Pruning for vigour managementIndustry Best Practice
Optimising vigour is critical for growing high yields of top-quality fruit. Pruning plays an important role in vigour management.
Yield in woody, fruiting crops depends on efficient light capture to maximise photosynthesis, then through efficient partitioning of photosynthates between vegetative growth and fruit, turning this light into high quality marketable fruit. Excessive vigour competes with fruit for photosynthates, while on the other hand low vigour fails to produce sufficient tree leaf and canopy volume to capture and utilise available light. Under Australian growing conditions both high and low vigour problems can occur, so it is necessary to define the tree vigour status before developing a pruning strategy. Optimum vigour for a well-balanced mature fruiting canopy, irrespective of root stock, is to produce growth of annual shoots in the 20–30cm length range.
The growing season for some pome fruit trees in Australia is longer than many Northern Hemisphere growing areas so it is unwise to adopt Northern Hemisphere growing practices without first trialling them under local growing conditions. With our long growing seasons, such as those found in the more temperate Australian climates, it is possible to build productive capacity more rapidly than often quoted for shorter Northern Hemisphere growing seasons. An example of this is the ability we have, under favourable conditions, to crop on the auxiliary buds of one-year old laterals. For us, the cropping life of a modern orchard should be able to commence in the second growing season after planting. In the absence of limiting factors to growth, young trees can carry commercial crops without compromising canopy development and future cropping potential.
With some varieties, e.g. Nicoter (Kanzi®) and Scifresh (Jazz), high vigour in young trees causes bare wood due to bud, jump problems in the second growing season. This is a serious problem for intensive plantings because the bare wood pushes fruitful bud beyond the trees allocated space. The key to high productivity in intensive orchards lies in maintaining the cropping zone within the tree’s allocated space.
Dealing with excess vigour
Potential excess vigour problems need to be identified early before the growth becomes excessive. The canopy needs to be set up to allow removal of problem branches without disrupting the canopy balance. Our experience in young orchards indicates that a steady build up of canopy volume as the tree grows by utilising as much as possible of suitable branch growth each year is preferable to forcing excessive tree growth with the objective of filling the canopy rapidly. Well grown nursery trees planted early into well-prepared ground, will generally make sufficient extension growth to achieve adequate canopy development as well as commence cropping in their second growing season.
It has also been our experience that ongoing tree vigour management in intensive orchards has been much easier when laterals coming off the leader at planting time have been retained and cropped in the following year. The trees have not parked up, and the second year crop load has moderated tree vigour minimising problems of excessive shoot vigour which often occur when there is no crop in the second year, as well as reducing biennial bearing risk. Crop load is the best vigour control management tool. Where trees are growing well, a fruiting lateral will not develop sufficient bud sites for vigour control until it comes into its third growing season.
This means that any fruiting lateral needs room for two seasons of annual extension growth. Where row-spacings are close, annual lateral shoot growth may be too strong for the spacing. In this situation the strongest laterals should be eliminated, leaving the calmer shoots to become fruiting laterals. Where the remaining laterals are too long for the planting distance, delay pruning back the shoots until two to three weeks after terminal bud break. These laterals need to be shortened by a quarter to a third of total length. Post bud break heading will have a lower vigour response than dormant pruning. Also, there will be less bud break inhibition so more, but weaker side lateral shoots terminating in fruit buds will grow, and more fruitful buds will grow behind them.
This minimises bare wood and markedly increases fruitful bud load so the excess vigour problem can be brought under control. In older trees with excess vigour problems, remove high vigour branches, while retaining weaker, less vigorous, more fruitful branches. In tree canopies there is a close relationship between branch diameter and its length. Once branch diameter, close to the leader or main branch exceeds about 2.5cm per metre of fruiting wood, annual shoot vigour tends to become too strong. Avoid shortening strong branches because this usually stimulates even more strong annual shoot growth. Strong branches are often shortened to allow picker and machinery access. Where this has been done, try and remove the entire branch during the winter pruning.
Which branches to remove?
The pruning objective should be to remove the least fruitful branches irrespective of where they occur in the canopy. As trees age, it is normal to remove strong, lower canopy branches and allow weaker, more fruitful pendant branches, higher in the canopy to fall into their place. Higher in the tree, take out any large, vigorous shading branches. Where canopies have become too dense, up to a quarter to a third of the total branch number can be pruned out in any one year without upsetting tree vigour balance, providing the remaining branches are pretty much left unpruned, apart from removing any strong upright shoots and very weak, shaded wood underneath the branch. In spindle bush, centre leader and double leader tree forms, side branches need to become progressively weaker towards the top of the tree to allow good light penetration into the lower tree.
