Walk into any nursery and you will see apple trees labelled M9 or MM106, pears marked Quince A, plums on St Julian A. These codes are not a secret language, but they decide more about how your tree behaves than the variety name on the label. Choose badly and you will spend the next decade pruning something that wants to be four metres wide in a space built for two.

The rootstock is the bottom section of the tree, below the graft union. It is chosen for its roots, not its fruit, and the variety you actually want to eat is grafted on top. Growers use it to control vigour, and therefore final size, but it also shapes how quickly the tree crops, how well it anchors itself, and how it copes with your soil.

What a rootstock actually does

Size is the headline, but it is not the only effect. A dwarfing rootstock produces a small tree that crops young, often within two or three years of planting, and crops heavily for its size. The trade-off is a smaller root system: it needs staking, watering and feeding, and it will sulk in thin, dry soil.

A vigorous rootstock does the opposite. The tree takes longer to settle into fruiting, sometimes five or six years, but it grows deep, wide roots that find their own water and hold the tree steady without help. On good soil it will also overshoot the height on the label. Nurseries quote sizes based on reasonable conditions; in a cold, exposed garden you may get a tree half that size, and on deep loam against a warm wall you may get one noticeably bigger. Treat the figures as a guide, not a promise.

Apple rootstocks, from tiny to towering

Apples have the widest range of rootstocks of any top fruit, which is why you can buy everything from a metre-high stepover to a full standard. The ones you will see most often in UK catalogues are:

• M27 – the most dwarfing apple rootstock in general use, giving roughly 1.2–1.8 m. Ideal for containers, raised beds and stepovers. It needs a permanent stake, regular watering and genuinely good soil.
• M9 – dwarfing, around 1.8–2.5 m. The classic choice for cordons, espaliers and small gardens. Crops early and heavily. Stake it for life, and thin the fruit or branches will break under the weight.
• M26 – semi-dwarfing, around 2.4–3 m. A forgiving middle ground for the average garden, though still happier with a stake.
• MM106 – semi-vigorous, roughly 3–4 m. The traditional garden apple. Free-standing after a few years and useful as a half-standard. It dislikes wet, heavy soils, where root rot can set in.
• MM111 – vigorous, 4–5 m. Tough and drought-tolerant, good on poorer or windy sites. Slower to crop.
• M25 – very vigorous, 4.5–6 m or more. For large gardens, orchards and traditional standards. It needs space and patience.

You may also come across the Geneva series – G11, G41, G935 and others – bred largely for disease resistance and now more widely sold here. G11 behaves a little like M26 and G41 like a slightly more vigorous M9, and both are worth considering where fireblight or replant disease is a worry.

Pears: quince, or something sturdier

Most garden pears are grafted onto quince, which keeps them compact and brings them into cropping early. The catch is that quince roots are fussy: they want fertile, well-drained soil, they dislike lime, and they need staking for life.

Quince C is the most dwarfing, giving a tree of about 1.8–2.5 m – good for a small garden or a large pot. Quince A is the safer all-rounder, reaching roughly 2.5–3.5 m and compatible with a wider range of varieties. Quince Eline sits between the two and copes better with cold, though it is less widely stocked.

If your soil is heavy clay, chalky or exposed, look for pears on Pyrodwarf or a pear seedling rootstock instead. Pyrodwarf gives a semi-vigorous tree of around 3.5–4 m that stands on its own roots without a permanent stake, and it is compatible with nearly every variety. Seedling rootstocks, often sold as Kirchensaller, make big, long-lived trees of 5–7 m or more – the right choice for perry pears and anyone with room to spare.

Plums, gages and damsons

Plums usually arrive on one of three rootstocks. Pixy is dwarfing, producing a tree of roughly 2–2.5 m, which makes it the obvious pick for a small garden or a container. It needs a stake for life and reliable watering. Fermentina, sometimes sold as VVA-1, is similar but slightly more vigorous and arguably easier to keep happy.

St Julian A is the standard garden rootstock, giving 3.5–4.5 m. It anchors well, crops reliably and only needs staking for the first few years. If you want a damson or a big culinary plum and have the space, Brompton is more vigorous still at 4.5–5.5 m. Myrobalan B is the thug of the group at 6 m and upwards, but it is one of the few sensible options on heavy, wet ground. It also suckers enthusiastically – pull the shoots away rather than cutting them, or you will simply get more.

Matching the rootstock to your garden

Start with the space, then the soil, then your own patience.

• Small courtyard, balcony or pot: M27 or M9 apple, Quince C pear, Pixy plum.
• Average garden, one or two trees: M26 or MM106 apple, Quince A or Pyrodwarf pear, St Julian A plum.
• Large garden, exposed site or poor soil: MM111 or M25 apple, seedling pear, Brompton or Myrobalan plum.
• Training against a wall or over an arch: dwarfing types such as M9, M27, Quince C and Pixy, because you need growth you can cut back each year.

Be honest about your soil. A dwarfing rootstock on shallow, sandy ground will produce a stunted, drought-stressed tree that never crops properly, while a vigorous one in a small bed will need constant summer pruning to stop it shading next door's greenhouse. On heavy clay, plant on a slight mound, improve the planting area generously, and consider MM111 or a quince-free pear.

Staking, feeding and the first few years

Buying the right rootstock is half the job. The other half is giving the tree the conditions it was bred for.

• Stake properly. Use a sturdy stake for dwarfing trees and drive it in before planting so you do not spear the roots. Tie with a soft, adjustable tie and check it each spring.
• Water for the first two summers. Dwarfing rootstocks have small root systems. A thorough soak once a week in dry spells beats a light sprinkle every evening.
• Mulch, but keep it clear of the trunk. A ring of well-rotted compost or bark, 5–8 cm deep, holds moisture and keeps weeds down.
• Thin the fruit. Heavily cropping dwarf trees snap branches. Leave one or two apples or pears per cluster with 10–15 cm between clusters, and space plums 5–8 cm apart.
• Prune to the rootstock's habits. Trees on M9 or Pixy respond well to winter spur pruning; vigorous trees need summer pruning to slow them down.

Buy from a specialist nursery that names the rootstock clearly, and check the label when the tree arrives. A fruit tree sold without a rootstock name is a mystery you will spend years solving.

Photo: vinsky2002 / Pixabay