Cluster thinning and leaf pulling are separate and distinct grower functions which, if performed at all during the season, are performed at different times. Despite their differences, however, we have both topics on this page because of the way they interrelate.
The grower, the vine pruning/training method and the trellis design should, in combination, establish a fruit zone. The fruit zone permits quick and efficient cluster thinning and leaf removal. Establishing a fruit zone about four feet above grade means that pruning and trellising results in all annual growth beginning slightly below this level. If there is fruiting wood higher than this fruit zone, you are going to have fruit in the canopy and that just causes you and the vine extra work.
Your pruning methods may produce variations on the fruit zone described above. There is such a thing as a split canopy (lyre trellis or Scott Henry training) which produces two fruiting zones. Then there is the high fruit zone as in the case of Geneva Double Curtain training. But these variations still create a zone that is observable, workable,
"pickable" and one that facilitates the two functions addressed here.
Based upon your knowledge of the varieties you cultivate, cluster thinning is a function that you may or may not perform. You may have a shy vinifera variety that is growing a little bit north of where it would like to be, and five pounds of fruit per year makes you very happy. On the other hand, you may have an American hybrid that comfortably produces 45 pounds of fruit, year after year. Within this range of possibilities (and if anyone is doing better than 45 pounds per vine, please let us hear about it!), you have to know the characteristics of the variety from books, other growers and your own experience.
A vine that has been allowed to overbear will give you a full report at pruning time. Canes that have not hardened properly and dropped off during the winter, late season tendrils that look wimpy, and a disturbing lack of fruiting buds for the new season are all indications that cluster thinning is advisable.
If you remove clusters, you are probably going to do it soon after berry set and before the foliage becomes thick enough to obstruct your vision. If you have trellised and trained to establish the fruit zone, the work progresses rapidly. Absent the fruit zone, the task is grueling and only slightly more possible than leaf pulling.
Leaves contribute to the development of the bud at the base of their stem. An immature bud that loses its leaf in the spring should not be expected to produce a healthy, fruitful and vigorous shoot the following year. For this reason it is a good idea to avoid removal of the first leaf on a shoot: The bud it is guarding and nurturing is likely to be one you will wish to save at pruning time. Similarly, if you want to save a fruiting cane that is strategically placed for next years crop, its fruit clusters should be removed and all of its leaves left in tact.
Leaf pulling in the fruit zone is designed to permit more light and air around the fruit. Light helps the ripening fruit and air drainage helps admit spray material and discourages pockets of mildew development.
There are two types of leaf removal in the fruit zone. Type one is removal of the lateral shoots in the fruit zone and this may be done at any time.
Type two is removal of leaves just above, at, and just below each cluster. The timing of type two leaf removal is important. It should be completed within two weeks of berry set. Later leaf removal increases the chance of sunburned fruit.
If you do not establish a fruit zone, leaf removal to expose fruit is probably impossible. Similarly, if you and your trellis permit the canopy to shade the fruit zone because it is too low, inadequately supported and/or hanging over one side of the vine row, you can forget about leaf removal as an assist to fruit ripening.
The best book currently available on the subject of trellising, canopy management and measuring canopy density in the fruit zone is Sunlight Into Wine, by Richard Smart and Mike Robinson. It is available on-line, and may be ordered by clicking on the book icon on the Home Page.
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