How to Choose the Right Apple Tree Rootstock

What the Rootstock Decides

To choose an apple rootstock, start from the tree height your site allows and whether you are willing to stake the tree for life. For a tree of 6 to 10 feet, pick a dwarfing rootstock such as M.9, B.9, M.26 or the fire blight resistant G.11 and G.41, and plan on permanent support. For a free-standing tree of 10 to 15 feet, pick MM.106 or MM.111. Then check the shortlist against your winters, your soil and the diseases common in your area.

Every nursery apple is two plants joined together. The fruiting variety, called the scion, is grafted onto the roots of a different apple chosen for what it does underground. University of Maryland Extension lists what the rootstock controls: tree vigor, planting density, cold hardiness, and resistance to insects and disease. The variety decides what the apples taste like. The rootstock decides how big the tree gets, how soon it crops and whether it can stand up without help.

That is why the same variety can be sold as a 6-foot tree or one more than twice that height. If you are still deciding what form to buy, our comparison of bare-root and potted fruit trees covers that choice.

Dwarf, Semi-Dwarf and Standard Defined

The three size classes are measured against an apple grown on seedling roots. Maryland’s definitions are that a standard rootstock reduces final tree size by less than 15 percent, a semi-dwarf by 25 to 50 percent, and a dwarf by 55 to 85 percent. A dwarf tree is therefore well under half the size of a seedling tree, and the classes are wide enough that two “dwarf” trees can differ a good deal.

Size class also sets the wait for fruit. University of Maine Cooperative Extension gives these bearing ages:

• Dwarf trees begin to bear two to three years after planting.
• Semi-dwarf apple trees begin to bear four to five years after planting.
• Standard apple trees can take as much as seven to ten years.

The trade for early fruit is support. Maine states plainly that dwarf apple trees have weak roots and will not hold themselves up once they bear fruit, so they need a stake or trellis.

Apple Rootstocks Compared by Size

The Malling and Malling-Merton rootstocks, labeled M and MM, are the long-established series, and the Royal Horticultural Society publishes a height, spacing and staking figure for each. The figures below are the society’s, for trees grown in Britain. Treat them as a guide to relative size, since vigor varies with soil, climate and the variety grafted on top.

Rootstock	Ultimate height	Spacing between trees	First fruit	Staking	Soil
M.27	4 to 6 ft	4 to 5 ft	After two years	Permanent	Needs good soil, unsuitable for poor soils
M.9	6 to 8 ft	8 to 10 ft	After two or three years	Permanent	Needs good soil
M.26	8 to 10 ft	8 to 12 ft	After two or three years	Permanent	Tolerates average soils
MM.106	10 to 13 ft	12 ft	After three or four years	5 years	Wide range, including poor soils
MM.111	13 to 15 ft	15 ft	After four or five years	None if planted young	Most soils, including poor ones
M.25	Over 15 ft	20 ft	After five or six years	None if planted young	Most soils, especially poor ones

The pattern in the table holds across the series: the smaller the tree, the earlier it fruits, the longer it needs a stake and the better the soil it demands. Spacing also decides how many trees fit, which matters when a variety needs a pollination partner. Two M.9 trees at 8 to 10 feet apart fit in the 20 feet that a single M.25 needs. See how much space fruit trees need and self-pollinating versus cross-pollinating fruit trees before settling the layout.

Rootstock Profiles for US Gardens

American nurseries sell the Malling series alongside two others: Budagovsky, labeled B, and Geneva, labeled G, a Cornell University and USDA series known for high resistance to fire blight, woolly apple aphid and Phytophthora root rot. The descriptions here are from University of Maryland Extension.

M.27. A very dwarfing rootstock, known for early ripening, whose trees require permanent support. It is the choice for the smallest spaces and the best soil.

M.9. One of the most popular dwarfing rootstocks in the United States and Europe. It always requires leader support, and it is highly susceptible to fire blight.

B.9. A dwarfing rootstock regarded as a replacement for M.9 in colder climates. Its trees are 25 to 35 percent smaller than those on M.9 and need to be supported.

G.11. Similar to M.9, cold hardy, and resistant to fire blight and crown rot. It needs trunk support in the early years.

G.41. The most widely available of the Geneva series, similar in size to M.9, and highly resistant to fire blight, woolly apple aphids and Phytophthora.

M.26. Sits on the line between dwarf and semi-dwarf. It is susceptible to crown rot and fire blight and produces many burr knots. Penn State Extension describes these as clusters of partially developed root initials that usually form on the exposed above-ground part of the rootstock.

G.935. Slightly larger than M.26, with production efficiency similar to M.9. It is resistant to fire blight and crown rot.

M.7 and G.30. M.7 is a moderately precocious semi-dwarf. G.30 gives a tree of similar size, precocity and productivity, and is highly resistant to fire blight and Phytophthora.

MM.106. A highly productive semi-dwarf that does not require support and is cold hardy. It is susceptible to fire blight, root rot and crown rot, so Maryland does not recommend it for poorly draining sites.

G.890 and G.969. Semi-dwarfing, cold hardy, and resistant to fire blight, Phytophthora and woolly apple aphid.

MM.111. Does not require support, tolerates drier soil and is very cold hardy.

Which Rootstock for Which Yard

Work from your tightest constraint. Where space is the limit and the soil is good, M.9 gives the smallest practical tree, and G.41 or G.11 give a similar tree with fire blight resistance. In a small yard with hard winters, B.9 is the dwarf to look for.

If a permanent post is out of the question, move up a size. MM.106 and MM.111 both stand without support once established, at the cost of a taller tree and a wait of three to five years for fruit by the Royal Horticultural Society’s figures. On dry or poor ground, look at MM.111, which Maryland describes as tolerant of drier soil. Where fire blight is common and you want a mid-size tree, G.890 and G.969 carry resistance, while MM.106 is listed as susceptible. Our guide to fire blight in fruit trees shows what an outbreak looks like.

Heavy, slow-draining soil calls for a rootstock resistant to Phytophthora root rot. G.41, G.890 and G.969 are described as resistant to Phytophthora, M.26 as susceptible to crown rot, and MM.106 as unsuited to poorly draining sites.

Planting Height and Support

Plant with the graft union above the soil, but only just. Burr knots usually develop on the exposed above-ground portion of the rootstock, so because M.26 produces many of them, Maryland advises that its union be no more than 1 to 2 inches above the soil level. It gives the same limit of 1 to 2 inches above the final soil line for MM.111, and warns that a higher union affects how the tree establishes.

Set the stake at planting time for any dwarf tree. The Royal Horticultural Society lists M.27, M.9 and M.26 as needing a stake permanently and MM.106 for 5 years. MM.111 and M.25 need none if planted as one-year-old trees, or 3 years if planted at two or three years old. Once the tree is in, our guide to pruning a young apple tree takes it through the first three years.

Questions to Ask Before You Order

A catalog listing that says only “dwarf” or “semi-dwarf” leaves out the facts that matter, since a dwarf can be anything from 55 to 85 percent smaller than a seedling tree. Before buying, find out:

1. The rootstock by name, such as M.9 or G.41.
2. The expected height and spread in your region.
3. Whether the tree needs a permanent stake or trellis.
4. Its resistance to fire blight, crown rot and woolly apple aphid.
5. Its cold hardiness, if your winters are severe.

Your state extension service can tell you which of those diseases are common locally. If the seller cannot name the rootstock, choose a supplier who can.