We occasional offer some apple trees on an "interstem" rootstock. This is a combination of two rootstocks grafted together, with the scion (fruiting) variety grafted on top. The resulting tree therefore consists of three parts: the primary rootstock (which makes up the entire underground portion of the tree, the interstem (usually just a few inches of wood), and the grafted scion variety (which makes up the main aerial part of the tree).
Why bother? The answer lies in the combination of the rootstock and the interstem. Although almost any combination is theoretically possible, in practice the primary rootstock will be a vigorous rootstock such as MM111 or Bud. 118, and the interstem will be a dwarfing rootstock such as M9, or Bud 9.
Vigorous rootstocks will produce free-standing trees, but are often impractically large, and take many years (5 or more) to start bearing fruit. However they will grow successfully on a wide range of soils including poor quality ones, and their large root systems mean they have some drought tolerance and are not bothered by competition from grass and other plants. Dwarfing rootstocks on the other hand produce nice compact trees which are very precocious and will start fruiting within a couple of years of planting - but they cannot support themselves so need a permanent stake, and require good soil conditions and irrigation.
The interstem tree combines the best of both worlds. You get the free-standing low-maintenance of the primary vigorous rootstock, coupled with the compact dimensions and precocity of the dwarfing interstem rootstock. Whilst commercial growers will prefer to use the true dwarf rootstocks because of the outright productivity they can achieve in irrigated supported orchard systems, for the backyard orchardist the interstem is often a better choice.
So why not just use M26 or a similar size rootstock like G30? Well we often do, as these are good choices in many situations. Interstems simply offer an extra option, and are particularly useful in difficult situations or where you definitely want a free-standing tree (M26 is not always free-standing).
How can you tell an interstem tree from a conventionally-grafted tree? This is usually quite easy on a young tree, you will see two graft unions, spaced 6" - 12" apart. In contrast a conventionally-grafted apple tree will only have a single graft union.
Note. When planting an interstem tree, you should normally make sure the soil line is below the interstem. However if you want a smaller tree, you can (although we don't necessarily recommend it) plant the tree deeper so that the soil line reaches about half-way up the interstem. By partly burying the interstem you activate an increased dwarfing effect. Make sure you don't entirely bury the interstem though, because then the scion variety may start to self-root.
The most common combinations of we use are as follows, the naming sequence is in the form interstem / primary rootstock:
M9 / MM111, G11 / MM111, B9 / B118, G11 / B118, B9 / MM111
As can be seen the Malling-Merton 111 (MM111) rootstock is the most common primary rootstock for interstem trees. It is not suitable for very cold zones (that is where B9 / B118 comes in) but in all other areas it is an excellent rootstock, with good disease resistance, good tolerance of both drought and flooding, and not greatly affected by competition from weeds and grass. It has just one vice - it takes a very long time to come into bearing. However a precocious interstem takes care of that.
All these interstem combinations produce a tree with a mature size intermediate between trees on M9 and M7 rootstocks, in other words roughly the same as the M26 rootstock. A typical spacing between trees would be 10ft.
In summary, the interstem tree is a useful option if you want a mid-sized tree that is self-supporting, and doesn't need as much attention as a true semi-dwarf tree.
Diagram shows an interstem tree with MM111 rootstock on the bottom, and a piece of M9 interstem grafted at point 1. The scion variety is bud-grafted on to the top of the interstem at point 2. Sometimes the bud-graft is at the bottom, but the principle is the same.