It doesn’t take long for anybody new to grape growing (or any kind of farming) to realize that, in the end, Mother Nature is ultimately in charge. In the northern half of the country, that is especially true during the winter — particularly this year.
Some of the coldest temperatures in many years have hit several grape growing regions in the East, from Indiana and Michigan to Ohio and New York. Even in places that are already considered “cold climates” for viticulture, like Minnesota and Vermont, temperatures have gotten low enough that there is the potential for significant injury to grapevine buds in many vineyards.
The temperatures that are capable of causing injury to buds, canes, and trunks vary depending on a number of factors, including the variety, crop load, and disease management practices the previous season. And each cold event is different: How cold did it get? How long was it that cold? Was the wind blowing or not?
All this is to say that the only way for growers to determine how much bud injury they have had in a given winter is to examine a sample of buds from their vineyards for damage.
Here’s the basic method for growers to assess bud injury (you can see videos demonstrating this method at our YouTube channel at http://www.youtube.com/user/cceflgp):
Step 1: Collect at least 50 buds (100 is better) from the block about to be pruned. Make sure that the buds and canes that are collected are representative of those that would normally be kept after pruning.
Step 2: Bring them inside to warm for 24 to 48 hours. This will allow time for tissues that have been
injured to desiccate, and make it easier to differentiate between healthy and injured buds.
Step 3: Using a sharp razor blade, cut cross sections of the bud until you can tell if the large primary
bud is healthy or injured (see pictures). Count the number of dead or injured buds, and divide by the total number of buds sampled to get the percentage of injury.
Once a grower has assessed just how much damage there is, then he or she can make educated decisions about how to adapt their pruning practices in order to compensate for the amount of buds lost. The ability to retain extra buds at pruning is much easier in spur pruned systems than it is with cane pruning. Trellises usually are not set up with extra fruiting wires to accommodate more canes, which means that two canes need to be wrapped out on the same wire (bad idea). Instead, growers should spur prune vines that need to have extra buds retained on them.
Dr. Imed Dami from Ohio State recently published a paper where he examined different pruning strategies after a major cold incident killed more than 90% of the primary buds in a Pinot Gris vineyard in 2009. He found that leaving long spurs provided the best balance of fruit production and vine growth in the following year compared to the other treatments (standard spur pruning, hedging to two bud spurs, and minimal pruning).
Once the injury assessment is made, growers can then make adjustments to their pruning practices based on those results:
• 0-14% bud mortality — No adjustment necessary
• 15-34% — Retain 35% more buds
• 35-50% — Double the number of buds
• 50%+ — Hedge pruning; retain long spurs.
Retaining extra buds at pruning can have implications on vineyard management practices later in the season. The increased number of buds per foot of canopy will likely increase the need for additional shoot thinning, leaf pulling, and hedging in order to allow adequate sun and air penetration into the denser canopy for disease control and fruit quality purposes.