Pruning: setting up the tree canopy for future crops
Research & Extension
Author: John Wilton
AgFirst
A good pruning job is one of the best foundations you can lay down to maximise orchard performance.
Two years ago I wrote two articles on pruning in Australian Fruitgrower. These cover the topic well and can be found in the May 2012 issue, pages 14 to 17, and in June 2012, pages 16 to 19. The main points made then still hold today.
The number one objective is to develop a uniform productive fruiting canopy that captures 60 to 70% of the available light.
Establishment pruning
When the orchard is being established, the focus has to be on rapid establishment of the fruiting canopy. Good tree vigour is required and the first objective should be growing tree height rapidly. I like to see a tree reach full leader height by the end of the fourth growing season, and this height needs to be at least equal to between-row spacing, or even a little taller if yield is to be maximised.
Although not strictly part of pruning, a robust tree support trellis is necessary, even with moderately strong rootstocks. The reason for the good tree support is that crop load is your best growth retardant, consequently, the tree needs to be fully cropped long before its leader has the strength to support the crop load. Once you have established a well-supported leader, the next task is to furnish it with calm, fruitful branches.
Experience shows that slightly pendant, well spurred, low vigour branches should populate the canopy at a density of roughly 6 to 8 per metre of leader for intensive plantings. Laterals need to become progressively smaller in the upper tree as it is essential to preserve a pyramid tree, or in the case of multi-leader systems a pyramid leader form, to allow good light penetration into the lower tree branches.
Once you have grown your fruiting canopy, the main objective is to keep it open, and fruitful. Vigour is now your enemy, so the first branches to go will be those showing excess vegetative growth.
Pruning for fruitfulness
As branches age, they thicken, and eventually become vegetative rather than fruitful. Once branches begin growing vigourous, upright, annual shoot growth, it is time to remove them. Much of the within-canopy shading problem is caused by vigorous annual shoot growth coming from the stronger branches within the canopy. Cutting them out goes a long way toward solving light penetration problems, not to mention reducing pruning costs.
Provided there is good light penetration within the canopy, fruiting sites have a relatively long life, so it is not essential to grow large amounts of new lateral growth for replacement of spent fruiting wood in pome fruit. Furthermore, excessive annual shoot growth in or beyond the outer canopy is usually of little value, because that is not where replacement fruiting wood is required.
Replacement fruiting wood, particularly for intensive plantings, needs to come from the inner canopy where there is room for it to develop and become fruitful before growing beyond the tree’s allotted space.
Upright shoots, irrespective of their length and vigour, generally become vigorous and eventually behave as leaders, so unless you are looking for a new leader in that position they must be completely removed.
Systemisation
Systemisation is a simple approach to pruning that adheres to a set of rules based on how the plant functions and responds to its environment. It is worth quoting directly from the June 2012 article on systemisation, which is one of the keys to efficient pruning.
“This is one of the keys to efficient pruning. Orchard performance is maximised when the fruiting canopy is full and uniform over the whole block. In general, pruning practices can be described in four or five simple rules, and once established and adhered to, it is not long before every tree in the block looks and performs the same.”
Some of the key rules are:
- Maintain a pyramid shape for the tree, whether two dimensional or three dimensional. The gap between the trees allows light penetration into the lower canopy and addresses the problem of apical dominance leading to upper tree branches becoming dominant.
- Keep fruiting units simple.
- Only leave upright shoots or branches where a new leader is required.
- Control tree shape by branch removal or manipulation, not by shortening cuts.
- Maintain sufficient separation between branches and laterals to allow the passage of transient light through the canopy.
- Eliminate high vigour branches, or manipulate them into a pendant position that will devigorate them.
- In more intensive orchard systems, systemisation can go as far as prescribing branch numbers per tree, their length, and even bud numbers.
- Have pruning targets. Here are examples from the June 2009 Future Orchards 2012 pruning notes:
- 35 km fruiting wood per hectare with one fruit every 8 cm = 80 tonne/ha.
- At 2,850 trees/ha, 18 branches/tree, average 65 cm long = 33 km fruiting wood per hectare, and at one fruit every 9 cm = 66 to 73 tonne/ha
Keep it simple
Sometimes referred to as the KISS (Keep it simple, stupid) principle.
Over the years numerous planting and pruning systems have evolved for growing pome fruit. They vary greatly in form and detail, but in general, if well executed, all give satisfactory orchard performance. Be very wary of complicated capital intensive systems. Over the years it has been shown that simple production systems, well executed and managed, perform best and because of their lower inputs generally have lower risk because their debt loading is less than more complicated, capital intensive systems.
My general impression from our OrchardNet™ database is that medium density orchards planted in the range of around 1,500 to 2,000 trees per hectare are producing some of our best crops. They may take an extra year to come into high production, but have much lower establishment costs, sometimes by more than $20,000/ha and I have seldom seen the very first crop of a very intensive orchard generate this level of profit to compensate for the additional capital outlay.
