Apple seeds are genetically unique, resulting in trees widely different from the parent tree. Seeds from a Red Delicious apple may produce a tree with crabapple-like fruit, and a seed from a sour crabapple may produce a delicious eating apple. Because of this diversity, nurseries propagate apple trees through grafting. A branch, or scion, is cut from the parent tree and grafted to a hardy rootstock. Growers select rootstock for their qualities under certain growing conditions and disease resistance, and to control the mature tree's size.
An old rootstock that dates back to 1688, M7 produces semidwarf trees. Apple trees on this rootstock grow 15 feet tall and begin producing fruit within five years. Although this rootstock produces many suckers, it is very cold-hardy and results in a heavy-bearing tree. M7 is also resistant to collar-rot (Phytopthora cinnimoni) and fireblight. This is a good choice for heavy or wet soils.
M9 is another venerable rootstock that traces its history back to medieval France. M9 is the most important dwarfing rootstock in the world, according to Ian Merwin of Cornell University, and produces trees averaging 10 feet tall. It tolerates heavy soils and is cold-hardy. Nurseries value M9 not only for its dwarfing effects, but also because the apples are larger and mature earlier than the same scion on another rootstock. However, M9 is not fireblight-resistant and has weak roots. Growers usually stake M9 apple trees to relieve the stress on the roots.
Popular in well-watered orchards, M26 produces apple trees smaller than the semidwarf M7 but slightly larger than the dwarf M9. Like M9, M26 has shallow roots and requires support. It produces good-quality fruit, sometimes bearing apples one year after planting. The University of New Hampshire Cooperative Extension points out that burr knots grow at the base of M26. These nodes are vulnerable to fruit tree borers. M26 also has little resistance to fireblight and collar-rot.