Root Pruning of Apple Trees 



James R. Schupp 



Highmoor Farm, University of Maine 



In 1989, the public furor over Alar™ effectively 

 eliminated the use of that chemical by apple growers. 

 This left a substantial number of growers looking for 

 alternatives to Alar as a means of reducing tree growth, 

 reducing preharvest drop, and improving fruit color 

 and fruit quality. Mechanical root pruning is one 

 option that some growers considered. 



Root pruning has been studied in some detail at the 

 Ohio Agricultural Research and Development Center 

 since 1982, and at the University of Maine since 1988. 

 The results of these research efforts are summarized 

 here and are used to make the recommendations that 

 follow. 



Effects on Bearing Trees 



Table 1 contains a list of the effects of root pruning. 

 To summarize, root pruning of bearing apple trees 

 reduces the total growth of the tree. This growth 

 suppression is season-long, thus pruning time is re- 

 duced. Fruit set is not affected, so the net effect is a 

 smaller tree with just as many apples. Fruit size is 

 reduced, while fruit color, firmness, and sugar content 

 are increased. Preharvest drop is reduced. 



Effects on Non-Bearing Trees 



The effects reported above are for bearing trees. If 

 the trees have little or no crop, either from frost damage 

 or because of alternate bearing, the effects will be much 

 less dramatic. For example, in one of our studies in 

 Ohio, root pruning reduced vegetative growth in bear- 

 ing trees by over 40%, while the reduction in growth in 

 non-bearing trees was only 14%. Studies with young, 

 potted trees showed that such trees needed root prun- 

 ing twice in one season to achieve season-long reduc- 

 tions in growth, even when the pruning was quite 

 severe. Pruning the roots of non-bearing trees in the 

 orchard more than once in a single season has not been 

 tried and the possible benefits, as well as the possible 

 problems, are not known. 



When and How? 



All these effects are dependent on timing. The best 

 time to root prune is from bloom until two weeks later. 

 Pruning too late will increase preharvest drop instead 

 of reducing it and will not reduce the growth of the tree 

 in that season. 



Root pruning is done with a sharpened subsoiling 

 blade mounted on a tool bar such that it extends out 

 beyond the right rear tire of the tractor. The 3-point 

 hitch, attachment pins, and tool bar should be heavy- 

 duty. A chain or cable extending to the front axle lends 

 stability to the offset blade. Tractors in the 40 to 50 

 horsepower class have proven satisfactory, however 

 the blade cuttingthrough the soil has a tendency to pull 

 the tractor into the tree row, particularly the first 

 season the treatment is applied, and especially if the 

 cutting depth is greater than 12 inches. Our studies 

 have shown that pruningto a 12-inch depth is adequate 

 to produce the desired effect, but if a grower wants to 

 prune deeper, or if the going is tough even at a 12-inch 

 depth, the steering problem can be corrected by: a) 

 running two tractors in tandem; or b) pruning the roots 

 once at a shallow setting, then going back through the 

 original cut a second time to the final depth. Root 

 pruning in heavy sod is much more difficult than in a 

 herbicide strip and almost always will require a second 

 pass to finish the job. 



In order to take effect, root pruning must be done 

 on both sides of the row, to a depth of 12 inches. The 

 amount of vegetative growth control can be adjusted 

 somewhat by adjusting the distance from the cuts to 

 the trunk. Root pruning of overly vigorous Melrose/ 

 M.26 trees at 24 or 32 inches from the trunk produced 

 dramatic reductions in growth. In Maine, I have ob- 

 tained satisfactory results on Mclntosh/MM.lll trees 

 pruned on 2 sides at 40 inches from the trunk and to a 

 depth of 12 inches. 



What Can Go Wrong? 



In addition to the problems of the blade pulling the 

 tractor into the tree row and the difficulty in root 

 pruning through thick sod, there are some things that 

 can go wrong. Low, hanging limbs get scraped up 

 badly, and if they are large, they will present an effec- 

 tive barrier to progress down the row. Long overhang- 

 ing limbs that stick out into the drive row can make the 

 tractor driver feel as persecuted by apple trees as was 

 Dorothy in The Wizard ofOz. 



If there are large roots just under the surface, 

 occasionally one will catch on the blade instead of 

 cutting cleanly. When this happens the root will be 

 pulled off the tree and often a good chunk of the bark on 

 the trunk will be stripped away with it. Dave Ferree 



Fruit Notes, Winter, 1990 



