Progress report on
The best current knowledge about training of young walnut trees is described in David Murdoch's excellent chapter in the New Zealand Walnut Growers' Manual. Instructions on the modified centre leader system and a minimum pruning option are included in the chapter. The NZWIG research sub-committee aims to build on this knowledge by comparing different approaches to walnut tree training. Our experiment includes hard, medium and light training options (these are defined below), and we want to measure how much each of these impacts on the growth and form of young walnut trees, for trees on fertile and less fertile sites. One treatment also has a high level of boron fertiliser added, to see if this affects form (it is used to increase apical dominance in pine trees on some sites).
Sites and trees
Blocks of young walnut trees were dedicated to the experiment at each of three Canterbury orchards. All the trees are cultivar Rex. The experiment began in June 2004. At two orchards, the trees were 2-years-old at the start of the experiment (i.e., had been planted in spring 2002), and at the third, the trees were 3-years-old (i.e., had been planted in spring 2001).
Progress to date
Progress to date
In this article we are reporting on data from the first two seasons. We have made some interesting observations, but will continue to collect data to check our findings, and observe the trees as they get older.
The training treatments
Hard training: Remove all branches. Head the leader back to round wood - this usually means cutting below the closely spaced nodes at the top of the early-season growth.
Medium training: Remove branches if they exceed half the diameter of the trunk or are directly competing with the leader. Head the leader lightly, removing highly fluted material but staying in the late-season growth above the closely-spaced nodes.
Medium training with elevated boron: Training as above. Boron added as ulexite in September and as a foliar spray in October.
Light training: Remove branches if they exceed two-thirds the diameter of the trunk or are directly competing with the leader. Do not head the leader (but nutlets are removed from terminal bud in spring).
As the trees get older, we have also been selecting and retaining the first few permanent framework branches. We will not go into the details of their selection in this article. However in terms of the training treatments, all rules above that apply to leaders also apply to permanent branches.
In young walnut trees (pre-cropping) we are aiming for:" Good growth, in terms of both tree height and trunk diameter " Good form, i.e., trunk straightness for strength and so that well-spaced permanent branches can be easily selected.
Thus, for the experiment, we are measuring:" Trunk diameter at a height of 600mm from the ground " Total tree height, to highest point " Length of chosen leader (past season's growth only) - this is not necessarily the highest shoot " Total shoot extension (i.e., summed length of all shoots that grew in past season) " Estimated angle of leader
We also photograph every tree before and after pruning, and write a prose description noting things like broken branches, whether the highest shoot was chosen as the leader, or whether the top shoots had poor angle and were therefore not chosen.
Results and discussion
Hard training promotes vigorous shoots which offer good options for a strong, straight leader. In comparison, the terminal bud seldom grows in a lightly trained tree; rather, another shoot grows from below and overtakes last year's terminal (figure 1). Lightly trained trees often have poor form, and to keep the tree straight, the pruner may have to choose a short but straight shoot as leader rather than a longer but angled shoot.
However, our measurements showed that overall tree height and diameter are no greater with hard training than light training. Up to about year 3 or 4, the vigorous shoots in the hard training option make up for the greater length that has been cut off, bringing the total tree height to, on average, the same as for the light training option. This is illustrated in figure 1 and the data is given in table 1.
The medium training option usually (though not always) produces a usable leader, and also results in the same tree size as the other two options up to about year 4.
As the trees get older (past 3 to 4 years), our measurements show reduced growth from the hard training option. While hard-trained trees still have long vigorous shoots (and therefore better form), the tree height and trunk diameter are less than with the light training option (figure 2). We have given some thought to the reason for this.
In the first two to three years, the amount of wood cut off, even with the hard training option, is not great. The total shoot length cut off is usually less than the height of the tree (see table 1). However as the tree gets older, the amount of wood cut off in the hard pruning option increases, since all the branches (except permanent framework branches) are removed. At 4-years-old, the total shoot length cut off is usually well over twice the height of the tree for the hard training option. Even with medium training, total shoot length cut off can be 1 to 2 times the height of the tree. A tree's stored energy is in its branches, trunk and roots. If you cut off a large volume of branch material, you are removing this energy, and we hypothesise that this is why we see reduced growth with the hard training option for older trees.
We have observed more branch breakages and twisting when hard training is applied to older trees, as compared to light or medium training. Hard training results in long, vigorous shoots (particularly long and vigorous in older trees where a large shoot volume has been cut off) and these are more prone to twisting and damage in the wind, compared to the shorter length of current-season's growth in the light training option.
We have not found any observable effect of added boron on tree growth or form.
Figure 1: Typical tree response to previous winter's training cuts, as observed at end of growing season. In the early years, light and hard training produce on average the same sized tree (height and diameter) but tree form is better with hard training, providing better options for leader selection.
Figure 2: Typical tree response to last winter's training cuts, as observed at end of growing season. From about year 4 onward, our measurements show that hard training (removal of all branches) reduces overall tree growth (height and diameter) though long, vigorous shoots with good form are produced.
Though we are only part-way through our experiment, we can draw some tentative conclusions and recommendations from our observations to date.
We applied training treatments in July 2006 and will re-measure the trees in winter 2007 and analyse the data. We will then assess whether to continue the trial. In the July 2006 treatments at the Caldwell orchard (5-year-old trees) the hard training treatment has been replaced with a modified medium training treatment. In this treatment, branch removal follows the medium training rules but the leader is headed back hard. Hopefully this will result in straight vigorous shoots at the top, but without the growth reduction associated with removal of large volumes of branch material.