The following lists rootstocks in ascending order of size, smallest first. The numbering scheme for rootstocks does not run in an intuitive sequence!
Whilst most authorities quote the mature size and spread of a tree (as do we) when comparing rootstocks, it is not necessarily as simple as that. An apple tree on M9 can readily grow to the same height as a tree on MM106 if you let it or encourage it. However the M9 tree will have far fewer branches than the MM106 tree, and the branches will not be as thick or have as many secondary branches, and will not form as dense a canopy of leaves in the summer.
M27
The most dwarfing of all apple rootstocks, M27 produces a tree which will be smaller than a person standing with their arms slightly spread out. Some authorities suggest that M27 is "difficult" because it is so small, but in our experience growing M27 apple trees is straightforward provided you attend to their needs - regular watering, good soils, mulching and weed suppression. On the other hand, if you want an apple tree you can just plant and forget about, M27 is not the right choice.
The great thing about M27 is that it allows you to grow a lot of different apple trees in a small space. This means you can choose varieties that crop at different times, to get a spread of ripe apples throughout the season. You could plant 5 trees in a row 7m x 1m or 10 trees in a space 7m x 3m - the same space that you would need for one ""traditional"" standard apple tree. The yield depends very much on the variety and conditions, but you should get 4-5kg of apples per tree - equivalent to 5-7 supermarket poly bags.
Apple trees on M27 also have another interesting characteristic. Unlike every other rootstock, these trees are the same scale as humans - all other rootstocks produce trees that are much taller than us. This creates a very different effect in the garden situation because an "orchard" of M27 apple trees does not take over an area of the garden in the way that fruit trees on other rootstocks would.
Advantages: Produces a very small apple tree, reaches its mature size within 2-3 years. Allows you to grow several different apple varieties in a relatively small space. Very easy to manage because everything is within easy reach. Useful for growing in patio containers.
Disadvantages: Needs a permanent stake. Requires regular watering and good soil conditions. Mulch or clear the soil around the tree - do not allow competing plants to get near.
Article: in praise of the M27 rootstock.
M9
Widely used by commercial orchards, and also ideal for the garden. M9 was one of the first modern apple rootstocks, released in 1917 as a specific classification of an old French "Paradise" rootstock called Jaune de Metz.. The "Paradise" rootstocks were used in Europe since the the Middle Ages.
M9 is a great choice for most garden and small orchard situations in the UK and western Europe, especially where you want to maximise your crop and range of apples in a small space. The only slight drawback is that each tree will need a permanent stake, but this is a small price to pay for its ability to produce lots of apples in a small amount of space, and from a relatively young age. Apple trees on the M9 rootstock are also easy to look after and pick from ground level - most of the apples are borne on branches less than about 2m / 7ft high. In addition, M9 apple trees generally have spindly branches and are quite compact, so they don't tend to take over or dominate a small garden.
Its close relation with the old-fashioned Paradise rootstocks which were in widespread use before the 1900s means M9 is a good choice for achieving period authenticity in pre-20th century garden projects.
Advantages: Although M9 induces a small dwarf tree, the apples are usually slightly larger than for the same variety on other rootstocks. It also encourages precocity - in other words, the tree will bear fruit at a young age, you should get a few apples in the 2nd or 3rd year.
Disadvantages: Requires a permanent stake. The ground around the tree should be mulched or kept weed free, because it cannot compete with other plants. Not the best choice if you just want to plant an apple tree and then forget about it.
M26
M26 is a good general-purpose rootstock, which can be used for a wide variety of purposes, from medium-sized bush trees, to cordons and espaliers. Best considered for the same situations as MM106 (below) but where you want a smaller tree, although vigour increases in warmer climates.
Advantages: Produces a good productive tree. Good choice for cordons. One of the most cold-hardy of the Malling-series rootstocks.
Disadvantages: Usually needs a permanent stake, but this need not be as prominent as it is for the smaller rootstocks. Susceptible to fireblight (not usually an issue in the UK) and woolly aphid. Some authorities classify it as susceptible to collar rot (which is an issue if planting in damp ground).
M7
One of the original Malling-series rootstocks, produces a tree intermediate between M26 and MM106. Like M9, M7 was a specific selection of an original ""Paradise"" rootstock.
Advantages: Popular in the USA because it turned out to have reasonable resistance to fireblight and crown rot - before other rootstocks had been developed specifically for these conditions.
Disadvantages: Rarely available in the UK.
M116
A new rootstock which is similar to MM106 in most respects but produces a slight smaller tree (although larger than M26). It was developed in the 1960s from MM106 crossed with M27. Although it is not widely planted yet, it can be considered as an improved MM106, with roughly similar productivity to MM106 (even though the tree is likely to be smaller) but adaptable to a wider range of soil conditions. It is most suitable for large bush-trained trees and central leader forms, but also has potential to form a half-standard in good soils if the scion variety is vigorous (such as Bramley).
Advantages: Resistant to Phytophora (the fungus which causes brown rot / collar rot) and woolly aphid. Resistant to re-plant disease.
Disadvantages: Relatively new, so there is less knowledge of its performance with different scion varieties and in different conditions.
MM106
This is probably the most versatile rootstock for growing specimen apple trees in Europe, producing a tree of 3m-5m in height and spread - often known as a "half-standard". It is also the best choice for producing large trained forms such as espaliers. Staking is useful after planting (more so if growing in an exposed situation, less so in a garden situation), but the tree will become free-standing within a few years and the stake can be removed. Trees on MM106 do not require much attention apart from watering in dry spells, and once established will tolerate other plants or grass growing around them.
Advantages: The best rootstock for most garden and small orchard situations in Europe, particularly when grown on good soils and well watered. Useful for both free-standing trees and for large fans and espaliers. Does not need staking, and can be left to its own devices once established.
Disadvantages: Whilst much of the crop should be accessible from ground level, you may need a ladder to pick fruit from higher up the tree.
MM111
A semi-vigorous rootstock, producing trees somewhat larger than MM106.
MM111 is well-known for its ability to grow in both heavy and light soils and to tolerate drought and damp conditions. However it has never achieved the popularity of MM106 because it is slower to start bearing. It remains a good choice for difficult soils.
Advantages: Doesn't require much looking after.
Disadvantages: Can take many years (4+) before fruit is produced in quantity.
M25
M25 is the most vigorous apple rootstock. It produces a "standard" apple tree of up to 6m height after 10 years or so in good conditions, and is the best choice for old-fashioned traditional orchards, as well as locations with poor soils.
Advantages: Doesn't require much looking after, and ideal for growing traditional large apple trees.
Disadvantages: You will need a ladder to pick the apples.