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Fundamentally in fruit growing you are farming light. Satisfactory bud development and production of high quality fruit needs exposure to 60 to 70% of the available light. Pruning management is the primary husbandry practice to ensure that these light conditions within the canopy are achieved and maintained.
Shade is a major cause of poor fruit quality. Within the tree shade can come from two main sources:
- Branch shading - ie too many branches
- Within branch shading
Branch shading occurs when too many branches are present, or where complex branches are allowed to develop. Optimum branch density ranges from 4 to 6 branches per meter of leader extension in the lower tree to 10 to 12 laterals per meter, as branches get smaller in the upper tree. Strong complex branches in the upper tree are particularly damaging to light penetration into the lower tree canopy.
Within branch shading occurs when the canopy at individual branch level develops depth and becomes dense. This problem is overcome by confining growth at branch level to a single horizontal plane and keeping branches narrow without significant sub-branches.
These days' fruit trees tend to be planted at higher densities than would be optimum for them when left to develop naturally. These higher planting densities are only possible through vigour control and containment pruning to confine the tree to its allotted space. Furthermore unwanted vegetative growth once the tree has filled its allotted space is very unproductive and made at the expense of fruit production.
Containment pruning practices need, therefore, to manage tree vigour as well as canopy light penetration.
Good pruning requires skill and in particular needs a good understanding of tree growth behaviour.
Reading the tree is an important skill. It's the ability to understand how the tree will respond to your pruning. As in many things, past behaviour is a good guide to the future so studying the way the tree responded to last year's pruning will show how the tree reacted to particular pruning practices. If you want to know what a particular lateral will look like next year, look for a similar lateral one year older growing under similar conditions nearby.
Take particular note of how laterals with different gradients behave when pruned or left unpruned.
The pruning operation can be systemized by adopting a number of simple rules. These are:
- Always prune out steep gradient laterals. These will tend to be vigourous and unfruitful.
- Remove shaded pendant (hanging) wood and buds from underneath the main branch axis
- Observe the 3:1 rule. Branches coming off the main leader or sub branches on main branches should not exceed one third of the diameter of the main branch. Where vigour is particularly high, an even wider branch size differential may be needed. This relationship may only hold well for younger trees.
- The 3cm/m rule. This rule describes the ability of a branch to produce vigorous water shoot growth. When branch diameter is greater than 3cm per meter of length the branch tends to become vegetative rather than fruitful and will produce water shoots rather than fruitful buds. Water shoots compete with fruit for photosynthate and shade as well.
- Never shorten into upright growth. When shortening a branch always shorten with horizontal or pendant wood.
- For modern, hedgrown orchards always maintain the pyramid tree shape. Within the row only the lower tier of branches should meet or intermingle with the branches from the next tree.
- Pruning is the first cut of the thinning programme. Use it to limit fruit bud numbers and maintain fruit bud strength. As trees age they tend to loose vigour and will require more detailed pruning to maintain fruiting bud strength.
- Balanced mature cropping trees should make 20 to 40 cm of 1 year shoot growth in the season.
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