Pruning establishes the number of buds retained per vine. Buds produce shoots on which the clusters are born, so the greater the number of buds per vine, the greater the potential yield. (The actual yield will depend on flowering, berry set, crop thinning and other techniques discussed later). Since shoots produce leaves, pruning determines the vine’s leaf area and therefore the vine’s ability to produce sugar—the building block for aroma, tannin and color compounds essential to wine quality. Proper pruning will create a balance between the vine’s leaf area and the number of clusters. If leaf area and crop level are not balanced, the vine is either undercropped or overcropped. If pruning is not severe enough, too many buds are retained and with a vigorous variety such as Sauvignon blanc or Cabernet Sauvignon planted on a deep, fertile soil, the vine is overcropped which stresses the vine causing weak growth and incomplete ripening. If pruning is too severe, let’s say in an effort to reduce yield, the result is equally detrimental to quality, contrary to popular belief. Because the reduced number of clusters concentrates the vine’s energy into fewer shoots. This overly vegetative condition creates a dense canopy, closing it off to both light and air circulation causing shading and poor ripening. Improper pruning decisions not only affect the quality of the current season’s crop, but affect the quality of next year’s harvest by affecting “bud fruitfulness.”
The buds that are retained at pruning in January will produce shoots. These shoots will produce more or less clusters depending on the amount of direct sunlight and heat the buds received during the previous spring. During that time, the microscopic grape clusters—flower clusters at this point—are being formed in the minute, developing bud. High temperatures and abundant direct sunlight—normal conditions in California—favor clusters and higher yields. Lower temperatures and cloud cover—more common to Bordeaux and Burgundy—diminish fruitfulness and yields.
Andy Bledsoe from the Robert Mondavi Winery recalls the influence these factors had on the ’95, ‘96 & ‘97 vintages: “The Spring of 1995 was horrendously cool and cloudy. Vines throughout California did not get enough light and temperature to encourage the vines to develop clusters the following year. Fruitfulness was very poor and therefore the cluster count was very low the following year (1996). Wine writers mistakenly attributed the low yield in 1996 to poor set. It did rain in some areas during bloom which caused a poor set in those areas.” Bledsoe continues, “by contrast, the spring of 1996 was excellent—a lot of sun and warm temperatures. We guessed that the fruitfulness would be very good in 1997, and we were right. We are seeing it right now—the cluster count is very good.”
Mondavi’s vertical trellis system and management practices such as leaf pulling and shoot positioning, promote fruitfulness by maximizing sunlight exposure onto the buds. The same results are achieved by more elaborate trellises on high vigor sites which open up the canopy by dividing it in two, maximizing light penetration. Factors such as excessive pruning, irrigation, fertilization or choosing the wrong combination of rootstock/variety/clone will also result in an overly vegetative vine and diminish bud fruitfulness, yield and grape quality. A low vigor site combined with a vertical trellis and leaf pulling opens up the canopy maximizing sunlight exposure and therefore fruitfulness (in addition to improving air circulation preventing rot).
Flowering or bloom occurs in April or May in California and June in Bordeaux and Burgundy. The flower cluster (which becomes the grape cluster after fruit set) contains up to 1,000 flowers. Bloom is the opening of the flowers in the spring following budbreak. Sections of a vineyard will bloom earlier where the soil warms more rapidly due to more favorable sun exposure. Cooler weather delays flowering and can be detrimental to quality as Burgundy grower Etienne Grivot points out, “In the ‘95 vintage we had a proportion of bad grapes. The flowering took place over a three-week period which was not absolutely perfect. The first grapes to flower were perfect at the crush, but the second part, maybe 5–6% of the grapes, had bad maturity and we had to make a selection and cut off these grapes.”
Fruit set occurs when a pollen grain fertilizes an individual flower, and the grape berry starts to grow. Usually only 20-30% of the flowers on a cluster set, the exact percentage determines the berries per cluster. Set is delayed by excessively low or high temperatures (water stress), rain or high humidity. Poor berry set (coulure in French) not only decreases yield but affects quality by preventing uniform ripeness within a cluster at harvest.