The cold snap late last week illustrates the judgment required to know when and how much to prune grapes in Iowa. It is essential to prune grapes annually in the spring because without pruning, they would attempt to produce far more grapes than possible without damaging and possibly killing the vine. And, pruning is a time-consuming job because the pruner is making judgments about the health of each vine, when to replace a trunk or one or even two cordons, and how to orient the canes in the right direction. Depending on how much has to be removed from the dormant vine, it can take as much as ten minutes to prune each vine. Considering that it takes nearly 200 vines to produce a ton of grapes, any commercial vineyard is going to consume lots of hours of pruning.
So, considering that a vineyard operator will want to finish pruning by about May 1 so he or she can move on to other vineyard tasks, pruning needs to start by early March. But, in early March, the probability of a later freeze is close to 100%. One more thing is that pruning stimulates bud growth. So, how do you decide when and what to prune?
At Tassel Ridge, we start with varieties that tend to bud out late and end with varieties that bud out early. This year, all pruning before April 15 will be long pruning. If we have a hard freeze and buds on long-pruned vines are damaged, we can just cut them off when we short prune. After April 15, we will start short pruning. Long pruning is much more time consuming than short pruning but after we have short pruned, we don’t have a plan B if we get a hard frost.
It turns out that the more developed the buds are, the more exposed we are to a hard frost. Once leaves have formed, temperatures in the 27–28°F range will kill about 50% of the primary buds.
So, where are we in the vineyards at Tassel Ridge? Sap is definitely flowing in some varieties but not in others as of today. As a result, we are starting to see bud swell in Marquette, La Crescent, LaCrosse and a little evidence of bud swell in Edelweiss. Because the vines are very sensitive to temperatures on a day-to-day basis, the cold nights we are experiencing right now will slow down development of the buds immediately. Similarly, a couple of 80°F days will be enough to get bud development moving quickly. So, we’d prefer to see cool to moderate temperatures (50–60°F days and nights in the 30°F range) for at least a couple of more weeks. That would allow a normal bud break about mid-May when we are at a 90% chance of no more freezes during the growing season. We are using the V-Mech mechanical pruning system to eliminate the long pruning step in the more robust varieties. We set the machine tighter to go right to a short prune so we avoid the labor in the two step process. The varieties we are about to prune in this way are Brianna, Sabrevois, St. Croix, and Frontenac.