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New Zealand Redwood Lumber

Pruning the lower stem
Pruning the lower stem

Pruning the lower stem

December 2011 – Simon Rapley, NZRC General Manager –

Several California native tree species are grown commercially in New Zealand and between them make up around 99% (by area) of our production forests (there is virtually no harvest of NZ native trees now). These California natives include Monterey or radiata pine (89% by area), Douglas-fir (6%), with Monterey cypress and coast redwood making up the remainder. We estimate there are 16,000 acres of coast redwood in NZ and interest is growing fast.

If readers of the Jill Poke know anything about redwood in NZ, it is that they can grow quickly. In fact all of the species listed above grow faster in NZ than in the US, and that is the main reason we grow them. Fast tree growth leads to widely spaced growth rings in the wood and therefore we know that the final product will differ in appearance from that produced in California.

What does this mean?

This question can be answered firstly by looking at other examples of NZ lumber. NZ has been exporting Radiata pine appearance lumber to the United States of America for many years now in direct competition to US grown Ponderosa pine and sugar pine. When this fast grown Radiata was first tested in the US markets, I am informed that the price was set around 20% lower than that for Ponderosa and sugar pine, largely because of the different appearance. Over time, this has changed and in US lumber price publications, we see NZ Radiata selling at or above US pine prices. There are a number of reasons for this but primarily, consumers are choosing a product that is fit for its intended purpose. Reliability of supply is also important and there is anecdotal evidence that some consumers will buy imported lumber if they think fewer trees will be cut down in their home country.

Is fast grown NZ redwood going to be fit for its intended purpose?

Valuable clear and sound knot grades from pruned NZ redwood

Valuable clear and sound knot grades from pruned NZ redwood

Wood quality can be measured in a number of ways. The most widely used measures are:

  1. Basic density or specific gravity. This is a guide to the strength of the wood.
  2. Dimensional stability. Described by rates of shrinkage during processing. Does the wood warp or split while being processed or in use?
  3. Durability. How long the wood will last in use.
  4. General Appearance. Mainly the colour of the wood and ring width. Consumers associate quality with appearance.
  5. Presence of knots. A strong visual indicator of quality.

Numerous samples of NZ redwood have been tested and we know the following:

The basic density of NZ redwood is comparable to that of young growth CA redwood. A recent study of a 38 year old NZ stand of redwood showed average density to be 8% lower than CA young growth redwood. NZRC is only propagating varieties where the density of the wood is known to be above a predetermined threshold. Dimensional stability is actually superior to that of CA grown redwood. Stability is important for uses such as joinery and for siding and decking where the wood is exposed to the weather.

Basic density (kg/m3) Radial shrinkage (%) Tangential shrinkage (%) Heartwood Durability Class
CA Old Growth redwood 400 2.6 4.4 Resistant or very resistant
CA Young growth redwood 350 2.2 4.9 Low or moderately resistant
NZ 38 yr old redwood 323 1.3 3.1 Moderately durable (class III)

Durability of redwood is a result of the tree storing extractives (chemical compounds) in the heartwood and is known to vary with the age of the trees. Heartwood formed when the tree is young has a lower durability but rapidly improves from around age ten onward. The study of the 38 year old trees showed moderate to very durable heartwood formed in the trees from age 10 onward.

The colour of redwood varies wherever it grows and redwood trees produce darker heartwood as they get older. This is obvious when comparing old growth redwood to young growth. The market seems to prefer darker heartwood as there is a perception of superior quality. NZRC is only cultivating varieties known to have darker-than-average heartwood.

In a market survey by Kent and Associates, CA, (jointly commissioned by NZRC) respondents considered that NZ redwood with three growth rings to the inch or more would be accepted into the CA market.

NZ heart redwood siding

NZ heart redwood siding

The presence of knots in many older NZ redwood has been problematic. Branches are larger than young growth redwood stands in CA and therefore do not self-prune causing severe downgrade in the lower logs due to the presence of black or loose knots. This problem becomes an opportunity as most NZ redwood growers are pruning their trees to remove lower branches before they die and downgrade the lumber. Lower logs will yield valuable ‘clear’ lumber instead of ‘merch’ grades which sell at a significant discount. Upper logs will yield mostly lumber with sound knots (‘construction grade’) which are popular and sell for high prices. The ability to prune in NZ will lead to the production of very high quality logs in terms of knots.

The acid test for the demand of any product in a market is to place the product in the market and see how it sells. NZRC has arranged the sale of two separate consignments of NZ redwood into CA. The lumber was noted as being a little different in appearance but was noted as being a quality product. The only negative comments were of the number and size of black knots. We can remove this problem by pruning.

NZ redwood is now sought after in NZ by a number of sawmillers and end users. Just as in CA when young growth redwood was trickled into a market dominated by old growth lumber, there was a disappointment with NZ redwood on account of the growth ring width and colour relative to old growth. NZ redwood is now recognised as a naturally durable, stable and attractive wood and this is reflected in prices at least as high as those in CA for young growth redwood.

In summary, NZ redwood lumber scores a little lower than CA young growth in some appearance attributes such as ring width. However it scores higher in dimensional stability and presence of knots (pruned trees).

For attributes such as colour, density and durability, we can make continual improvement through selection of superior trees we choose to propagate.

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