Recent rains have provided some relief, but California is still in the depths of historic drought. Here in North Coast wine country, there are several potential consequences for Vintage 2014 and beyond.
A large number of established vineyards in Sonoma, Napa, Lake, and Mendocino counties are "dry farmed." That is, the vines grow during the summer and grapes ripen in the fall entirely from naturally available moisture. As it always rains very little in the summertime in California, that means the vines are supported by soil moisture left in place by winter rains or underlying groundwater supplies. If the drought conditions mean less stored groundwater for such a vineyard, the grower must make some adjustments to preserve the vines. Of course, if an alternate source of irrigation water were available, that could be supplied. In a drought, spare water is hard to find. Furthermore, irrigation of winegrape vineyards is usually through a drip system supported on wires in the trellis structure. Retrofitting a vineyard with a drip system after the vines mature would be very expensive and is an unlikely course of action.
Instead, dry-farm vintners facing water shortages will probably reduce crop. For cordon-trained and spur-pruned vines, the number and length of spurs sets in place the likely crop load. For instance, in my little 1/3-acre hobby vineyard, my vines are six feet apart in the rows and the rows themselves are nine feet apart. I have trained them as cordons, permanently tied to the fruiting wire about 38 inches above the ground. As I do my annual pruning (usually in February), I typically leave about 14 spurs on the cordon, each cut back to two buds (pictured above). Each bud makes a shoot and each shoot will typically bear two clusters of grapes, so I am pruning for about 28 clusters per vine.
In a drought year, a vineyard owner might cut back on the number of spurs or, more likely, the number of buds. Leaving the usual spurs mean the following year's pruning can more easily return to normal if the drought eases. Eliminating spurs means that new renewal spurs would need to be selected and developed in the future. Instead, cutting each of the existing spurs (in my case, 14) back to one bud instead of two will greatly reduce the amount of foliage and the size of the crop. A grower would lose tonnage and income, but in a dry year the smaller canopy might survive the low soil moisture and live on to produce again when rains return. Making such pruning decisions is fraught with anxiety. If I cut back to one bud in February because of drought, then March and April bring abundant rains, I have irretrievably reduced crop for no good purpose. But if I don't cut back and soil moisture drops too low in a dry farmed setting, the health of the vines may suffer....