Our vines have just started to flower and, hallelujah, the weather is calm and clear! In the photo, you see a lower cluster that has just begun to bloom and an upper cluster that is just about to. Grapes self-pollinate. In the first hour after one of the flowers bursts open, microscopic pollen grains fall from the tips of the five white filaments onto the the would-be berry at the center where they hopefully attach to the stigma (the white circle with the green dot at the center if you zoom in on the photo). This sets in process a chain of events that culminates in a grape berry. If it is raining when the flower opens, the pollen can be washed away, or if it is too hot and windy, the pollen can dry up and blow away. When this occurs, only a fraction of the berries set and this has implications throughout the entire growing and ripening season.
The amount of fruit on the vine controls the canopy vigor. A poor fruit set causes the vine to put too much energy into shoot growth. Come the ripening season, the vine has trouble diverting its energy from the canopy into the ripening clusters. So a light fruit set can actually lead to under ripe fruit at harvest, which is perhaps opposite from what you may have thought. If there is too much fruit on the vine, most commonly due to too many clusters, the canopy will not grow enough and there will not be enough photosynthesis to ripen the clusters. So there is an optimal balance to achieve between fruit mass and canopy growth. The main goal of high-quality viticulture is to achieve this optimal balance each growing season. Many factors influence this balance including: choosing the proper rootstock for each soil type, how the vines are trained and pruned, whether there was damage from a spring frost, how much water the vines receive, and quite importantly, the weather conditions at pollination. One thing is for certain: each growing season is unique.
In those years where Mother Nature has conspired to alter the optimal balance of the vine, all is not lost. But it means considerable human intervention can be required to get the vines where they need to be. Fortunately, so far in 2009, Mother Nature is in a benevolent humor: the vines are optimally poised for a great vintage.