May 14, 2015

A renewed interest in root pruning

Recent weather conditions are promoting bud break and bloom in stone and pome fruit. The bloom is concentrated and pressing everyone’s schedules with multiple activities simultaneously. Michigan State University Extension has a project in tart cherries where one of our treatments is to prune roots to see if the practice on this crop can stymie tree vigor so that we can more easily harvest with a berry harvester. This is one of different approaches to canopy management to make canopies more compact. This will be the second year we have tried this practice and look forward to the results.

In spring 2014, we pruned roots using an implement we borrowed from a couple different fruit growers. In discussing this practice with Phil Brown of Phil Brown Welding, he indicated there is a renewed interest, especially in apples, in using a root pruner, and he is building new machines. Some orchardists are even asking for one that can do two rows simultaneously (one pass). The practice became popular when apple growers were using semi-dwarfing rootstocks and finding that trees were shading neighboring trees due to tight spacing. Studies have demonstrated that restricting root extension and volume by pruning with a sub-soiling knife can reduce canopy volume and vigor of fruit trees by nearly 30 percent.

The studies conducted on apples and sweet cherries in Hungary (Bruner, 1986) and apples in Ohio (Ferree, 1992) demonstrated that bloom was the optimum time to prune roots, and both studies also indicated fruit size was diminished by 20-25 percent. The practice calls for pulling a sub-soiling knife alongside tree-rows to a depth of 14-18 inches and 12-18 inches out from tree trunks on both sides of trees parallel to tree rows. The only problem with this practice is that the pruning is done without knowing for sure exactly how much of the root volume is affected (removed) since it is done below ground. The theory indicates that likely one is removing about 30 percent of the rooting volume. The practice soon became less interesting to apple growers with reports of significant reduction in fruit size. Secondly, many growers began transitioning to dwarf rootstocks, eliminating the need to prune roots.

Why is the interest returning? According to Brown, growers are using the machine to reduce fruit size for large fruited varieties, which is helping meet marketing preferences for smaller fruit size. Secondly, some of the new Tall Spindle plantings are becoming more vigorous than intended on sites with fertile soils.

Our study, which was just deployed for the first time in spring 2014, demonstrated a canopy volume reduction of close to 30 percent on Montmorency/Mahaleb rootstock, conducted at the Northwest Horticulture Research Center. Roots will be pruned once again at bloom in the plot (next week). In a separate study, we compared root pruning at bloom and at pit hardening (three weeks later) to determine the difference. We will be trying the comparison again this year. There is little to no reports of previous studies conducted on tart cherries.

— By Ron Perry, Michigan State University Extension, Department of Horticulture