Canopy Management is a very official-sounding term for pulling leaves off of vines. Also referred to as "leaf thinning" or "leafing" it is the practice of managing the green growth in a vine to keep in balance with the fruit growth. Too much vegetation, and the grape clusters are hidden in the shadows and will not ripen evenly or productively.
Another benefit leaf thinning offers is assistance with air circulation. Lack of air circulation from too many leaves is a factor in the development of powdery mildew (which can also lead to bunch rot).
Interestingly, more vegetal growth does contribute to a wine's "vegetal" taste, but not in how you'd think. Chemical compounds are responsible for vegetal or herbaceous characters in some wines, chiefly the class of pyrazines called methoxypyrazines, or MP. MP creates the familiar green pepper/green bean aromas associated with many Bordeaux-type varieties. These chemical compounds break down in light. Several studies have revealed a strong relationship between exposure to light and decrease in MP content. So if the grape clusters receive light, not only do they ripen and gain red/black berry aromas, jam aromas, fresh fruit aromas, and fruity flavors, but the "green taste" that we associate with unripe grapes also dissipates.
So, now we know more light is good, but this doesn't mean we want the clusters to get sunburned, either. It can easily get over 100 degrees in Amador county, and if not protected, the rays can damage the fruit, as well as the overall plant. If the climate is hot and sunny, like California's, the goal is for the clusters to be shaded by leaves for about half the daylight hours.
Deciding where and when to remove leaves is a complicated task. The vineyard managers sit down with teams and look how can they can achieve the desired "appled" light effect - or get good light penetration to the clusters at different points of the year. They will strategize where and how they're going to leaf each plant because there are many row directions as well as elevations and exposures. For instance, if you have an east west row direction, then they would typically leaf on the North side which will allow the afternoon sun, or the South side, to be protected from the high-intensity head of the afternoon and evening.
And this might change over the course of the year. They might look at a vineyard in July when the sun might be hottest on the morning side, but by September or even into October the heat might be on the opposite side of the vine. So then they might go in and do a second "leafing" to remove leaves on the interior part of the canopy to create this nice micro-climate environment for the grape cluster.
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