THE KITCHEN GARDENER
If you want to grow beautiful fruit, don’t forget your roots, writes SIMON RICKARD.
Rootstock.
When we choose fruit trees, we tend to select them based on the type of fruit they will produce. Perhaps we prefer a Granny Smith apple over a Fuji, or a Navel orange over a Seville. The fruiting growth is only half of the tree, however. Each fruit tree has another half which is just as important, and can mean the difference between success or failure for the home gardener: its rootstock.
Fruit trees are propagated by grafting, a 3000-year-old technique practised by the Persians and Chinese. Put simply, grafting is a method of propagation which entails joining parts from two different plants together to form a single unit. The aboveground part of a graft is called a ‘scion’. It is made from a twig, or a slip of bark with a bud on it, from the desired plant. The underground part of a graft is called the ‘rootstock’. The rootstock has to be from a related species to the scion, so that they are genetically compatible.
The scion and rootstock are both cut to expose their sapwood, and bound tightly together for a period of time. If all goes well, they will knit together and become a single organism.
Grafting is used to propagate plants which are difficult to strike from cuttings or reproduce in quantity. But why should it matter to home gardeners? Because just as grafting a scion from a Granny Smith would produce green, tangy fruits, or a scion
of Fuji would produce honey-sweet pink fruits, different rootstocks confer their own different attributes on a tree. Some rootstocks result in a tree which only grows to half the normal height, or even a quarter. Some make the tree tolerant of particular soil types. Others lend the tree resistance to particular diseases such as woolly aphid.
Indeed, it was only by grafting grape vines onto American wild grape rootstocks that the French wine industry was saved from the root-sucking phylloxera bug in the 19th century, to which the old French grapes had no resistance whatsoever.
Rootstocks tend to have very unmemorable names, but it’s important to understand their effects. Let’s say you want to train a Jonathan apple tree as an espalier to save on space, and the need for a ladder. It’s vital that this tree is grafted onto a suitable semi-dwarfing rootstock. For example, an apple tree grafted onto ‘MM102’ rootstock results in a tree only two-thirds normal size. If you wanted to train a Jonathan as a miniature ‘stepover’, or grow it in a pot, you would need to select a very dwarfing rootstock such as ‘M27’ or ‘M9’.
If you want to train a Williams pear as a full-size orchard tree, you might choose vigorous ‘D6calleryana’ for its rootstock. Conversely, if you wanted to espalier a Williams, you would choose a semi-dwarfing ‘Quince A’ rootstock, which has much less vigour. It will grow a little slowly initially, but eventually will be the ideal size for an espalier.
Citrus grafted onto ‘Trifoliata’ or ‘Flying Dragon’ rootstocks have good tolerance to cold and heavy soils, and result in a tree only head-high. At three metres tall, cherry trees grafted onto a semi-dwarfing ‘Colt’ rootstock relieve you of the need to own a cherry picker in order to prune, net and harvest them. The list goes on.
In the northern hemisphere, it’s normal to buy fruit trees which stipulate not only the fruit variety, but also the rootstock it has been grafted onto. Unfortunately, this is not yet the case in Australia and New Zealand. You will need to shop around to find rootstocks suited to your needs, but it is worth any amount of extra effort. If you don’t, be prepared to fight a losing battle against a tree that doesn’t want to comply with your requirements. ●
Some rootstocks result in a tree which only grows to half the normal height, or even a quarter. Some make the tree tolerant of particular soil types.