Where blackwoods go wrong
Ian Brown
New Zealand Tree
Grower February 2005
The
photograph at the bottom of the page is of a blackwood which we planted
in 1980 in the Far North. At 24 years, its diameter is over 50 cm, and
it is nine metres to the first branch. It is of course one of our best
trees – no one shows off their failures. However, although clearly
selected it does indicate the potential for blackwood growth. On an
adjacent trial plot the mean top DBH at the same age is just over 50
cm, with a trajectory that suggests this will reach 60 cm before 35
years.
Good in patches
Our forest, in which we planted over
8,000 blackwoods, is a typical farm forester’s curate’s egg – good in
patches, not so good in others. This may not be a bad thing. The
familiar mix of good and bad provides those of us who are amateurs
in a field dominated by professionals with a special advantage – by
making more mistakes we get on to a fast learning curve. In my case the
mistakes have been mainly errors of omission. Because of the distance
from home, which meant infrequent visits, and various distractions when
we got there, the work never got quite finished. As a learning
experience this was perhaps fortunate in that it provided a comparison
of treated and untreated trees. It was in effect a farm forester’s
version of a controlled trial, in which the number of the control, or
untreated trees was determined by the demands of the family, and by the
quality of fishing nearby.
After looking critically at these trees
over time, and at the efforts of other blackwood growers, it is clear
to me that we all make the same mistakes. It is not difficult to grow
good blackwoods, but there are a few fundamental areas that need
attention if we are to grow them successfully.
Survival and performance
Blackwoods
will grow almost anywhere in New Zealand. In Australia, where they mix
with some pretty tough competitor species, they have a wide natural
range. This has led growers at times to plant them in inhospitable
sites, where they have conspicuously failed to thrive.
If we are to
make a profit from these trees we should make a clear distinction
between survival and performance. In blackwood, more than most of the
trees we grow, performance is greatly influenced by site factors.
For
a species with a reputation for toughness, blackwoods are surprisingly
demanding. On dry exposed locations they are generally slow growing,
with a short trunk and disorderly branch architecture. They do not
tolerate heavy frosts. They grow best in warm areas. Of particular note
is that, although they respond well to moisture, they do not
tolerate waterlogged soils. This may surprise some growers who
know that the Blackwood Swamps of north west Tasmania produce most of
the commercial crop.
The Blackwood Swamps are in fact river
floodplains, and are dry underfoot for much of the year. Blackwoods are
dominant there as they will tolerate occasional flooding, provided the
water is flowing and oxygenated. This is quite different from our
version of a swamp, in which the water is stagnant and never dries out.
When planted in such conditions blackwoods are
unlikely to survive.
Blackwoods
like decent, well-drained soils, need good shelter, and respond to
warmth and plenty of moisture. Ideal sites are gullies and lower valley
slopes. They are therefore best suited to mixed plantings where more
exposed sites are occupied by other species.
To get the best from them blackwoods should be regarded as highly site
selective.
Pruning
The
worst thing that can be done with blackwoods is to neglect to prune
them, and simply plant and hope for the best. The inevitable
outcome is the blackwood we all know, with a bole too short to be
useful, and massive branches. Almost as bad is to leave them a few
years, and then call in a commercial gang. They will prune them like
radiata, and convert a shambles into a disaster.
Programmed to frustrate tree growers
Blackwoods
are genetically programmed to frustrate tree growers. As they grow they
periodically abort their leading shoot tip, and later replace it with
lower branches, which then tend to form double or multiple leaders.
This may suit the biological interests of the tree, but not the
commercial interest of the grower. Although most conspicuous in open
grown trees, malformation also occurs when blackwoods are grown in
mixtures. Growing blackwood in mixtures, although theoretically
appealing, is not as easy as it might seem in practice.
Malformation in blackwood is something we have to confront, and I do
not believe we will solve the problem by tree selection.
Learning a few tricks
This
means that we have to use a system of form pruning aimed at producing a
straight stem of about five or six metres in length.
The basic
concept is easy – it is simply a matter of selecting and removing
competing leaders. There are a few tricks which can be quickly learnt.
The
branches to be removed can be selected either visually, or by
measurement using a gauge. Form pruning can be done quickly, and with
little effort. The first three metres require no more than a few visits
with a pair of secateurs. This is not difficult to learn for growers
with no forestry experience. However the method is unfamiliar to
growers who have spent some time pruning radiata pine.
The best results
To get the best results from form pruning there are a few basic rules.
- Start pruning early, preferably no later than the second summer
- Form prune once a year – the best time is probably late spring
- Aim for a straight stem of five or six metres, but a shorter stem
of good diameter will be very acceptable.
- Follow through with clearwood pruning, which usually starts at
about year four.
- Remove any branches over three centimetres.
The
key message is that unless the grower is able and prepared to carry out
regular form pruning, it is a waste of time to plant blackwoods.
Thinning
Farm
foresters generally make a good job of establishing and tending their
trees. The price we pay is that, in doing so, we become too fond of
them, and are then reluctant to thin on time. For some species a delay
in thinning may not matter too much. With blackwood it certainly does
matter, and the effect is dramatic.
A common mistake
When
blackwood crowns come into contact, the live crown can retreat with
disconcerting speed, often within a year or two, resulting in a small
high crown. The effect of this is a marked slowing in the rate of
diameter growth, and a much longer rotation. This is one of the most
common mistakes in growing blackwood. Good examples can be seen on many
woodlots, and wherever blackwoods have been planted by city or regional
councils. On reasonable sites, thinning should start when the crowns
begin to compete at about year six, and be complete before year 10,
leaving about 200 stems per hectare.
Reluctance to thin is
understandable. It means sacrificing some very good trees, and the
remaining trees, which are about seven or eight metres apart, can look
very lonely. But it has to be done. If you are squeamish about
thinning, employ a steely-eyed contractor to do it, take a holiday, and
come back when it is all over. Blackwood millers like fat trees. To
grow them fat they must be thinned on time.
Great potential
In
summary, when growing blackwood and having selected a suitable site,
the first principle is to prune on time, and the second is to thin on
time. There is of course more to it. Over the last 20 years a lot of
good information has emerged, mainly as a result of excellent research
trials carried out by Ian Nicholas and Ham Gifford of Forest Research,
together with much anecdotal experience by farm foresters.
We now
have enough knowledge to consistently grow high quality blackwood
trees, and the
AMIGO
group is happy to share it. There is no need to
repeat the mistakes of the past. As a species that will deliver high
quality decorative timber for a niche market place, blackwood has great
potential.
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