Thursday, September 8, 2016

Painting a new bark graft union

   Earlier this year, I posted some photos of grafting onto a sprout growing from a coppiced tree.  Four months later the scion has sprouted a new shoot over 6 feet in height. Its amazing how a 15-year-old root system can push the growth of a scion. However, developing a nice new central leader tree from a thicket of stump sprouts takes numerous trips back to the tree all summer long. New stump sprouts continuously popped up which need to be pruned off. The scion was grows so fast it's hard to keep up with the removal of stalked buds. The results of all this work is a nine-foot-tall pole of a tree (photo at right). At this point, I'm not concerned about the total lack of lateral branches. New branches will sprout next spring from secondary buds and the tree will begin to fill out.    

    This week I came back the tree to not only prune off stump sprouts and stalked buds, but to unwrap the graft union and give it a coat of white paint. The photo at left shows the graft union still covered in aluminum foil and plastic. Even before unwrapping, I could tell the scion on this bark graft had already grown to cover over one-half of the rootstock stump. 

   Once I uncovered the graft, I could see the the scion was growing over the rootstock stump and the rootstock had developed a ring of callus tissue around its upper edge (photo at right). Now that you can see the graft union let me point out a couple of things.
   As the scion grows in diameter it never does unite with the woody portion of cut surface of the rootstock. In the photo at left, the yellow arrow points to a gap that forms between scion callus tissue the woody center of the rootstock. The scion never joins with the wood at this point but simply grows over the stump, sealing over the dead woody tissue.  This is why bark grafts remain structurally weak until the stock is completely covered by the scion.

    On the back side of the graft union you can still see the staples I used to make the graft. In the photo at right each staple is marked by a yellow asterisk on its right. The bottom-most staple is still clearly visible but the other four staples are becoming buried in the bark. The tree will eventually cover over all these staples and they will forever be sealed inside the trunk.

    I like to remove all the wraps from my grafts in late summer so ants and dogwood borers don't make permanent homes under the aluminum foil. Since the graft union has been covered all summer long I like to paint the union with white paint to provide protection from sun-scald (photo at left). I use white, exterior-latex house-paint to reflect the sun and leave a highly visible record of which trees have been successfully grafted.  
    I apply paint to both the stock and scion (photo at right). Once the paint dries, I'll reattach the bamboo support stake to the tree to help prevent wind damage to the scion.