How To, Jorge, Plants, Trees

How to Choose The Perfect Apple Tree Rootstock

Scrumptious on a MM106 (left) and M27 rootstock. Note the difference in foliage and stem thickness.

Nobody should purchase an apple tree without first considering its rootstock. Rootstocks determine a tree’s size, resistance to disease and time to fruit. Primrose recommends you buy a dwarfing rootstock if you are interested in maximising productivity. For most gardens in the UK, an apple tree grown on its own roots is simply too large!

What is a Rootstock?

A variety grafted onto a rootstock.

Rootstocks are plants, usually of the same species, selected for their interactions with the soil. It is these plants that your selected apple variety will be attached in a process known as grafting. Thus, the Cox apple tree that you buy isn’t entirely a Cox as its roots are a different variety.

Apple trees have to be produced this way as their seeds aren’t true to type. Apple trees need to exchange chromosomes to produce offspring just like humans. Thus, if you plant a Cox apple and it germinates your resultant tree won’t be a Cox, but a mix between a Cox and another apple variety.

Not only are rootstocks necessary for propagation, they affect the variety they are attached to. One of the most important effects, is how a rootstock affects a tree’s eventual size. Thus, rootstocks are categorised into dwarfing and vigorous rootstocks, with dwarfing producing smaller trees and vigorous larger.

fruit tree rootstocks
The commonest rootstocks of some popular fruit tree species.

Dwarfing rootstocks produce smaller trees as they are worse at gathering resources. This may sound like a bad thing, but it is actually a huge benefit. Trees will put more effort into fruiting at the expense of vegetative growth as they want to pass on their offspring. This makes your tree more efficient. A higher percentage of carbohydrates go into fruits than leaves and stems.

This leaves us with this simple rule of thumb. If you are interested in maximising space and maximising output, dwarfing rootstocks are a must. If you want to grow your tree for ornamental value, buy a vigorous rootstock.

Is there anything else I need to know about rootstocks?

As rootstocks affect other traits than a tree’s size, such as disease resistance, you should factor such traits into your calculations. Importantly, it is hard to determine how a rootstock, variety and local conditions interact before planting. Thus, if you wish to establish an orchard, buy multiple varieties on different rootstocks to test the waters before any big outlay.

With dwarfing rootstocks, it is important that they are supported with ties, stakes or poles. Such trees produce heavy crops relative to their size and will lean or fall over; their roots are shallow and brittle and liable to uprooting in severe weather.

History of Apple Rootstocks  

Use of rootstocks goes back thousands of years, but it wasn’t until the 1800s when they were referred to by name. It is this period that provides us with two examples – Paradise (French Paradise) and Doucin (English Paradise) – of which the latter was more vigorous than the former. Common as these rootstocks were their ubiquitousness may have been due mislabelling and mis selling, rather than widespread propagation. One author from the late-1800s sussed this out, describing 14 different kinds of Paradise rootstocks.

It wasn’t until the early 20th century when the East Malling Research Station set upon determining the trueness of the Paradise rootstocks. Unsurprisingly, the institute found numerous misnamed and mixed collections of plant materials and decided to abandoned their names entirely assigning a number to each, giving us the original M (Malling) series of 24 rootstocks. Unfortunately, they weren’t numbered according to tree size with M9 smaller than M2.

Controlled crosses produced the famous M26 and M27 rootstocks. The MM106 and MM111 were produced once the John Innes Institute of Merton joined with East Malling in developing rootstocks resistant to wooly aphids. Thus here, MM means Malling-Merton.

In the 1960s work began to remove viruses from the rootstocks as to reduce incompatibility. (Viruses can cause a graft not to take). The first rootstock cleaned was the M7 which became M7A. Unfortunately, removing viruses led to a loss of size control with the dirty M9 producing smaller trees than the cleaned M9 (known as M.9EMLA with EMLA standing for East Malling & Long Ashton research stations). Eventually, all M series rootstocks were cleaned.

Other countries around the world set on developing rootstocks suited to their climates with the Budagovsky series developed at Michurinsk College, Russia and the P series developed in Poland, both seeking cold hardiness. (As cold hardiness is unnecessary in the UK, these rootstocks are not widely used by nurseries).

A more research breeding program led by Cornell University has produced the Geneva series of rootstocks with a special emphasis on resistance to fire blight. This series you are more likely to see in the future, due to the emphasis on disease resistance.

List of Apple Rootstocks By Size (Smallest to Largest)

apple tree rootstock infographic
Approximate apple tree eventual heights by rootstock. We have taken growing conditions as optimal and presumed the tree is a Cox.

Note: standard refers to a tree grown on its own roots. Thus a rootstock that produces a tree 30% of standard will produce a tree 70% smaller than if it was grown on its own roots.

M27

Perfect for small gardens, M27 will produce a tree 30-50% of standard and is very precocious (produces early in its life). With the Cox variety, you can expect a tree 1.5-2m high.

It is highly susceptible to fire blight but resistant to crown and root rot. It produces few burrknots and root suckers.

Permanent support is recommended. Leader support will produce larger trees.

The tree was produced in 1934 at East Malling and is a cross of M13 and M9.

M9

M9 will produce a tree 45-50% of standard and is very precocious. With the Cox variety, you can expect a tree 1.8-2.5m high.

It is very susceptible to fire blight and woolly aphid, but fairly resistant to crown and root rots.  It produces a moderate amount of root suckers and many burrknots.

Permanent support is recommended. Leader support is necessary. Unsuitable to wet sites.

The tree was selected from a group of French genotypes “Juane de Metz” in the late-1800s. It is the most widely used rootstock in the world.

M26

M26 will produce a tree 55-60% of standard and is precocious. With the Cox variety you can expect a tree 2.5-3m high.

It produces many burrknots and is susceptible to to crown rot and fire blight.

Usually requires support. (Possibly forgo in sheltered sites.) Unsuitable to wet sites.

The tree was produced in 1929 at East Malling and is a cross of M16 and M9.

MM106

MM106 will produce a tree 70-75% of standard and is somewhat precocious. With the Cox variety you can expect a tree 3.5-4m high.

It is resistant to woolly aphid, susceptible to crown and root rot and fire blight and hypersensitive to tomato ringspot virus. It produces few burrknots and root suckers.

Support is unnecessary. Unsuitable to wet sites.

The tree was produced in 1932 by John Innes and East Malling and is a cross of M2 and ‘Northern Spy’.

Rootstock Spacing

M27 needs 1.5m between trees, M9 2.5-3m, M26 3-3.5m and MM106 3.5-4m.  Using the higher estimate is recommended for triploid varieties, which are known for their vigour.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

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