by Jovita Turan, 24 May, 2020
There are many factors, which affect vinegrape quality, many of which are climate-related and not readily controllable. However, one important factor that is largely controllable is crop yield (Coia and Ward, 2014). According to International Horticultural Congress (2002), inappropriate application of irrigation can provoke excess yields and be inadequate for wine quality. Cluster thinning can be an effective way to control yields in some years because cluster thinning provokes a decrease in yield. However, depending on the season and intensity of cluster thinning, it is possible to find a final compensation in production at harvest.
CLUSTER THINNING AND ITS BENEFITS
Flower-cluster or fruit-cluster thinning is one of the spring-summer pruning techniques. Flower-cluster thinning has an effect similar to delayed winter pruning in permitting a more precise adjustment of fruit load to vine capacity. While fruit-cluster thinning can be even more precise in this regard, however, it is more difficult (Jackson, 2000).
Grapes typically set more clusters than they will have the energy to develop. Removing some of these groups of fruit will allow the vine to concentrate on maturing viable clusters and individual fruit (Grant, 2016).
Cluster thinning removes undesirable, small and sparse clusters, increases berry size and cluster weight, directs the vine’s capacity into the remaining clusters, and helps a vine balance foliage and fruit (Sorkel, 2004,). In some studies, a total sugar increase due to the cluster thinning has been observed. Cluster thinning can contribute to a better ripening process and better must and wine quality on vineyards under non-appropriate conditions (International Horticultural Congress, 2002). The procedure results in a greater amounts of anthocyanins, a higher total phenolic index, greater flavonol, proanthocyanidin, and polysaccharide concentrations, and lower titratable acidity. Furthermore, cluster thinning allows light and air into the plant, which enhances overall health (Grant, 2016). Most cultivars require no thinning, while others benefit from it (Sorkel, 2004,).
TIMINGS OF CLUSTER THINNING
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. Over-cropping of young vines will reduce the size of their root system. Fruit should not be retained during the first and second growing season unless vines are very vigorous. 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.
Clusters can be removed from the vines anytime during the growing season (Skinkis, 2012). White (2013), recommends removing flower clusters early when the shoots are 30 cm long, after fruit set because it can increase cluster size and compactness. Waiting until 4-6 weeks after fruit set will minimize this plant compensation factor. According to Sorkel (2004), cluster thinning should start two to three weeks before bloom (four-week window), preferably no later than one week past bloom and other resources say that Thinning of grape clusters is done immediately after the flowers have dropped and berries are set (Grant, 2016).
However, the impacts to the remaining fruit and the vine will vary depending on when it is done. Usually, most fruit thinning is done between fruit set and near the end of veraison. One potential impact of thinning closer to fruit set is that the vine will compensate for the removed crop by producing larger berries on the remaining clusters. By removing competing sinks from the vine, more energy is directed to the growing berries that are undergoing cell division, which will create larger berries.
Waiting to thin until lag phase or the ripening phase of berry development generally will not result in larger berries. This is not always the case, however, if vines are vigorous and the rate of cell division is not limited by the larger crop already, cluster thinning would not impact final berry size and weight (Skinkis, 2012).
Another question that gets asked in relation to timing is how long to let the additional fruit hang on the vine if it is being used to slow down shoot growth. To answer this, the topic of the concept of competing sinks for nutrients and photosynthates should be discussed. During the vegetative stage of development (before veraison), shoot tips and clusters compete for the vine’s resources. Reducing the strength of the sink that competes with shoot growth, by removing clusters, will enable more resources to be devoted to shoot development. In other words, the longer that fruit hangs on the vine, the longer it competes with the shoots as a sink for resources from the vine. Therefore, in this case, fruit should be removed when the vines reach veraison.
Cluster/crop thinning will cause a ripening response if it is done up through veraison. Thinning fruit after veraison is a game of diminishing returns as it gets later in the season. Both average temperature and day length are declining at this point in the season, so thinning later means that there is less opportunity for the vine to take advantage of the reduced crop load. So, to maximize the potential impact to ripening, fruit should be thinned at or prior to veraison (Walter-Peterson, 2013).
HOW MUCH TO THIN
Grape cluster thinning is not difficult. It simply means removing any clusters that are small, misshapen or even overly large (Grant, 2016). The amount of cluster thinning is determined by cluster size and vigor. There should be left one or two clusters per shoot, the basal (primary) cluster and maybe the secondary cluster should be retained (Sorkel, 2004,). Although crop reduction can lead to increased fruit quality, thinning to very low levels can cause reduced fruit quality as vines become under-cropped. Thinning to levels that are very low may result in green flavors in some cases.
The best way to determine appropriate crop levels for the vineyard requires experience with a given cultivar and vineyard block over time and maintenance of detailed records. Research conducted in various regions indicates that 0.8-1.2 square meters of leaf area are needed to support and ripen 1 kg of fruit. This equates to 16-18 leaves to support a single grape cluster. Of course, this is just a general metric and more leaf area may be required in cool climates. (Walter-Peterson, 2013).
REFERENCES
Coia, L. and Ward, D., 2014, Cluster thinning effects on crop yield and quality of Cabernet Sauvignon and Cabernet Franc grown in the Eastern US, USA, file:///C:/Users/X-PC/Downloads/FNE11-708CoiaFactSheet.pdf
Grant B. L., 2016, Tips For Improving Grape Fruit By Thinning Grapes http://www.gardeningknowhow.com/edible/fruits/grapes/thinning-grapes.htm (January 1, 2016)
Gil, M., Esteruelas, M., González, E., Kontoudakis, N., Jiménez, J., Fort, F., Canals, J.M., Hermosín-Gutiérrez, I. and Zamora, F., 2013. Effect of two different treatments for reducing grape yield in Vitis vinifera cv Syrah on wine composition and quality: berry thinning versus cluster thinning. Journal of agricultural and food chemistry, 61(20), pp.4968-4978.
International Horticultural Congress, 2002, Viticulture - Living with Limitations, Toronto, Canada.
Jackson., R.S., 2000, Wine Science, Academic Press, Orlando, Florida, 647 p.
Skinkis, P., 2012, Crop Thinning: Cluster Thinning or Cluster Removal, Oregon State Universityhttp://articles.extension.org/pages/31767/crop-thinning:-cluster-thinning-or-cluster-removal (July 26, 2012)
Sorkel, K., 2004, Commercial Grape Production in Kansas, Kansas State University,
Walter-Peterson, H., 2013, Fruit thinning in wine grape varieties, Finger lakes Grape Program http://nygpadmin.cce.cornell.edu/pdf/newsletter_notes/pdf28_pdf.pdf (July 22, 2013)
White M., 2013, Crop Load Management - To Thin or Not to Thin?, Iowa http://midwestwinepress.com/2013/09/26/crop-load-management-thin-thin/