Q. Hello Peter, My Beauty Plum, Italian Prune, and Stellar Cherry trees all have noticeably smaller diameter rootstock than the grafted on main trunk. These trees are all 5-8 years old and the difference seems to be getting more pronounced. Is the rootstock likely to catch up or is the upper tree going to snap off in a windstorm someday? When selecting fruit trees, should I be choosing trees with matching root stock and upper tree diameter sizes? My apple tree trunks and rootstocks all look normal with no noticeable differences in diameters. Thanks, Henry Raynor – Puyallup, WA
A. I guess the first thing I should do is explain why fruit trees need rootstock in the first place. People have grafting the scions of cultivated varieties onto rootstock for over two thousand years. The Greeks were known to graft varieties that they chose for taste or fruit sized and grafted them onto seedlings produced from local fruit trees of the same species. This gave the cultivars the vigor of the local trees with the benefits of the cultivated variety (cultivar). Over the centuries the grafting has become more complex and is used to control the growth habit of the cultivar including size and fruit production, disease or weather resistance. Grafting also allows people testing new cultivars to bring seedlings into production much faster than if they grew on their own roots to maturity. This can save years on the release of a new cultivar. For home orchards many rootstock have been developed for varying degrees of growth limitation. When you see dwarf, semi-dwarf or standard this relates to the rootstocks ability to control the ultimate size of the tree. Also rootstock does not necessarily have to be from the same species as the scion wood although they are usually within the same genus. Many times there can be graft incompatibility and the union between scion and rootstock breaks down. Compatibility and effects of a particular rootstock on a particular scion cultivar is determined by field tests. There is no other way to determine compatibility. What you are describing is a reverse taper, which indicates that the scion is outgrowing the rootstock. This is the case when a very vigorous scion is placed on a very dwarfing rootstock and causes a growth mismatch. I talked with Sam Benowitz of Raintree Nursery (raintreenursery.com) and he said that although this is not common he has heard this can happen when Western Sand Cherry Rootstock (Prunus besseyi) is used. Since you don’t know what the type of rootstock was used it is difficult to say anything for certain other that there is a growth mismatch. Reputable growers would use proven compatible rootstock. The reverse taper is not necessarily an indication that there is a graft incompatibility. And no the problem won’t reverse but is likely to become more pronounced with time since you are dealing with different growth rates (implying time). But it is likely your trees will remain healthy and upright for many years to come. So get used to living with your “bipolar” tree. If nothing else you have a conversation piece for backyard parties.