There is nothing quite like the joy of eating a fresh apple. Growing your own can open up a whole array of choices in varieties, and can enhance your home landscape. However, in the rush of enthusiasm for raising this time-honored fruit, gardeners often overlook a critical aspect of apple growing: the correct choice of rootstocks.
Occasionally, a gardener will devote a great deal of energy into apple growing, carrying out all the best horticultural practices, only to find that the tree doesn't seem to "behave" as expected. Perhaps it is in a soil where the tree's rootstock isn't suitable, or the rootstock isn't sufficiently cold hardy.
Knowing about apple rootstocks, asking the nursery which rootstock a tree is grafted on, choosing the rootstock most compatible with your site, and following the best cultural practices for each rootstock will help the ecological gardener to ensure that his or her trees will be healthy and productive.
Rootstocks are one of the unique aspects of apple growing. There is no other above-ground crop where so much attention has been devoted to roots! There are about 100 rootstocks for the major tree-fruits, and more than 20 for apple. Some of these rootstocks-like M.7 and M.9-can be traced back hundreds or thousands of years in history. In this bulletin we describe the relative strengths and weaknesses of apple rootstocks commonly available from fruit-tree nurseries.
Apple varieties are propagated by taking vegetative buds from a young shoot (scion) of the desired variety (i.e. a 'McIntosh,' 'Jonagold' or other named cultivars) and grafting those buds onto another tree branch or small sapling. This is necessary because the seeds of each apple are the result of pollination from a different apple tree species or variety. This makes each seedling a genetically unique individual with unpredictable traits; for example, seedlings sprouted from 'Granny Smith' apples might produce tiny red crab apples!
Rootstocks are usually necessary for grafting and propagation of apple scion varieties. In past times, seedlings that sprouted naturally in pomace piles around cider mills were often dug up, and buds from known scion varieties were grafted onto these seedlings for planting new orchards.
Since the genetic traits of these seedling rootstocks were unknown, their performance was unpredictable. Fruit trees in seedling rooted orchards were usually large, vigorous, slow coming into production, and many died off because their roots were not adapted to the particular soil or climate conditions in each orchard.
To avoid these problems, most orchards today are propagated from "clonal" rootstocks-that is, they are grafted onto rootstocks that are genetically identical offshoots or clones of a mother rootstock type with certain desirable characteristics such as disease resistance, tolerance of winter cold, seasonal flooding and summer droughts, or reducing tree size. Clonal propagation ensures that the important traits of each rootstock will make the resulting orchard more manageable and productive.
Most of the important apple rootstocks used today were derived from collections and selections by East Malling Research Station in England, during the early 1900s. Pomologists at East Malling collected and characterized the clonal rootstocks that had been developed by farmers during many centuries in Europe. They assigned numbers to each clone, and subsequent rootstocks have been developed by hybridizing these clones, or breeding them with other apple species and varieties. The following Malling or Malling-Merton hybrid rootstocks are important and widely available from nurseries:
M.9 -- The most important dwarfing rootstock worldwide, descended from 'Juane de Metz' in medieval France, and the famed 'Paradise' apple tree of ancient Persia. M.9 is precocious, fairly cold-hardy, tolerant of wet (but not droughty) soils, and compatible with all scion varieties. Increases fruit size, and advances fruit maturity about one week. Its main problems are susceptibility to fireblight, and brittle roots which require a pole or trellis to support the tree. Produces a tree about 25% of full size, usually about 10 feet tall at maturity.
M.9/MM.111 Interstem -- An interstem tree consists of three parts: the scion variety, the interstem piece, and the understock. M.9/M.M.111 has all the advantages of M.9, but a stronger root system and somewhat better drought tolerance. Should be planted deep, with only the interstem piece showing.
M.26 -- A hybrid cross of M.9 and M.16. Very popular in irrigated orchards on well-drained soils. Very precocious, requires tree support, good cold hardiness. Susceptible to crown rot ("wet feet") and fireblight. About 35% of full size.
M.7 -- Descended from 'Doucin Reinette' in France, around 1688. Very popular in areas with cold winters, and deep well-drained soils, like the Lake Champlain region. Suckers profusely, prone to crown gall, resistant to fireblight, tolerates wet soils but does best where it can root deeply. Trees are 50% of full size, or so-called "semidwarfs."
MM.106 -- A hybrid of 'Northern Spy' and M.1. Precocious, well anchored, does not produce many root suckers. Fruit matures late, and trees grow late into Fall, making them prone to winter injury. Susceptible to crown and root rot. Trees about 50% of full size.
MM.111 -- 'Northern Spy' hybrid. Quite drought tolerant. Not very precocious, minimal suckering, fairly tolerant of wet sites, prone to burr knots. Perhaps best as an M.9/111 interstem rootstock. About 75% of full tree size. A new "G" series of rootstocks developed at Cornell's Geneva Experiment Station may offer the best traits of the M-series, without the major problems like suckering and disease susceptibility. These are just becoming available from nurseries, and have not been widely tested but are worth a try.
G.16 (G.5-A) -- Similar to M.9 in tree size and precocity, but more resistant to Phytophthora root rot and fireblight disease. Still requires tree support.
G.30-Similar in size to M.7, but more precocious, without the root suckers.
Another useful rootstock is Bud.9, which was developed by the Budagovsky program in Russia. This is a mixed hybrid of Malling clones, crab apples and some traditional Russian varieties. It has all the good traits of M.9, but is sturdier, more winter hardy and less susceptible to fireblight.
The depth of planting on rootstocks influences subsequent tree growth. As more of the upper rootstock is left above ground at planting (i.e. the shallower a rootstock is set in the ground), the tree's ultimate growth is reduced. On the other hand, if the graft union between scion and rootstock is buried, the scion itself will produce roots, and the rootstock's size-controlling effect will be lost-resulting in a very large and vigorous fruit tree. Usually it is best to have 2 to 4 inches of the upper rootstock above ground when the soil has settled after planting.
Knowing how to choose and plant rootstocks, you can match your own site, soil type, pest and disease problems, and desired tree size to the characteristics of available rootstocks. Be sure to inquire from the nursery exactly which rootstock their trees are grafted onto, because general terms like "dwarf" or "semidwarf" do not provide you with the information needed to select the best rootstock for your situation.
Ian A. Merwin, Associate Professor, Department of Fruit and Vegetable Science Cornell University, Ithaca, NY, 14853