Scion

When you buy an apple or plum tree, you're getting two different plants fused into one. The scion is the top part: the variety you actually chose, the bit that grows the shoots, leaves, blossom and fruit you want to eat. It is joined to a separate root system, the rootstock, by grafting — fusing a bud or short length of stem from the chosen variety onto the roots of another tree.

Why two plants instead of one

The simple reason is that the scion and the rootstock each do a different job, and growers want to control both. The scion decides what you grow; the rootstock decides how big the tree gets.

Think of it as a division of labour:

• The scion decides the apples. All the fruiting character — variety, flavour, colour, when it ripens, whether it's a cooker or an eater — comes from the scion. Graft a Bramley scion on and you get Bramley apples, every time.
• The rootstock decides the size. The roots control vigour, so the same scion can become a 1.5m patio tree or a 4m orchard tree depending entirely on what it's grafted onto.

This is brilliant news for UK gardens, where space is usually tight. You can have the exact variety you love kept to a sensible size — full-flavoured Cox apples on a tree small enough for a large pot — because the two decisions are separated.

Why varieties aren't grown from seed

You might wonder why nurseries bother grafting at all. The answer is that most fruit varieties don't come true from seed. Plant a pip from a shop-bought apple and you won't get that apple back — you'll get an unpredictable seedling, usually a poor one. The only reliable way to reproduce a named variety is to take a scion from an existing tree of that variety and graft it on, so the new tree is genetically identical to the parent. Every Bramley in the country traces back, by grafting, to one original tree.

Spotting the join

You can usually see where scion meets rootstock as a slight kink, bulge or change in bark texture low on the trunk, a few centimetres above the soil. Keep that graft union above ground when you plant — burying it can let the scion put out its own roots and undo the rootstock's size control. Sometimes a rootstock sends up its own shoots from below the join; rub or pull these off, as they're the wrong variety and will never give you the fruit you wanted.

So when you read a fruit tree label, remember it's telling you both halves: the variety is your scion, the code beside it is the rootstock. For more on choosing well, see how to grow apples and the wider fruit growing guides.