The key to success with any of these systems lies in the attention to detail, and elimination of any limiting factors to tree performance such as water stress, poor drainage, hunger or debilitating diseases.
The same can be said for pruning; the key is knowing where you are heading and executing the job with careful attention to detail.
Pruning the older orchard
In recent years there has been a strong emphasis on orchard intensification which involves capital intensive new orchard plantings. While there is good data to confirm the trend towards orchard intensification is the best way forward, the major limiting factor of capital availability for orchard redevelopment means that it will be many years before older, less intensive orchard plantings are replaced. In the meantime, we have to maintain their production at a level of performance that makes a profitable contribution to the orchard business, otherwise the capital will not be there when it is time for their replacement.
Furthermore, the introduction of good vigour control techniques, and development of reflective mulches to improve lower canopy light levels have closed the performance gap between modern intensive orchards and older, less intensive orchards.
For instance, the basic fruiting unit, the weak, slightly pendant, fruitful branch should be similar among different planting densities. The fundamental plant physiology that determines fruiting performance is pretty much the same, and independent of planting density. What is different is the arrangement of these fruiting units within the canopy.
With intensive plantings, the leader is usually the only main structural branch, with only weak fruiting units growing from it.
At wider tree spacings, lower and middle tree branches off the main leader become partially structural and begin to take on the role of the intensive tree leader in order to increase canopy volume required to fill the tree’s allotted space.
Where trees have been trained with a formal tier structure, excess vigour along aging branches becomes a real challenge. The more you prune them, the more they will want to regrow, and the more costly they are to maintain as productive branches. The best long term solution to this problem is to progressively remove these high vigour/high maintenance branches and replace them with calmer, pendant branches that are more fruitful.
Where trees are outgrowing their between-row spacings leading to machinery access problems, removing those lower branches running towards the row alleyway responsible for the machinery access problem is a good start. As a rule, they do not carry much high quality fruiting wood and what they do carry can be easily replaced with higher quality fruiting wood coming off branches running along the tree row.
It is important to maintain a single layer of fruiting laterals on these larger branches to minimise within branch shading problems.
Also, avoid cutting laterals and branches back into shoots and laterals coming off the top of branches. Sometimes these are known as “lifting” cuts and they usually respond with more vigorous growth. The tree will produce much calmer growth and be more fruitful if stronger upside growth is removed to maintain a straight main branch axis without the presence of lifting cuts.
Higher up the tree the branch structure needs to become progressively weaker, as well as being pendant. Emphasis needs to be on removing the stronger branches that are producing the most annual shoot growth.
At wider between-tree spacings, branches can become more complex in order to build canopy volume, even in the middle and upper tree. The key is making sure that they do not become too dense with numerous overlapping layers of fruiting wood.
While branch removal is the most cost-effective way to deal with problem branches, this is only possible where there are suitable replacement branches. Where there is little choice and problem branches have to be retained to maintain canopy volume, they can be tamed down by training them into pendant positions along with removal of any dominant high vigour upright sub-branches coming off them. But, remember, if you want to settle them down into low vigour fruitful branches, sufficient cropping capacity needs to remain on them to utilise the photosynthates their leaves produce in growing fruit rather than shoot growth.
A year or two back it became fashionable to slim down branches by removing side laterals to try and turn the branch into a fruiting rod. Where this was done to excess, too much fruit bud was pruned away, leading to high branch vigour.
There are also good relationships between fruitfulness, branch length to diameter ratios and branch vigour.
As a general rule, fruiting laterals need to be smaller than about 2.5 cm diameter per metre of length and once their diameter begins to approach 3 cm per metre of length, they become less fruitful and more vigorous. Heavy pruning back into branches destroys this relationship and should be avoided if possible.
Alternative to dormant season pruning
While most pruning needs to be and is best done over the dormant season, there are advantages in doing some pruning over the growing season which should not be overlooked.
These are:
- Water shoot removal – best done in late spring pre-hand thinning when succulent young water shoots can be easily pulled out, taking with them any adventitious basal buds to reduce regrowth. It is much easier to pull these shoots than prune them out later and their removal improves the light environment within the canopy.
- Leader topping, and branch tipping. If this pruning is delayed until after fruit set, the crop weakens vegetative regrowth so tree height can be reduced with less risk of a strong regrowth response.
- Annual growth tipping to reduce vigour and stimulate fruit bud development. This is usually done in late spring/early summer and is a good way to turn excessively strong annual shoot growth into useful fruiting wood. In Europe, hedging when annual shoot growth is between 6 and 10 leaf length is the basis of their mechanised fruiting wall pruning systems. As varieties and rootstock vigour and growing conditions all appear to impact on response, the technique will need to be tested under local conditions to determine timing.
- Post-harvest removal of vigorous shading branches – very relevant for early harvest varieties.
