What is an "own-root" fruit tree?
The vast majority of fruit trees produced world-wide are propagated on rootstocks. The fruit tree therefore consists of two parts, the rootstock which is mostly below ground, and the scion (fruiting variety) which is the main part of the tree visible above ground. If you look carefully on a young tree you can see the kink in the stem just above the ground where the main tree has been grafted on to the rootstock. Fruit trees are therefore very different in this respect from other trees - the typical oak tree growing in a field will have grown from seed, on its own roots.
The photograph shows from left to right:
- A 1-year bud-grafted apple tree. You can clearly see where the rootstock was "headed" to allow the grafted bud to grow away. After a few years the bud will grow right over the top of the rootstock.
- A 1-year bench-grafted apple tree. The grafting wax is still visible, and the stub of the rootstock can be discerned in the centre of the tree (the graft slants from top right to bottom left in the centre of the stem).
- A 2-year own-root plum tree. Note how there is no graft union on the own-root tree.
The technique of using rootstocks for growing fruit trees has been known for hundreds of years, but it was not until the early 20th century that standardised virus-free rootstocks became available, at which point the use of rootstocks quickly became the standard way of producing fruit trees. Before that time, the propagation of fruit trees was more haphazard, with a good number being grown simply on their own roots.
Thus own-root fruit trees are in some senses a more "natural" way of growing fruit trees. There has been a resurgence in interest in own-root fruit trees, partly as a result of the work of English apple breeder Hugh Ermen in the late 20th century.
Advantages of own-root fruit trees
Hugh Ermen was a great believer in the benefits of own-root fruit trees. He argued, with some justification, that the artificial joining of two different types of tree (the rootstock and the scion) created a degree of incompatibility. By growing the fruit tree on its own roots this incompatibility was removed and as a result the tree would be more healthy, live longer, and the fruit would have more flavour. However it should also be said that the degree of incompatibility between rootstock and scion also varies considerably. For example, most dwarf pear trees are grafted on Quince rootstocks - a different species altogether - but most semi-vigorous and vigorous pear trees are grafted on PyroDwarf rootstock, which is of the same species.
Unfortunately Hugh's work has not been continued so the true extent of these benefits is not really known. It is fair to say that an own-root tree is closer to its natural state than a conventionally-propagated fruit tree, and a fruit tree grown on its own roots might more fully express the inate characteristics of the variety, without any influence from the rootstock.
Disadvantages of own-root fruit trees
There are two main issues:
- Firstly, one of the main benefits of a rootstock is that it controls the height and size of the tree. Own-root trees do not have this control and if no alternative measures are taken, will grow quite large over time. As a guide, they are likely to achieve a similar size to a conventional tree on the MM106 rootstock. However the size of any given own-root tree varies more widely than a tree propagated on a rootstock and it is difficult to predict the mature size with the same degree of accuracy.
- Secondly, own-root trees take longer to grow and will not start fruiting as early in the life of the tree as the same variety grown on a rootstock. This is because most rootstocks, especially the more dwarfing ones, are precocious - they encourage the tree to reach maturity and start fruiting earlier than it otherwise would.
For these reasons it is unlikely that own-root trees would ever be used commercially, but in a garden situation they can make an interesting alternative to conventionally-propagated trees for the enthusiast.
It is also possible to counter the drawbacks described above, through careful choice of variety. The problem of excessive tree size can be addressed by selecting varieties that are naturally of low-vigour. Similarly, the problem of having to wait longer for the own-root tree to start producing fruit can be addressed by selecting varieties that are naturally precocious and like to fruit early in their lives.
There are also some fruit tree species which are natually small and precocoius and can therefore be grown on their own roots without these drawbacks. Morello cherry trees are a good example of this.
How are own-root trees propagated?
Many fruit tree species, particularly apples, do not grow true from seed. In other words, if you plant a pip from a Discovery apple tree, the resulting tree will not be a Discovery - it will be a cross between its mother (Discovery), and whatever other apple tree variety pollinated it (almost certainly not another Discovery tree). Whilst this makes it easy to raise new varieties, it is a problem if you want an apple tree of a particular variety growing on its own roots.
Hugh Ermen developed several techniques for propagating own-root trees. One of the most successful involves grafting a section of the desired apple variety on to a conventional M27 rootstock, which acts as a temporary "nurse root" to get the own-root tree started. The resulting combination is then planted out in the nursery, but much deeper than usual. The M27 rootstock provides the initial growth, but the scion soon develops roots of its own and because these are naturally very vigorous, they soon out-compete the M27 rootstock which eventually dies away. The tree then continues to grow vigorously on its own roots.
It is relatively easy to tell if a young tree has been grown on its own roots, because there will be no kink at the base of the stem where the rootstock would join the scion as in a conventional fruit tree. Instead there will be a continuous smooth stem from the roots up.