Fruit plants produce fruit as a means to distribute seeds, and the seeds contain a mixture of the genetic material from both parents. Grafting, the process by which tissue from one tree is attached to another tree, is used to propagate desired varieties. In a basic case this could be a wild seedling or another tree in the orchard of a less desirable variety, but over time people found that the selection of the “base” tree (known as the rootstock) had an impact on the overall tree performance and, as with the fruit varieties, the best rootstocks were propagated and classified.
When buying a fruit tree from most retail outlets the rootstock is not specifically mentioned; the tree will generally be identified as being “Standard”, “Semi-Dwarf”, or “Dwarf” which refer to the final tree size and depend on the growth characteristics of the rootstock. When buying from a specialty nursery, however, the same variety may be available on several different rootstocks, such as “Foxwhelp on MM.106” or ‘Foxwhelp on Bud. 9” which allows for better matching the tree to both the desired size and the growing conditions it will be planted in.
Regardless of where purchased, understanding tree size is important. A standard tree is one that will grow to the natural full height for the given variety, which for most apples is on the order of 20-25 feet. While some standard trees are on their own roots, there is also a group of rootstocks which do not substantially affect the growth of the tree and provide for standard sized grafted trees. A semi-dwarf tree uses a rootstock that slightly reduces the growth of the tree and will be roughly 70% of the size of a standard tree. A dwarf tree will usually be 50% or less of the size of a standard tree. Smaller trees generally improve access for tree maintenance and fruit harvest, tend to require less time after planting to produce fruit, and can be planted more closely together, however they also tend to be less cold-hardy, more susceptible to drought, may require permanent trellising or other support, and do not live as long as larger trees. Tree yield is largely a function of tree size, with a standard typically yielding up to 10 bushels per tree, a semi-dwarf 5 bushels, and a dwarf 1 bushel, however as the smaller trees can be planted closer together the overall yield per square foot is roughly consistent regardless of tree size. Beyond tree size, rootstocks have several other attributes which may be important to consider such as the soil type they grow best on, cold and / or drought tolerance, disease resistance and moisture requirements.
For my purpose semi-dwarf apple trees were the best option. Dwarf trees would have allowed me to grow more varieties and simplified pruning, spraying, and harvesting, but they also would have required an investment in trellising and, more importantly, would have had most of their fruit in the range where deer could easily browse it. I initially opted for M.7 and MM.106 as I wanted trees on the smaller side of the semi-dwarf spectrum. MM.106 has performed well for me, however the trees I have had on M.7 have all had some issues which the neighboring trees on other rootstocks have not had – two of them blew over in storms and the remaining two have begun to lean to the point that they have to be supported. Once I found out that I had to worry about deer browsing, I have opted for the larger MM.111 rootstock in order to put more or the tree above the reach of the deer.