Blackwood – the start of a learning curve

by Ian Brown

For over 30 years I have been on a learning curve for blackwood. I planted my first blackwoods in 1980 on a small abandoned farm property I had bought in the far north of New Zealand. It was a long way from home, but it was cheap, and the nearby coast was appealing for family holidays. It was located in an elevated valley, sheltered from prevailing winds, an adjacent range provided good rainfall, and had clay loam soils based on ancient volcanics. In response to a recent study carried out by Ian Nicholas at Forest Research, who had been working on a project to select a limited number of species to supplement our monoculture of radiata pine, my brother and I decided to trial some alternative species, including blackwood.

Blackwoods have been planted in New Zealand from the 19th century. It had been generally assumed that to produce good form they should be interplanted with another species, and many had been planted in native bush, or in mixtures with eucalypts or pines. There had been little interest in pruning blackwood for form correction.

1980 trial planting.

We planted 500 blackwoods in groups of 4 trees, 8 metres between groups, and interplanted with pines. When we visited the site in the first summer the blackwoods were growing strongly, and were about chest height, but some of them were developing double leaders and competing branches. It seemed logical to trim these back, leaving a single leading shoot. Without a clear agenda, we set to with secateurs. However competing demands for time (fishing) meant we did not finish the job. In the following summer, two things were apparent: the trees we had pruned were much better in form than the unpruned trees, and the pruning had not affected their growth rate. And the blackwoods were growing faster than the pines, which were clearly having no influence on them. Over each of the following summers we continued with form pruning, directing attention to competing shoots near the top of the trees, and from about year 4 followed through with clearwood pruning from the base. When the trees were above 6 metres we thinned them to one per group.

The pines lagged behind, and made no contribution until about year 4, and by year 6 were suppressing the blackwoods. We then felled the pines. This left a thick layer of slash, which made access difficult for further silvicultural work. The pines had clearly been more trouble than they were worth.

I have a trial plot on the site, and in 2010 at age 30 the mean diameter was 55 cm. It should be close to 60 cm by 35 years, when I hope to mill them.

The message we got from this block was that on a good site, blackwoods respond well to annual form pruning, without the need for a nurse. So we decided to try something novel.

1982. Open grown planting.

In 1982 we planted blackwoods, again in groups at 7 to 8 metre spacing between groups, in the open, and undertook annual form pruning, but without a nurse species. These are probably the first blackwoods to have had this form of intensive annual treatment. Of the systems we have tried, it proved to be the simplest and most effective, and with some modifications it has been the method I have used since then.

At the same time we planted blackwoods in holes cut in an area of regenerating native scrub. This worked well, but the trees still needed an annual visit for light form pruning and remove overhanging branches. I tended to get lost when locating the trees, and spent some time wandering about in circles. This might have been avoided by cutting lines in the scrub.

1983. Blackwoods and eucalypts.

   In 1983 we returned to orthodox management, and interplanted blackwoods in a mixture with with E. saligna. This worked well for the first few years, and we started to clearfell the eucalypts at about year 5. Half way through the program we encountered a problem familiar to growers who have tried this regime: we were seduced by the eucs, which were growing strongly, and looked too good to fell. So we kept them, in the hope of eventually milling both species. It didn’t work out. The blackwoods became badly suppressed, and the eucalypts thrived. Where we had thinned the eucs, the blackwoods grew well.

The message here was that nurse species provide some benefit for a limited period, but although improving form, they do not eliminate the need for form pruning. They add costs and complicate the management, and to avoid suppression have to be sacrificed on time.

In subsequent planting in the Waikato I have relied on form pruning on open-grown trees, planted in groups of 3 or 4 at final spacing. This has worked well, and has done so on other plantations in NZ that I have looked at, provided one essential condition is met: to grow blackwoods in the open and without a nurse you must have a good site, one that will encourage rapid growth. This means warmth, shelter, adequate moisture, and decent soils. On sites that are cold, dry or exposed, it is very difficult to control form in open grown trees. In those conditions you might get away with it by using a nurse crop. Or it might be better to simply plant another species.

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Photos are from the Northland planting, taken in April 2010. The top photo is from the 1980 planting. The scrub is natural regeneration. The tree marked 7 (below) is from the 1982 planting, open planted and annually form pruned DBH 70 cm at age 28. Naturally it is one of the bigger trees.

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Thanks Ian for a great contribution. I hope it generates some discussion amongst readers.

