Tree size

The mature size of a fruit tree depends mainly on:

(a) Site characteristics - some sites are more favourable for fruit tree growth than others.

(b) Management practice. Competition from grass and any other nearby vegetation, particularly trees, reduces vigour (as does root pruning or partial girdling), while mowing, mulching, herbicide within the row, watering and fertilizing generally increase vigour.

(c) Variety. Apple varieties noted for their vigour are often triploids, like ‘Bramley’ and ‘Jonagold’, while others like ‘Discovery’ are much more compact.

(d) Pruning/training. This usually aims to increase crown width while reducing crown height. If trees were allowed to grow relatively tall, crown area would tend to be correspondingly less. Pruning can also restrict crown area, as in containment pruning and in trained forms such as the cordon and espalier.

(e) Rootstock. Knowing how big you want your fruit trees to grow, and evaluating (a) to (d) above, you have to choose a rootstock that will achieve the desired size. Particularly for apple, there are several rootstocks that vary from the least invigorating (dwarfing) to the most invigorating.

 Rootstocks

The table below, derived from published sources, gives consensus values for crown area by rootstock (the commoner ones).

 Table 1. Areas to allow per tree by rootstock, depending on anticipated vigour. Trees with low vigour (owing to site factors, variety or management factors) achieve a lesser mature size and may therefore be planted more closely together.

Apple M9 is the commonest rootstock in intensively managed commercial orchards. Permanent staking is required owing to the weakness of the roots. Often promoted to garden owners but can be disappointing owing to insufficient vigour since it needs good soil and freedom from weed and grass competition.

Apple M26 gives compact growth in many gardens, largely tolerating average soils and growth in grass (if mown).

Apple MM106 is widely used in wetter climates, for varieties with low vigour and for trained forms in poor soil. It tolerates a wide variety of soils as well as grass competition.

Apple MM111 is vigorous, tough and cold-hardy, tolerates drought relatively well and is said to have some resistance to waterlogging.

Apple M25, the most vigorous apple rootstock, is suitable for the standard form on good sites.

'St Julien A' for plum, 'Quince A' for pear and 'Colt' for cherry are common (if traditional) choices. The more dwarfing rootstocks for these fruits can easily have insufficient vigour unless the site is favourable and management good.  

 Planting distance

If in doubt choose a more rather than less invigorating rootstock. It’s easier to thin trees or restrict growth than to invigorate weakly ones.

Settle on planting distance, hence the number of trees you have room for, before deciding on varieties. This is a relatively sensitive decision if you want your trees (when mature) to touch within the row, as you would to maximize fruit production per unit of ground area.