Jim Gremel was our guest for the February General Meeting to demonstrate how to graft shimpaku foliage onto juniper stock. Jim is known all around the Bay Area and California as an expert grafter and for his superb trees. It was our great pleasure to have Jim as a guest and make the long drive down from Occidental on a rainy Thursday night.
Of the many useful techniques to enhance trees trained for bonsai, grafting is one of the more powerful techniques, allowing addition of new roots, branches, and foliage to a tree. For our demonstration, Jim focused on grafting new branches onto the trunks of established junipers. Members of the Bonsai Society of San Francisco are all familiar with the benefits of grafting shimpaku foliage on juniper stock. Our logo tree is the complete product of 30+ shimpaku grafts 50+ years ago by Mas Ishii. So several of our members were anxious to begin.
Prior to grafting, Jim emphasized that a candidate tree must be extremely healthy and completely unstressed. That means no recent repotting, wiring, or pruning performed on the tree several months prior to grafting. A healthy tree will accept grafts readily and heal quickly.
Grafting is relatively simple and only a few tools are needed. Jimís grafting tool kit includes a grafting knife, wooden block, rubber bands, copper wire, small plastic baggies, aluminum foil, and a syringe. To begin, Jim checks the sharpness of his grafting knife. A sharp blade is required to slice cleanly through the cambium layer of the material without squashing or disrupting the sensitive tissue. To achieve the finest edge on his knives, Jim uses a bench grinder equipped with a cardboard disc as the finishing surface.
When choosing scion material (the scion is the new plant material added to the stock plant), Jim suggests that the new branch be a similar diameter to the area that will accept the graft. By matching the diameter of the branches, the cambium layers should align easily.
At the graft site, a shallow, angled cut is made into the bark through the cambium and slightly into the xylem of the tree. The cut begins distal to the trunk and moves toward the trunk of the tree approximately 2 Ė 4cm. On the scion, two angled cuts are made on opposite sides of the branch with one at a slightly steeper angle. The steeper angled side will face outward from the tree while other fits snuggly against the trunk.
The cut scion is fitted into the slice on the trunk. Gently, the cambium layers are aligned. If the layers are properly aligned, fluids will travel freely between layers allowing the new branch to be supplied with water and nutrients from the stock plant.
Once the cambium layers are matched, Jim wraps a 5mm diameter rubber band around the branch to secure the scion to branch of the tree. A small baggie is then placed over the scion and the main branch, then secured with a small piece of #18 copper wire. The baggie acts as a mini-greenhouse and keeps the scion from drying out over the coming months. A small amount of water is injected into the baggie to help maintain the humidity around the graft. Jim recommends attaching a small piece of aluminum foil around the new graft to prevent exposure to direct sunlight.
Within a few weeks, if the graft is successful, the scion will begin to grow. Over time, the graft must be protected from any movement (wind, animals, etc) that may disrupt the grafted area. Also, the new growth on the native branch should be pinched back. Hormones that inhibit back budding along the branch may also inhibit growth in the scion.
The key to successful grafting is practice and post-care of the tree after the procedure. Proper grafting of the new scions is only the beginning of the technique. Over the next few weeks and months, you must observe the growth and maintain proper humidity in the baggie. As the scion begins to grow, you may poke a few holes in the bag to release water. After a year, you can remove the bag and allow the new branch to grow freely. However, do not forget to carefully to wire the new branch into form. Once it thickens, you risk disrupting the branch if large difficult bends are attempted.
As inspiration, Jim brought along a wonderful juniper that he grew in the ground from a cutting. After successfully grafting a number of new scions onto the tree, Jim has begun to wire the branches into place and remove the native foliage. Expect to see this tree in future shows around the area.