Mechanical Blossom Thinning In Peaches Using a Darwin String Thinner

Hand thinning is a major expense for stone fruit growers, so research on chemical and mechanical methods is critically needed. In 2009, research using a Darwin String Thinner was shown to effectively remove flowers throughout the canopy at UC Kearney Agricultural Center (UCKAC) and in several orchards in California’s San Joaquin Valley. When adequate thinning was achieved it reduced considerably the follow-up hand thinning costs. The Darwin equipment attaches to a tractor and has dozens of 24” long individual strings along a 10’ spindle that easily removes blossoms when it rotates.  The best results were obtained in trees trained to “V” and quad “V” orchards of heavy setting varieties. We are reporting on two separate studies conducted in 2010. The objectives of Study 1 were to determine the Darwin’s effectiveness in extra early to late cling peach varieties with renewal pruned vase, quad “V”, or semi-quad “V” training systems in California’s Sacramento Valley. Study 2’s objectives were to quantify the effects of various parameters such as spindle rotation speed, ground speed, string type and string configuration along the spindle to define the settings for achieving a given level of thinning. In Study 1, the Darwin plus follow-up hand thinning was compared to hand thin only using eight 2-tree replications in 5 different peach varieties at three sites.  Data included percent blossom removal and fruit set; fruit diameter, weight and time to hand thin; and fruit size and yield at harvest. For all varieties, fruit on blossomed thinned trees was significantly larger than fruit on the control trees at hand thinning time. Blossom thinning resulted in higher saleable yields in the quad “V” and renewal pruned vase-shaped trials whereas saleable yields were reduced in the semi-quad “V” trials. Blossom thinned trees had fewer fruits per tree than those hand thinned only indicating that trees were over thinned by hand.  Study 2 was conducted in 5 freestone peach or nectarine “V” or quad “V” systems near UCKAC. Within each of these orchards we tagged many individual flowering shoots. Flower counts were made before and after treatment. Generally, 150 shoots were tagged for each individual treatment. We have concluded that increased thinning can be achieved by faster spindle rotation, slower tractor speed, thicker strings and by leaving no gaps between strings. We also found differences among varieties and stages of bloom. Thus, 50% flower removal could be achieved under many different conditions.