Crop Load Management- To Thin or Not to Thin?
How many clusters should I have on my grape vines? Good question. It varies by age of the vine, vineyard site, weather, cluster size, cultivar and individual vine vigor. This is the gray area of viticulture where art meets science.
I don’t think we ever know what that perfect cluster number is. Only through experience and knowledge of our individual cultivars and site can be narrow in what that perfect cluster number per vine might be. Last year we were dropping some crop in response to the severe drought. This year we may want to reduce our crop loads to help hasten the harvest dates for a season in which bloom dates were 10+ days behind schedule. Over cropped vines tend to ripen later in the season.
The science of Cluster Thinning involves the removal of all clusters during the first two non-fruiting years and removal of excess clusters during fruiting years to keep from over-cropping. Over cropping of young vines will reduce the size of their root system. Unless vines are very vigorous, fruit should not be retained during the first and second growing season. It is recommended to remove flower clusters early when the shoots are 12″ long. If the vines exhibit a lot of vigor, a small amount of fruit can be left during the second season, but it should be only a few clusters per plant.
An early cluster thinning can be done when the shoots are about 12″ in length for table grapes and some wine grapes that exhibit poor fruit set. This early cluster removal should increase fruit set and increase berry size. Early cluster thinning is not recommended for cultivars that normally set tight clusters due to the increased chance of tighter clusters causing bunch rots.
For mature winegrapes thinning shortly after fruit set can also increase cluster size and compactness. Waiting until 4-6 weeks after fruit set will minimize this plant compensation factor. This practice works well for tight clustered cultivars like Seyval, Leon Millot and Vignoles.
The following rules of thumb for after bloom cluster thinning can be used :
– Remove all clusters from shoots less than 12″ long.
– Leave one cluster per shoot for shoots 12-24″ long.
– Leave two clusters per shoot for shoots more than 24″ long.
– Remove small or poorly pollinated clusters
– Remove late emerging clusters that probably will not ripen
– Remove small clusters on the distill end of shoots
A late cluster removal (green harvest) can also be done around veraison for a final crop load adjustment and promote ripening. Leaving a heavy crop load on very vigorous plants and then adjusting the crop load at veraison can be used to slow the growth of very vigorous plants. Removing all or most of the clusters on low vigor vines before berry set will help to increase vine vigor for future crops.
The chart above can be used to help determine how many clusters you want to retain per vine. It is taken from my Vineyard Pruning Cheat Sheet. This Cheat Sheet will give you approximate cluster sizes that can be expected for the different winegrape cultivars grown in the upper Midwest. This Cheat Sheet and my Canopy Management Concepts document can be found here: http://www.extension.iastate.edu/wine/viticulture
Mike White, CCA, CPAg, CSW, is the Viticulture Specialist at Iowa State University Extension and Outreach in Indianola, Iowa.