The 2011 walnut cultivar trial
Heather North & Anna Brenmuhl, NZWIG research committee, October 2015
Introduction – NZWIG cultivar trials
At present, most commercial walnut orchards in New Zealand are dominated by just two cultivars – Rex and Meyric – with other cultivars in only small numbers. Rex and Meyric were recommendations from the NZ Tree Crops Association selection trials at Lincoln University, which were planted in the mid 1980s and started producing results in the mid 1990s. NZWIG would like to see continuing progress in this area, with the goal being a wider range of good cultivar options available to walnut growers.
So in 2005, the first of our two cultivar trials was planted. It contains Serr (a USA cultivar imported in the 1970s); selections from Vernon Harrison, Jenny and Malcolm Lawrence, and Diana Loader; and Rex and Meyric for comparison. The research committee is currently involved in detailed assessments of walnut samples from this trial (looking at factors such as weight of nuts, visual attractiveness, shell seal and crack-out percentage). We also have multi-year data on tree growth, timing of budburst and flowering, and yield. In the near future we will be able to publish a report with initial recommendations on which of the cultivars are the most promising (our progress report from several years ago is listed below as North (2012)).
Then in 2011, a second cultivar trial was planted, and this is the one we are reporting on here. Because the trees are still young, we only have growth data so far. This trial contains three cultivars imported from Tasmania in 2009: Lara (originally from France), and Tulare and Howard (originally from USA). Information about how these cultivars were chosen for import can be found on the web page listed below (North, 2009). The trial also contains four further New Zealand selections and Rex and Meyric for comparison.
Design of the 2011 cultivar trial
There are three trial sites – one in North Otago and two in central Canterbury (Aylesbury and Charing Cross). We had been hoping to also include a North Island site but were unable to find one.
The nine varieties being trialled have been given code numbers so they can be assessed without bias: the code numbers are 220 to 228. Jenny Lawrence is holding the codes and name correspondences and, once we are ready to make recommendations from the trial, we will ‘crack the code’ and report them by name.
At the largest of the sites (Aylesbury) there are five replicates of each of the nine varieties being trialled (i.e. 45 trees in all). At the North Otago site there are five replicates of most varieties, but no trees of 228. At the smallest of the sites (Charing Cross) there are four replicates of most varieties but no trees of 226. Most trees were planted in 2011 but a few were added in 2012. In particular these were of varieties 226, 228 and 220 for which we had poorer grafting wood available and lower grafting success.
The varieties are randomly located within the sites, using randomised block layouts. They are labelled by their code number.
Progress so far
Each winter, from 2012 to 2015 inclusive, we have measured the trunk diameters of all the trial trees. Trunk diameter (measured at a height of 600mm) is our standard method of tracking tree growth. Table 1 shows the average trunk diameters in 2015 (averaged for each variety) after four years of growth. Figure 1 shows the same data in graphical form.
Table 1: Average trunk diameter (at 600mm height) in 2015 (mm) for each variety on each trial site. The cells shaded blue indicate those varieties where all, or a significant number, of the trees were planted a year later (in 2012). In addition to the shaded cells at the North Otago site, a number of other individual trees were planted or transplanted late. The varieties have been ordered from largest to smallest using the trunk diameters at the Aylesbury site
Figure 1: 2015 trunk diameter data from Table 1 in graphical form, ordered from largest to smallest by the trunk diameter at the Aylesbury site. Those varieties planted later (in 2012) are highlighted in blue in Table 1.
There are differences in tree growth between sites but this is not our main interest. What we wish to find out is which varieties perform the best and the worst within each site. There have been a few tree deaths at each site. In Table 2 we show the number of trees of each variety remaining alive at each site (out of the number planted).
Table 2: Number of trees remaining alive at each trial site out of number planted
In coming years we will continue to collect growth data and, as the trees get older, we will also collect information on budburst and flowering times, yield and walnut characteristics.
NZWIG research committee thanks Darrell Johnston of River Terrace Nurseries for propagating the trees in the cultivar trial, and the trial growers – Russell Hurst, Hugh & Jill Stevenson and Anna Brenmuhl – for making the sites available.
North H, 2012. Report on NZWIG trial of New Zealand walnut selections. Health in a Shell 79: 2–17.
North H, 2009. Choosing the cultivars to import for trial.