We have a tree in one of our research trials that was grafted to the wrong cultivar. What was supposed to be a Posey turned out to be a Pawnee. Back in 2007, this misplaced Pawnee tree was severely broken down by an ice storm leaving us with not much more than the tree's trunk. We trimmed off all broken limbs and let the tree sprout from the remaining trunk. What sprouted out was above the original graft so the tree remained a Pawnee. In the photo at right you can see the misplaced Pawnee full of leaves and catkins while the Posey tree right behind it is just starting to leaf out. This year, I decided to correct our grafting error and top-work this ice-storm-damaged tree over to Posey.
The photo at left shows one of the main sprouts that grew from the ice-damaged tree trunk. Note that there is a good chunk of dead wood on top of the original tree trunk above the point of new sprout attachment. The roll of callus tissue on the trunk marks the point of live tissue growth.
The first step in top-working this tree was to trim out the dead wood and then cut the limb sprout off in preparation for placing a bark graft (photo at right). There are two things to note in this photo. First, I trimmed the trunk at about a 45 degree angle, removing as much of the decaying wood as possible. And second, I left other limbs on the tree un-pruned to provide photosynthetic support for the tree's root system.
At this point, the tree is ready for bark grafting.
Preparing the stock to receive a scion is as simple as drawing a straight line. Using the tip of my knife, I sliced the bark straight down the stem for about 3 inches (photo at left). I made sure to cut all the way through the bark so the bark can be lifted away from the wood when the scion is inserted.
Bark grafting requires that the bark "slips" easily from the wood. The bark slipping period starts in early spring when new leaves are expanding. and continues through the pollination season. However, I've found that grafts applied early in the grafting season out-perform those made later in the spring.
Now its was time to work on the dormant scion. The first cut I made on the scion removed about 2/3 the diameter of the stick. In the photo at right, I present two views of the same scion: one from the side and one looking straight down on the cut surface. I made the cut parallel to scion except for the curved shoulder near the top of the cut. I call this first cut into the scion "the deep cut".
Turning the scion over, I make a shallow cut pealing off enough bark to enter into the wood. Note that I started the shallow cut below the point on the scion where I carved the shoulder of the deep cut. The photo at left is a composite of three views of the same scion. This shallow cut is made at an angle to the deep cut. In making the shallow cut, I start peeling of the bark in such a way as to leave as thin a strip of bark as possible along one edge of the scion (upper view). Turning the scion slightly, you can see a thick strip of bark on the other side of the scion (middle view). At this point my scion wood has a wedge shape when looking at it from the bottom (bottom view).
With the next cut, I expose cambium at a right angle from the deep cut (photo at right). I turned the scion so I was looking at the thick bark strip side of the stick. I then placed my knife at 90 degrees from the deep cut and shaved off the edge of the scion. This cut needs to be deep enough so you can see white wood all the way down the edge of the scion but not so deep as to remove all the bark from that side of the scion. After making this cut, my wood has a triangular shape when looking at the bottom of the scion (lower portion of photo above).
For the final cut, I created a chisel point at the end of the scion to allow easy insertion under the bark of the stock. This cut (red arrow) is made on the opposite side from the deep cut. With a fully carved scion its time to join scion and stock.
To insert the scion, I lifted the bark on the left side of the slice I had made into the stock. The deep cut is facing the wood of the stock, while the shallow cut is covered by the stock's bark. Note that the 90 degree cut fits smoothly against the slice I had made in the stock's bark.
I gently tap the scion down under the bark until the shoulder of the deep cut rests just above the wood of the stock. Note that the shallow cut is completely covered by bark.
I use a light duty staple gun with 5/16 inch staples to press and hold the bark of the stock tightly against the scion. In the photo at right, you can see that I used 4 staples to hold the bark flap against the scion. I put these staples in at a 45 degree angle so that both sides of the staple are fully driven into the tree.
I added three more staples along the left side of the scion to compress the stock's bark tightly against the scion (photo at left). It is very important to eliminate all air spaces between the scion and stock when grafting pecan trees. So feel free to use as many staples as it takes to bend the bark to fit around the scion.
I then covered the graft union with aluminum foil to block the sun from inhibiting callus formation (photo at right). The foil covers the entire top of the stock and fits tightly around the scion.
To conserve moisture around the graft union, I placed a plastic bag over the aluminum foil (photo at left). I tore a corner out of the bag and placed the bag over the scion. As I slipped the bag over the scion, I made sure that I didn't injure the buds on the bud stick.
I then used green grafting tape to seal the plastic bag over the graft union (photo at right). I first tied the bag around the scion, then at the bottom of the bag around the stock.
My next step in the grafting process was to seal the top of the scion from moisture loss with a dab of white glue. At this point, the graft is complete. However, there is one more step crucial for protecting the graft from bird damage.
When grafting in the field, I always attach a "bird perch" on every tree (photo at right). A single scion sticking out in the open becomes a very attractive location for male meadow larks or red-winged black birds to perch and sing their songs of romance. As small as these birds seem, they can land with enough force to break out the scion. To prevent bird damage, I attach a branch stick (cut from the removed top of the tree) to the tree using black electrical tape. The end of this "bird perch" is much higher than the scion and provides a more desirable location for birds to sing their mating calls.
In three weeks, I'll be able to tell if my grafting effort on this tree was successful. In the mean time, see related posts on
bark graft maintenance and the
anatomy of a successful bark graft.
