Like most amateur fruit-growers, I've had a tendency to ignore the rootstock when selecting new trees.  Reading The Holistic Orchard and The Grafter's Handbook, though, helped me realize that rootstocks are responsible for a lot more than the size of the tree, so I dropped an email to my favorite orchard (the source of a lot of my apples) to see what rootstock I currently have in my garden.

MM111 is one of the older apple rootstocks, and it's still around for a reason --- it's clearly a workhorse.  The roots do just what they should, anchoring the apple trees firmly and helping them survive in poorly drained, heavy clay soil (like mine).  While some other fruit trees have perished in our garden, my apples on MM111 keep plugging along.

On the negative side, trees grafted onto MM111 seem to be inclined toward burr knotting, and (as with all dwarfing rootstocks) the trees don't live quite as long as seedlings --- 40 years instead of 75, in this case.  If you're concerned about the disadvantages of MM111 (or of any other rootstock), you can plant the tree deep enough to bury the graft union and the scionwood will root, creating what's essentially a seedling tree (which may have its own disadvantages).

Of course, size does matter, not only because you might not fit a full-size apple tree into your homestead, but also because smaller trees bear much faster (4 to 6 years in this case compared to 8 to 12 years for a seedling tree).  MM111 produces a tree that's 70% to 85% the size of a seedling (aka standard) apple tree, with the lower figure referring to less vigorous, spur apples and the larger size referring to more vigorous varieties.  (This site gives information on the vigor of many common apple varieties.)  That means your tree is likely to get 14 to 20 feet tall (depending, again, on vigor of the scionwood variety), and trees should be spaced that distance apart.

Yellow Transparent is listed as very vigorous, but my other varieties are supposed to have medium vigor, so I think I've actually given my apple trees more room than they need (25 feet between each one).  That's good news since it gives me space in between for extra forest garden experimentation --- more on that in a later post.