence that they work, and plenty they may not work at all.
Delayed or conservative pruning: I have heard generally positive reports about the effectiveness of both delayed pruning as well as pruning to high bud densities to offset losses to frost. Remember – plans must be made to go back through and prune to appropriate bud densities and/or be prepared to shoot thin in earnest. Waiting too long to final prune the apical buds has negative consequences as well – the retained basal buds can produce lower crop with retarded maturity. Timing of the final prune and bud number retained seem to impact bud break and crop yield produced from the basal buds (see pictures below). More research is needed to understand the pruning methods that will effectively delay basal bud break and maintain crop yield.
Wind machines: These have already saved a lot of grape crop throughout Georgia this spring. Yes, upfront cost is high (roughly $35K), but perhaps not as high as the “cost” of losing a crop – especially if that crop is turned into wine and sold. Wind machines are rated to raise the temperature 1-3 F over 10-12 acres, and this will vary based on several factors including vineyard topography and the magnitude of the temperature inversion on a clear, calm night. It is not advised to run wind machines in windy conditions as they are not effective during advective freezes, but doing so could also damage the wind machine itself.
After the frost: Once a frost has occurred, there are some practical considerations.When the primary shoot is killed, a secondary bud can break and often bear some crop. While the secondary bud is certainly less fruitful than the primary bud, the secondary bud can bear 50% or more of the crop carried by the primary bud in some French hybrid cultivars. This, in turn, could be an important cultivar consideration if the vineyard is in a frost-prone site. Regardless of severity of frost injury, it is probable that vines will survive. It is important to care for any secondary and tertiary shoots that emerge and implement the perennial management tasks of shoot positioning and training system maintenance. This will ensure good leaf exposure for photosynthesis and will maintain spur positions for the following season. If there is a small crop, its maturity may be slightly delayed due to the late growth initiation and reduced leaf area. Use your chemical and sensory measurements to determine when to harvest the fruit and what it can be best used for in the winery (i.e. less-ripe fruit is often a good candidate for a rosé or sparkling wine, or used to boost acidity in other wines). Disease management is also an important post-frost injury consideration. In general, the spray program should proceed as if the frost injury didn’t occur – it is important to keep leaf area healthy regardless of crop level to ensure optimal carbon gain throughout the remainder of the season.
Good luck to those that are in higher altitudes in Georgia, and to all industry members in NC, VA, and all over the southeastern / eastern US. My sincere wishes for frost injury avoidance and a great start to the 2018 vintage.