Bridge grafting girdled fruit tree trunks
Bridge grafting is an option to help save trees with extensive bark damage inflicted by rabbits during the winter.
March 26, 2014 - Author: Ron Perry, Michigan State University Extension and MSU Department of Horticulture
Several fruit tree growers have reported rabbit damage on young trees below the highest snow line as a result of this winter’s deep snow cover. With a long cold winter like we have experienced, the damage can be severe as rabbits fed themselves on tasty bark of fruit trees. The most common target this past winter has been young apple trees. In many cases, the damage is being experienced above tree guards, most being set at 18 inches above the soil line. Snow lines commonly accumulated at 24-30 inches which made trunks and branches vulnerable to the bunnies.
In St. Johns Mich., Brian Levene of Agronomic Sciences, Agro-Culture Liquid Fertilizer reported rabbit damage on 3,000 trees where bark was stripped away at 100 percent over a length of 6-8 inches. He has considered bridge grafting but due to the fact that the block needs to be uniform for experimental trials, they plan instead to regenerate and select a shoot after heading low at 18 inches.
For fruit growers who do not require the research trial uniformity, bridge grafting is an option. The process involves grafting scion sticks 1/4-1/2 inch in diameter into live tissue of bark cambium above and below the injured area, thus bridging the damage. Grafting can’t be done until the bark begins to slip in Michigan about late April. However, scion sticks should be selected now, while dormant. Store scion sticks in poly bags with moist towels or shavings until ready for use. Cut smooth and straight scion sticks into 12-14 inch lengths.
Historically, bridge grafting was done where few trees were planted per acre and thus saving one tree could be important to the integrity of the orchard block. Trees were expected to live beyond 40 years before replanting. Today, we have many orchards established at 600-1,200 trees per acre at investments of $10,000-18,000 per acre. The pressure for return on investment is great. Therefore, the value of saving trees may be paramount to avoid the expense of replanting with $9-10 trees and a trellis system that is already newly established. Growers can head the trunk at a point just above the injured area and train a new regenerated adventitious shoot to become the new leader however the canopy needs to be redeveloped again after a few years of training and investment. Given this situation, growers may want to consider bridge grafting to avoid regenerating the canopy. I would recommend pruning some of the canopy back to reduce initial stress during bud break.
Bridge grafting can be professionally accomplished or, with practice, accomplished by a crew relatively new to the art. In his book “The Grafter’s Handbook” R.J. Garner recommends using one piece of scion-stick for every 1 inch of trunk diameter. Therefore, a 1 inch diameter trunk will need one stick. If you have not done this before, consider at least two sticks to insure a take. In our climate, confine bridge grafting to species tolerant of wood canker diseases such as apple, pear, European plum and sour cherry. Cutting into bark and vascular tissue in sweet cherry, peaches and apricots invites infection from Bacterial Canker.
The materials needed include a sharp grafting knife, small hammer, 1 x 18 inch wire nails, wax or grafting compound and practice. Some experienced grafters don’t recommend bridge grafting where injury extends beyond 50 percent of the bark. However, Garner indicates that it still can be accomplished where 100 percent damage is experienced. Either way, it is possible to perform the graft in late April and then reevaluate 6-8 weeks later to determine success. If not, adventitious water sprouts will arise from the live tissue below the damaged area and can still be selected and trained as the new leader in the tree.
Fruit growers who have trees with severe damage and are considering bridge grafting should consult publications that provide detailed guides through the process. Iowa State University’s article "Rabbit Damage to Trees and Shrubs" describes rabbit damage and shows bridge grafting illustrations. A step-by-step YouTube video describes the process but does not include painting the grafts with wax or grafting compound upon completion. Additional references include "The Grafter's Handbook" by R.J. Garner and "Plant Propagation Principles and Practices" by Hartmann, Kester, Davies and Geneve.
Additional articles on bridge grafting: