The answer to this question is not as simple as it appears. To some people, it may mean a tree propagated on any of the East Mailing rootstocks.
To others, it may mean a tree propagated on a specific stock, such as East Mailing IX; others still think a dwarf tree has an East Mailing interlock.
One can readily see that tree performance cannot be the same for the three categories and that a state of confusion exists about terminology concerning dwarf trees.
To have an idea as to what constitutes a dwarf tree, one must have an understanding of the East Mailing rootstocks. This series of East Mailing rootstocks, usually shortened to “EM,” is identified by Roman numerals rather than by name.
These stocks differ from seedling stocks because they are propagated by layering or cuttings. EM stocks are clones or rootstock varieties, each originating from a single individual.
Understanding EM Rootstock Numbers
The degree of tree vigor that EM stocks produce depends on which numbered stock is used. It ranges from true dwarf, semi-dwarf, vigorous to very vigorous.
For example, EM IX produces a true dwarf tree; VII is a semi-dwarf tree; XIII is a vigorous tree; and XII is a very vigorous tree.
The degree of dwarfing that a given rootstock will produce also depends on the variety that is propagated on it. Vigorous varieties will not have their growth restricted as much by a true dwarfing stock as weak varieties will.
Controlling Tree Size With Interstocks
Another method of controlling tree size is using an interlock of a true dwarfing stock such as EM VIII or IX. This method includes inserting a short stem piece between the seedling rootstock and the scion variety.
The degree of dwarfing obtained by this method depends upon the length of the stem piece and EM stock used. Trees that are produced in this manner will be larger than trees budded directly to the rootstock.
For example, if EM IX is used as an interstock, the trees will be larger than trees on EM VII or IT roots. In other words, trees propagated with an interstock are not identical to trees grown directly on a given EM stock.
Size and Yield of Dwarf Trees
The average homeowner seldom has enough space to plant anything but a full dwarf tree. The ultimate size of such a tree will vary with variety and location, but a tree on EM IX can attain a height of eight to ten feet with a spread of ten to twelve feet.
It is possible, however, to maintain a smaller tree by rather severe pruning. Trees propagated with an interstock of EM VIII or IX will be somewhat larger and may attain a height of 15 feet or more.
Yields of three to eight bushels per tree may be expected from true dwarf trees. Trees on EM VII roots may reach a height of 20’ feet and have a spread of 25’ feet or more. Trees of this size can produce 20 or more bushels of fruit per tree.
Purchasing Apple Tree
To ensure getting the desired size, the homeowner should specify variety and rootstock when purchasing apple trees. The identifying tag on the tree should include the variety name and the name of the rootstock.
Reliable nurserymen have adopted the following method of labeling dwarf and semi-dwarf trees: Mcintosh/EM IX; Mcintosh/EM VII.
It should now be possible for a homeowner to purchase apple trees of the desired size if he recognizes that the East Mailing stocks produce trees of different sizes and he orders his trees by EM number rather than asking for dwarf trees.
44659 by Prof. W. D. Weeks