Rootstocks and what they control

Apples do not come true from seed. Sow the pips of a Kingston Black and you get a set of new, unrelated and almost certainly worthless seedlings, because the apple is highly heterozygous and outcrossing. The only way to reproduce a named cultivar is vegetatively, and grafting is the method that has been used for that since antiquity. Every Dabinett tree in the world is a clone of one seedling found in a Somerset hedge.

Grafting the cultivar onto a root system of its own kind would give a very large tree, so nurserymen began selecting rootstocks for the behaviour they conferred. This was empirical and chaotic until East Malling Research Station in Kent collected, sorted and standardised the confused stocks then in circulation in the early twentieth century, producing the numbered Malling series that is still the international reference.

The result is that the modern orchard is designed at the root. Choosing a rootstock is choosing a tree size, and therefore a planting density, a training system, a harvest method, a management regime and a replacement date, before a single variety has been decided on.

The Malling stocks form an approximate ladder from very dwarfing to vigorous. The figures below are the conventional working guide: they describe a tree grown in reasonable soil under normal management, and they shift substantially with soil depth, cultivar vigour, pruning and whether the tree is grown in grass or clean ground. Treat them as relative positions on a ladder rather than as predictions.

The cider-relevant part of the ladder is the middle and upper end. Dwarfing stocks such as M9 give quick returns and heavy yields but require permanent support, careful irrigation and a level of management that a crop sold by the tonne cannot fund. Cider bush orchards therefore sit on MM106 and MM111, which stand unsupported, tolerate a grassed alley and withstand mechanical shaking. Standards and half-standards sit on M25 or on seedling stocks.

Precocity — how soon the tree bears — runs broadly opposite to vigour. Dwarfing stocks crop in their third or fourth year; a standard on seedling rootstock may take eight to twelve. For a commercial planting this is the whole of the investment case, because every year without a crop is a year of cost carrying interest.

Longevity runs the other way. Vigorous stocks make trees that live for many decades and can recover from damage; dwarfing stocks make trees with limited reserves that decline sooner. A cider grower who wants an orchard that will outlive them is choosing at the vigorous end by definition, and accepting a long wait as the price.

Soil tolerance is where the choice becomes site-specific rather than a matter of preference. MM106, the default cider bush stock, is susceptible to phytophthora collar rot and performs poorly on wet ground; MM111 is more tolerant of both drought and heavier soils; M9 is intolerant of drought because its root system is small and shallow. A rootstock chosen without reference to the soil pit is a rootstock chosen badly.

Anchorage matters for a specific cider reason. A tree that is going to be grabbed by the trunk and shaken by a machine every autumn needs a root system that will hold it and a graft union that will take the strain. This is one of the constraints that keeps cider bush orchards on semi-vigorous stocks rather than following dessert orchards down the ladder.

Perry pears are conventionally grown on pear seedling rootstock, which gives the enormous, extremely long-lived trees characteristic of the Three Counties. Quince rootstocks are used to control size in dessert pear growing, but many perry pear cultivars are graft-incompatible with quince or grow poorly on it, and where a smaller tree is wanted an interstem of a compatible pear is normally used.

This is a large part of why perry orchards look the way they do and why they are so hard to replace. The size-control options that transformed apple growing in the twentieth century were never fully available for perry pears, so a new perry orchard is still, in essence, a nineteenth-century proposition: big trees, slow returns and a very long horizon.