Lots of trees can be grafted—fruit trees, shade trees, and even small ornamental trees. Grafting is the art of putting together two different parts of trees to make one new tree. Unlike Frankenstein, the results are not monstrous, but instead the new tree will have some of the best traits of each of its parts. The rootstock can impart things like dwarfness, winter hardiness, and soil pH adaptability, among other positives. The top of a graft union, called the scion, may bring bigger flowers, more fruit, and/or a different form than normally found (think of a lilac bush grafted onto a standard). If a cutting can be slow to root, grafting to a root system can get the plant off to a quicker start, making for a saleable tree sooner rather than later.
Notice the left side of the tree has a reduced number of leaves. Photo by S. Sternberg
While nurserywomen and men seek out closely related rootstock-to-scion wood unions to ensure grafting success, sometimes issues of incompatibility develop. These issues can occur right away, or they can develop 15 years after the tree has been planted in its permanent home. Symptoms of graft union incompatibility are odd, with a range of curious developments. The trunk may develop girth far beyond the size of the rootstock, giving the plant a bulging top-heavy appearance. Another indication of graft union incompatibility is overgrowth of roots, with roots aggressively forming, one over the top of the other, gradually encompassing and constricting the scion itself. When this occurs, reduced leafing and branch dieback in the scion are typical. The important thing to remember with graft union incompatibility is that it is a location of weakness, leading to possible breakage at the juncture.
There are a few reasons why graft unions fail. Sometimes the rootstock and scion are not related closely enough. If the two components belong to the same plant family but not the same genus, then incompatibility can develop. Another possibility– disease-causing organisms–may be present when the parts are grafted, sealing in pathogens, and creating the perfect environment for their growth and development. A final possibility for graft incompatibility is the grafting technique itself. There are a number of grafting methods, each with its own benefit but not necessarily suited to all grafting endeavors.
Not all bulges at the base of grafted plants are indications of graft incompatibility. A slight swelling is normal. Roses are some of the most common grafted plants in our landscapes. Monitoring their growth is necessary to ensure sprouts arising from the graft union or below are removed.
More about graft union incompatibility may be found here: https://extension.umd.edu/resource/graft-failure .