## 2 Thinning Methods
### Mechanical Thinning of Sweet Cherry Flowers  
A gradual reduction of the use of chemical agents approved for thinning as well as a large amount of labour involved in manual thinning have resulted in an increased interest in mechanical thinning over the past 10-15 years. In addition, the use of chemicals may be ineffective due to its dependence on various internal and external factors (e.g., weather \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cultivar** & **Thinning** & \multicolumn{3}{c}{**Fruit Percentage in Size Categories (\%)**} & **References** \\ \hline \multicolumn{6}{c}{**Fruit Diameter (mm)**} \\ \hline \multirow{3}{*}{’Bing’} & Control & \textless{}21.4 & 24.2 & 25.4 & \textgreater{}26.6 & \\  & Thinning & 2 & 5 & 15 & 78 & \\ \hline \multirow{3}{*}{’Sweetheart’} & Control & \textless{}21.4 & 24.2 & 25.4 & \textgreater{}26.6 & \\  & Control & 18 & 14 & 22 & 46 & \\  & Thinning & 1 & 2 & 4 & 93 & \\ \hline \multirow{3}{*}{’Lapins’} & Control & \textless{}22 & 22–23.9 & 24–25.9 & \textgreater{}26 & \\  & Contin & 82 & 3 & 8 & 7 & \\  & Thinning & 30 & 21 & 21 & 28 & \\ \hline \multirow{3}{*}{’Hedelfingen’} & Control & \textless{}21.4 & 23.5 & 25.5 & 26.5\textless{} & \\  & Control & 54 & 37 & 9 & 1 & [66] \\  & Thinning & 33 & 43 & 21 & 3 & \\ \hline \multirow{3}{*}{’Van’} & Control & \textless{}21.9 & 22–23.9 & 24–25.9 & 26\textgreater{} & \\  & Contin & 6 & 33 & 32 & 29 & [67] \\  & Thinning & 2 & 13 & 36 & 49 & \\ \hline \multirow{3}{*}{’Regina’} & Control & \textless{}22 & 22–24 & 24.1–26 & \textgreater{}26 & \\  & Contin & 0 & 3 & 26 & 72 & [19] \\  & Thinning & 0 & 0 & 5 & 95 & \\ \hline \multirow{3}{*}{’Bing’} & Control & \textless{}21.5 & 21.6–26.4 & \textgreater{}26.5 & \\  & Control & 18 & 58 & 23 & [17] \\  & Thinning & 9 & 40 & 50 & \\ \hline \multirow{3}{*}{’Bing’} & Control & \textless{}21 & 21–25.4 & \textgreater{}25.4 & \\  & Control & 19 & 59 & 22 & [40] \\  & Thinning & 9 & 55 & 36 & \\ \hline \multirow{3}{*}{’Bing’} & Control & \textless{}22 & 24 & 26 & \textgreater{}26 & \\  & Control & 20 & 30 & 33 & 18 & [68] \\  & Thinning & 3 & 14 & 29 & 53 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Influence of thinning on the distribution of cherry fruit diameter. conditions). In addition, the declining availability of workers has made mechanical thinning an attractive alternative to both manual and chemical thinning [1]. This method reduces the time consumed by manual thinning by up to 85% [69]. The thinning device consists of a rod/spindle rotating on its axis with strings fixed to it, which can destroy part of the flowers on a tree [70]. However, in order to be effective, the use of a mechanical thinner requires a specific tree training system. Open and spindle crowns must be replaced by a hedgehog system in which fruiting formations are exposed to mechanical devices [5; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79]. The advantages of mechanical thinning are speed, independence from weather conditions, and suitability for ecological orchards. The only risk is tree infection through contact of pathogens with shoots and leaves damaged during the treatment [63]. Studies show that a portable flower thinning device can effectively remove flowers from apple trees and stone species, including cherries. Of all the device parameters evaluated, the spindle rotating speed had the greatest direct impact on the capacity to remove flowers. The device tested at the Washington State University Roza Research Orchard removed 61.1% of flowers (2500-3000 rpm), 30.8% (1500-1800 rpm), and 18.0% (500-800 rpm) with a swipe of about 0.5 m\(\cdot\)s\({}^{-1}\). In trees with a central leader, 50% of the flowers could be removed within 85 s. The efficiency is also influenced by the speed [70] and weight [50] of the device. Research carried out in recent years has focused on the optimization of technical solutions (actuator, frame, string material characteristics, rotational speed, and driving speed) adapted to the fruiting characteristics of different species and cultivars [51].