Apparently, contemporary apple trees are rarely planted on their own roots. Who knew? Instead, they are grafted onto one of several common rootstocks. Yes, it DOES sound violent (and there is a very sad picture in my organic gardening book of a crowd of the baby trees all taped up with what looks like a bandage around their joining point to their new roots). I had discovered the same thing about roses several years ago, but decided to buy roses that were grown on their OWN roots. There were very few places I could find that had a good selection, but I happened upon "The Uncommon Rose" website, whose market niche is that ALL their roses are grown on their own roots. The roses have really taken off and what REALLY mattered was that I felt much better about them. I'm not quite sure why it bothers me so much -- just another version of our modern Frankenstein sensibility that gives me the creeps, I suppose.
Well, back to those apples. Rodale's Illustrated Encyclopedia of Organic Gardening (yes, I had forgotten their name was so grand in previous posts and inadvertently shortened it) reports that there are five common apple root stocks: M27, M9, M26, MM106, MM111. Apparently, the effect of each grafted rootstock is to dwarf the tree, from "very dwarfing" (M27) through "semi-dwarfing" (MM106), or to allow a tree to be "semi-vigorous in poor soil" (MM111), although this same rootstock is apparently "vigorous" in good soil. "Own-root trees" are vigorous --yes, that is the awkward hyphenation - and how strange to make the natural tree the one that must be marked out with a special adjective.
Another benefit of the dwarfing rootstocks is that they fruit earlier. Own-root trees reportedly produce fruit in 3 or 4 years, while the dwarfing rootstocks produce fruit from "very early in life" to 2-3 years after planting.
What kind of heights are we talking about? M27, the "very dwarfing" rootstock, limits the trees to 5-6 feet, while the dwarfing M9 and M26 top out at 6-10 and 8-12 feet respectively. The semi-dwarfing MM106 may reach 12-18 feet, while the semi/vigorous MM111 rises to 20-28 feet. Own-root trees are over 28 feet tall, unless grown close together.
Luckily for my squeamishness about grafted roots, the book recommends own-root trees for cordons 30 inches apart, or M26 or MM106, so I will use own-root plants for the porch. For the chain-link espalier, the book states that "all rootstocks except M9 are suitable" -- likely meaning just the grafted plants. However, the implication of the text and table is that they are recommending the dwarfing rootstocks to cut down on pruning for espaliers. This is, that I may need to prune more with an own-root tree planted as an espalier. Despite the warning (after all, I need experience pruning, right?) I am leaning toward the own-root for the chain-link espalier as well as for the porch cordons, but the scientist in me is curious to see if there is a real difference. Unfortunately, the conditions of each site (soil, proximity to pavement, hours and postion of sunlight) are so different that an attempt to compare them really wouldn't work very well.