Shoot Thinning

by Jim Law

March, 2016

Shoot thinning is the most satisfying of all vineyard operations. There is immediate gratification in transforming a disheveled mass of random green shoots into an orderly, balanced canopy structure. This can all happen at about a minute per vine. Fingers are the only tools required. The weather is usually textbook perfect (early/mid May).

 

Shoot thinning is a continuation of pruning. Uniformity, spacing and direction of shoot growth are fine-tuned. Shoot thinning is arguably the greater determinant of potential yields. When done by skilled workers (as it would have to be), it sets the stage for the next year’s pruning and training decisions.

 

Timing

 

Six to eight-inch shoot length seems to be the sweet spot. The shoots snap off easily, clusters are visible (or not visible in the case of blank shoots), and shoot direction is apparent. In a wet early spring, if phomopsis was an issue, the infection scarring can sometimes be evident. Affected shoots can be culled out to some degree.

 

Frequency is dependent on training system, variety, and vine age. Cane pruning has a significant advantage over cordon trained vines. There are few latent (blind) buds in cane pruned vines. One thorough thinning with a second quick “sucker clean up” during leave removal is adequate. However, cordon training requires several passes with most varieties. Petit Verdot, Cabernet Franc and Sauvignon Blanc never seem to cease throwing out new shoots from older wood (trunks and cordons). In order to prevent a veritable mess in the fruit zone, cordon pruned versions of these varieties require up to three passes per year.

The shoots of Chardonnay and Petit Verdot remain very fragile, easily breaking off until around bloom time. It is possible to lose a third of retained shoots in one windy thunderstorm. To mitigate this we often leave a few extra shoots, especially in the renewal head zone of cane-pruned vines.

 

Strategies

 

Hardscrabble vineyard has a plethora of varieties, clones, soils, slopes, training systems and vine ages. Each block is treated differently. All vineyard workers have many years (in some cases decades) experience with these blocks. These same people pruned the vines and will manages the canopy and harvest the grapes. I cannot stress how important this is in achieving precision in any vineyard.

Perhaps the most important instruction in any given block is the number of shoots to retain per vine. This varies considerably and is mostly dependent on historical average cluster size. The best example would be our Chardonnay. Hardscrabble has two very different clones. Clone #4 (also known as clone 108) has massive clusters potentially averaging 250 to 300 grams (over half a pound). Clone #72 clusters are small, loose and often have hens and chicks. They average less than 100 grams. Clone #4 vines are 20 to 30 years old and can easily be over cropped. They are shoot thinned down to 2 shoots per foot. Most vines are spaced at 6’, so an average of only 12 shoots per vine is retained. On the other hand, clone #72 struggles to get economic yields, so we basically leave any shoot with a cluster, as long as the fruit zone does not get too crowded.

These are extreme examples, but generally Riesling, Sauvignon Blanc, Petit Manseng and Carmenère are allowed up to 4 shoots per foot, where as Cabernet Franc, Cabernet Sauvignon, Merlot, and Petit Verdot average about 2.5 shoots per foot.

Average is an important concept. We use a balanced pruning concept. The pruner evaluates the previous season’s vigor by assessing number and size of canes, and then adjusts the retained cane length accordingly. This serves as a way of communicating individual vine capacity to the person shoot thinning. Shorter canes will have less shoots retained. This concept is especially important if a block has many replants of varying ages and vigor capacities.