Pruning and training of trees on productive rootstocks, such as Gisela 6 or 12, require techniques that are completely counter to pruning trees on Mazzard rootstock. Growers producing cherries on Mazzard rootstock must consider how to encourage precocity and productivity in the tree, whereas growers producing trees on productive rootstocks must focus on reducing crop load.
According to research by Dr. Matthew Whiting, Washington State University, and Dr. Greg Lang, Michigan State University, the production of high-quality cherries requires a gross canopy-leaf-area-to-fruit ratio of at least 200 square centimeters per fruit, which roughly translates to five leaves per fruit. Trees that have a leaf-area-to-fruit ratio that is lower than five will be unable to manufacture enough carbohydrates to produce premium cherries. It is for this reason that tree vigor is important. More leaves mean more carbohydrate production and larger cherries.
Since cherry trees have strong apical dominance, lateral branches will not readily form without some intervention. Heading cuts are the most common technique used to force branching and the production of new leaves necessary to produce a crop of premium cherries. A heading cut does two things. First, it eliminates the flow of an inhibitory plant hormone called auxin to the lateral shoot buds and allows three or four branches to develop immediately below the cut. These branches multiply the available leaves capable of producing carbohydrates for developing fruit lower on the branch. Often, new shoot leaves are at least 50 percent larger than most spur leaves, thereby contributing significant photosynthetic potential as fruit development proceeds.
Lang and MSU graduate research student Marlene Ayala (now in Santiago, Chile) found that new shoots begin to export carbohydrates to developing fruit within three weeks of development. In addition, at 56 days after full bloom, in the middle of stage III fruit growth development (between pit hardening and harvest), the leaves on these new shoots are exporting approximately the same amount of carbohydrates to the fruit as fruiting spur and nonfruiting spur leaves. It is obvious from this work that these new shoots are important to the ultimate size of the fruit.
The second important function of a heading cut is to reduce the crop. When made into one-year-old wood, the cut reduces the future cropping potential of the branch. By tipping last year’s new growth during the dormant season and removing a third to half of every shoot, a substantial portion of the future crop can be eliminated. In fact, since terminal spurs produce more flowers than basal spurs and are closer together, removing a third of the new growth will reduce about half of the fruiting potential of that branch.
Research performed by Whiting illustrates the importance of this tipping cut from the first dormant season on. From previous work, Whiting had determined that approximately five leaves per fruit produced high quality cherries on Bing/Gisela 5 trees. When Whiting left the new shoots untipped, by the third leaf, the leaf-to-fruit ratio was only 2.5:1, and by the fourth leaf, it was down to 1.8:1 (see table, page 26). Tipping the dormant shoots for one year only gave a leaf-to-fruit ratio of 4.7:1, which was near the 5:1 ratio desired. However, since the shoots were not tipped the following year, the leaf-to-fruit ratio dropped to 2.1:1, well below ideal. Tipping the previous season’s growth each dormant season helped increase the leaf-to-fruit ratio, but was not enough to maintain the five leaves per fruit that were desired. These results indicate the need to not only reduce future crop load but also the crop that will be produced in the current season.
In addition to tipping the shoots, the current season’s crop can also be reduced by heading with stub cuts. All fruiting wood should be considered renewable. Stub cuts of three inches to two feet in length, depending on the position of the branch in the tree, will replace branches, renew old spurs, and reduce the current year’s crop. Adequate light must illuminate the area around the cut in order for a new branch to form. For this reason, branches located near the tree base must be cut to a longer stub than those near the tree top. The highest quality cherries are grown at the base of last year’s growth and on young spurs. Therefore, no spur should be older than five years old. In order to maintain spurs within this age range, 20 percent of all fruiting branches need to be stubbed back and renewed each year.
The third step to producing quality fruit on productive rootstocks is to thin out pendant or small-diameter wood. Typically, these branches overset and produce small cherries. Removing these branches in the dormant season can eliminate a significant amount of small fruit before they develop.
Finally, since only leaves in full sunlight can photosynthesize at maximum capacity, thinning cuts are needed to produce light channels to inner and lower portions of the tree. Branches in the top of the tree and on the perimeter should be reduced to a single shoot. This is the fourth and final step to properly prune trees.
Balancing the crop load to maintain the proper leaf-area-to-fruit ratio assumes that adequate irrigation and nutrient management is being maintained. This means that nitrogen is being applied at adequate levels including a low-biuret urea foliar spray (30 to 45 pounds of nitrogen per acre) in the fall just prior to leaf senescence. This application will increase nitrogen storage levels in the buds; Lang has preliminary data that shows it may help to produce larger spur leaves in the spring and thus more carbohydrate production for fruit growth. A little marginal leaf burn may occur at higher spray volumes, so this foliar spray method should only be used on a trial basis at this time.
Whiting, M.D. and G.A. Lang. 2004. Bing sweet cherry on the dwarfing rootstock Gisela 5: Thinning affects fruit quality and vegetative growth but not net CO2 exchange. Journal of American Society of Horticulture Science. 129 (3): 407-415.