Excerpt from How To Grow Apples in The Southern U.S. – Apple rootstocks
Here is an excerpt from How to Grow Apples In The Southern U.S. by Trey Watson.
Rootstocks for Southern Apples
All modern fruit trees are grafted onto a rootstock of a different variety. Apple trees are no different. They are grafted onto rootstocks that give the apple tree its final size. All the apple varieties that produce the great-tasting, large apples that gardeners are accustomed to have very weak roots. These apple-producing varieties are grafted onto other varieties that are bred and grown for their vigorous roots. These varieties are referred to as rootstock. The variety of apple grafted onto the top of the rootstock is called the scion. There are dozens of apple rootstocks commonly used in the orchard trade. In addition to the old standard rootstock, the hobby gardener or commercial grower has a number of compact apple rootstocks to choose from. What follows is a list of commonly used apple rootstocks that work well with low chill apple varieties.
Standard-sized trees are the most commonly encountered apple trees. Standard trees grow to 20-25 feet tall, with a crown spread of 15-20 feet. They produce prolifically. Standard trees are grafted onto apple seedlings. Since apple seedlings have wide genetic variability, the final size and shape of standard apple trees can vary slightly. Standard trees are great for orchard settings, but they may not be ideal for the home gardener with limited space. A typical suburban lawn can usually handle 2-3 standard fruit trees. Standard trees also bear fruit a year or two later than dwarf fruit trees, but they bear much more fruit over their growing life. Apple trees grafted on seedling rootstock have deep, strong roots once established. Apple seedlings have been used as apple rootstocks since ancient times. All other rootstocks are compared to the standard rootstock when final tree size is described.
MM111 is a rootstock that grows to about 80% of standard-sized trees. It has some disease resistance and it grows well in heavy clay soils. The MM111 rootstock imparts some drought resistance and has strong root anchorage. Apple trees grafted on MM111 are considered semi-dwarf. Semi-dwarf trees produce less fruit than standard trees, but they are more compact. Apple trees grafted onto MM106 grow to about 70% of standard-sized trees. MM106 provides some disease resistance, a fair amount of drought tolerance, and heat resistance. In wet locations, collar rot is a problem for trees planted with this rootstock. It is also sensitive to early winter freezes. Thanks to its drought tolerance and heat resistance, MM106 is a great rootstock for southern apple orchards.
M7 rootstock keeps apple trees at about 50% the size of standard trees. This is the smallest semi-dwarf variety and it is ideal for orchard growing in small spaces. M7 grows well on most soils, through it does best on well-drained, fertile sites. It is not drought tolerant, so trees planted with this rootstock must be watered during dry spells. It also tends to produce numerous root suckers that will grow out of the ground near the base of the tree. M7 has some disease resistance. M7 does well as a rootstock in the southern US.
M26 is the largest dwarf rootstock. It does best on fertile sites. On poorer quality soils it may grow spindly and need to be supported. Trees grafted on this rootstock grow to about 40% the height of a standard tree. M26-grafted fruit trees produce fruit at 3-4 years of age. It does not have much disease resistance, but it is a great rootstock for gardeners with limited space.
M9 is the most dwarfing commonly-available rootstock for apple trees. It reduces tree size to 20-30% of a standard size tree. It does not grow a free-standing tree, since it is so small. Trees with this rootstock are ideal container plants and they need to be supported by a trellis or wiring system. The root-anchorage of this rootstock is only moderately strong. M9 does have some disease resistance. Certain apple varieties do not survive long on M9 due to brittleness at the graft union. This is a great rootstock for people who live in small yards or even in apartments, though the tree should be examined at purchase to ensure the graft union is sound and not brittle.