4 responses to “Blackwood – the start of a learning curve

  1. Having been to NZ and seen Ians’ excellent blackwoods, plus numerous other sites, I can agree with what Ian says regarding site requirements. Of particular note are the following comments from Ian.

    “The message here was that nurse species provide some benefit for a limited period, but although improving form, they do not eliminate the need for form pruning. They add costs and complicate the management, and to avoid suppression have to be sacrificed on time.”

    “On sites that are cold, dry or exposed, it is very difficult to control form in open grown trees. In those conditions you might get away with it by using a nurse crop. Or it might be better to simply plant another species.”

    Private Forests Tasmania have a demo site near Sheffield in northern Tassie. Although a poor quality site for growing blackwood, it clearly shows the positive benefits of a nurse crop on harsh sites. I wouldn’t recommend that anyone grow blackwood on such a site, other than along streams for a combination of environmental protection and limited long term timber production. However, there are numerous sites throughout Tassie capable of good blackwood growth that would certainly benefit from shelter to enhance the production of a straight stem for 5-6m.
    If I were to plant blackwood again I would do the same as Ian and plant in groups of 4 with 7m b/w groups (200sph). However, eucs would be planted as a nurse crop at a stocking of 200sph, on the same 7m grid as the blackwood but offset. The result is that each group of blackwood are surrounded by 4 eucs. I agree 100% with Ian, it does add cost and complications, but I’m convinced that all but the most sheltered sites would benefit while the extra cost and management is worth it.

    My experience with pruning the blackwood at PFTs demo site is that height growth is enhanced, branch development is reduced, form pruning easier and more effective while the knotty core is reduced in size when taller eucs influence the blackwood growth habit. At the low stocking I’m suggesting the eucs start to influence blackwood form from age 3-4 and by age 10 are in need of removal (by which time pruning and thinning of the blackwood should be completed). Removal can be felling or stem injection with roundup. If a grower manages the form pruning and thinning of the blackwood as required I don’t believe a nurse crop adds too much extra work (unpruned eucs at 200 sph aren’t worth hanging onto for any future milling). I remember inspecting blackwood trails in NZ, one site had a plot of blackwood that were planted with eucs as a nurse crop (and removed on time) but unfortunately there was no data supplied for that particular plot. Another participant commented that the blackwood looked to be better trees than those grown without a nurse crop.

    Claude Road Trial 1

    Claude Road Trial 2

    I’m somewhat frustrated as I had applied for funding for trails of regimes as I’ve outlined above back in 2003. Unfortunately such trials have never been done. I was successful with my application and federal money was to be supplied, landowners and high quality sites lined up and ready to go. It only fell over at the last moment due to a particular organisation withdrawing a cash commitment at the last moment. A real shame as such trials would now be 10 years of age and we would have answers to questions such as what is the cost of a nurse crop versus the positives. Alas, the Kiwis are still way ahead of us!

    I know some would disagree with me but until a few trials are set up we will never know. By all means, if you have a very sheltered site then open grown blackwood will produce the goods if pruning and thinning is done on time. However, there are significant areas where additional shelter in the first 10 years will greatly enhance the area that could be successfully planted with blackwood.

    • If the initial planting is groups of 4 at 7m spacing that’s 14 x 14 = 196 groups = ~800 trees/hectare. Was the 200 sph the final stocking after thinning?

  2. I tend to agree with Ian Brown’s comment about cold and dry sites – try another species, not blackwood.

    As for exposed sites, I have seen enough examples to be confident in saying that shelterbelts work very well, where all the other factors are in blackwoods favour (eg. rainfall, soils, etc). The Carrabin plantation at Paradise (just around the corner from the above PFT trial at Claude Road) is a great example. This virtual ridge-top planting is protected on the western side by a cypress hedge, and the blackwoods are doing very well. In fact some of the biggest blackwoods are right up against the hedge. No need for a nurse crop at all, just a single-line planting of cypress on the windward boundary.

    This has enabled the owner to focus all his considerable efforts on managing the blackwoods without the distractions and complications of a nurse crop.

    There are lots of ridge-top sites like this in north west Tasmania that can grow commercial blackwood with the help of a simple shelterbelt.

  3. 200sph is final stocking after thinning. Cypress grows too slow if planted at the same time as the blackwood, fine if already well established prior to planting. Need something that is fast growing to provide shelter if planted at the same time.

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