Shoot and Cluster Thinning Grapes – Facts for Fancy Fruit

Shoot and Cluster Thinning Grapes

One of the most critical management practices for grapes is crop load management through shoot and cluster thinning. Many varieties grown in Indiana tend to produce a large number of “non-count” shoots from adventitious buds and basal buds at count nodes. This can lead to excess crop and shading in the canopy. In addition, some of the varieties we grow tend to be overly fruitful, producing more fruit than the vine can feasibly ripen. Carrying a large crop can result in reduced vine size and capacity, so careful thinning to balance fruit production to vegetative capacity is required.

Shoot thinning reduces excess shoot number to both adjust crop and reduce shading. Growers typically select 40-60 nodes per vine during dormant pruning. If delayed-double pruning was done, that number may be much higher. Now that the danger of frost is “mostly” past (wow, close call this week), it is time to go through the vineyard and assess shoot number and adjust it to the desired number. Five to six shoots per foot of row is generally considered to be the optimum density. That equates to 40-50 shoots per vine at typical 8 foot vine spacing. It is very easy to accomplish now while the shoots are short and tender. They are not attached to the vines very firmly so the break off easily. If you wait too long, the shoot attachment toughens and the shoots many need to be cut, greatly increasing the time required for removal.

Cluster thinning will also be necessary on most large clustered varieties. Typically each shoot is allowed to carry only one or two clusters. Excess clusters are removed, and all clusters from short, weak shoots are removed. Growers should try to adjust the crop to balance the fruit production for a “crop load ratio” of about 10. e.g. vines that average 2 lb of pruning wood should be able to produce 20 lb of fruit (and 2 lb of pruning wood again). By taking pruning weight data in your vineyard you can estimate the appropriate number of clusters to leave to produce the optimum yield. If you do not know the average cluster weights, see HO-221 Grape Varieties for Indiana for average cluster weight data from my trials and a discussion about crop load ratio. Large clustered varieties such as Chambourcin and Vidal typically have clusters that weigh 0.3 to 0.4 lb. That means you will have 1 lb of yield for every 2.5 to 3 clusters. If you leave 50 shoots per vine and each one produces 2 large clusters, you could have twice as much fruit as desired. So thinning is very important at maintaining vine size and producing high quality fruit. Cluster thinning for wine grapes is best done after bloom and fruit set. Waiting until then results in looser clusters. Thinning prior to bloom can increase berry set, resulting in full clusters that are very compact and potentially prone to bunch rots later in the season.

Share This Article
It is the policy of the Purdue University that all persons have equal opportunity and access to its educational programs, services, activities, and facilities without regard to race, religion, color, sex, age, national origin or ancestry, marital status, parental status, sexual orientation, disability or status as a veteran. Purdue is an Affirmative Action Institution. This material may be available in alternative formats. 1-888-EXT-INFO Disclaimer: Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may have similar uses. Any person using products listed in this publication assumes full responsibility for their use in accordance with current directions of the manufacturer.