A well-balanced tree in an intensive orchard only requires 18–20 fruiting branches, plus a few moderate annual shoots to become future replacement branches. Another useful recommendation is 7–8 branches per running metre of stem. Where replacement shoots are required, particularly in the lower tree, leave a two or three bud stub to guarantee regrowth. Flush cut where regrowth is not needed.
Where trees have excessive vigour there is a tendency for trees to become too tall. Optimum tree height is around a 1:1 ratio between tree row spacing and tree height for three-dimensional tree forms. There is some evidence emerging to indicate that two dimensional canopies can be a little higher than the 1:1 ratio, however, I suspect that in the future, health and safety laws may limit tree height, so we need to begin thinking about cunning ways to limit tree height while still retaining optimum canopy vigour balance. Where it is necessary to lower tree height, topping the leaders should be delayed until after fruit set to reduce the vigour response.
Low vigour problems
Low tree vigour usually indicates a site limiting factor usually associated with soil conditions, soil drainage, water stress at critical times in the growing season, soil compaction or in some situations a nutrient deficiency. High temperature stress can also affect tree vigour, especially if soil temperatures are too high where trees are planted on shallow rooted dwarfing rootstocks, such as Malling 9 (M9). Wind and weed competition will also weaken tree vigour. Weed competition in the spring/early summer period is particularly limiting for young
tree growth. The first step in overcoming a low vigour problem is to identify the growth limiting factors. Often there are more than one of these involved. Specific replant disease (SRD) is often overlooked and is a common cause of poor tree growth in replant sites. Once identified, these limiting factors to growth need to be rectified. Hard pruning will stimulate a tree vigour response, but unless the factors responsible for the low vigour are also addressed the tree vigour will tend to stall again once canopy volume reaches the size that is was before hard pruning the tree.
Pruning part of solution
Although pruning alone will not overcome a low tree vigour problem it can certainly be part of the solution. Trees with poor vigour usually set up large numbers of floral buds including terminal flower buds on most one-year old shoots. Flowering is very stressful and imposes heavy demand on the tree growth resources. In fact, it possibly has a greater dwarfing effect on the tree than that of the fruit load after initial fruitlet shedding. Pruning, or rubbing out surplus fruit buds along with limiting the number of shoots in the tree which directs the growth into fewer growing points will improve the vigour. The remaining shoots should be lightly tipped back to a leaf bud to remove the terminal fruit bud.
Try to retain as much canopy volume as possible and put the focus on thinning out shoots and floral buds, rather than harsh pruning cuts back into the remaining branches. Harsh pruning to stimulate new vigorous annual shoot growth will result in higher shoot vigour problems, but because canopy volume and light interception is markedly reduced, yield potential is also lowered. This will set back cropping for several years, while fruitful wood is being replaced.
Biennial bearing is an entrenched problem in many older orchards. It can also become a problem in younger canopies too if initial crop loads are not carefully managed. Experience shows that recently grafted trees can be very prone to developing biennial bearing. This is largely because a two or three-year-old graft contains a larger proportion of similar aged wood which is all going to crop in the same year triggering a strong biennial bearing response. Controlling the bud load in young grafts by selective pruning out surplus shoot growth to stimulate a mixed age of wood in the canopy will reduce the biennial risk. Where a biennial cropping pattern has become entrenched, adopting an alternative year pruning program will help deal with the problem, as well as significantly reduce your pruning costs. Prune biennial cropping trees in the winter prior to their “on” crop and leave them more or less unpruned in the winter before the “off” crop. The “on” crop year pruning should focus on a branch thinning programme taking out any strong large shading branches in the canopy so as to minimise the need for “off” crop year pruning. The “on” crop year will have surplus flower load even after branch removal, so additional pruning off of fruit bud spur clusters will be needed to bring floral bud numbers down to manageable levels. Coming into the “off” crop year, flowering is usually light and weak. For this crop as many flowers as possible will be needed so any pruning will need to be light and focuses on removing problem shoot growth such as vigorous uprights. Incidentally, some years ago East Malling Research Station in England investigated pruning every second year versus annual pruning. In their trials, which were conducted on mature trees, pruning every second year resulted in similar crops to those in the trees pruned annually, however pruning costs were halved.
Root pruning field demonstration (Batlow, NSW) – Kevin Dodds, NSW DPI
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