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Research on redwood wood properties is progressing

Patrick Milne, New Zealand Tree Grower August 2013.

Coast redwood has had a chequered history in New Zealand having been widely planted before 1940. However, most of these plantings failed for one reason or another. There are many successful remnant plantings of which the Redwood Grove in Rotorua is one. Some of these remnants have been important in demonstrating the potential of the species in New Zealand and rekindling enthusiasm for it.

In the United States the species is renowned for producing a durable, stable and versatile timber. New Zealand grown redwood has been the subject of detailed wood property tests by Future Forests Research and the Forest Research Institute. This article provides a brief update on information from this research as well as the current classification of redwood timber properties.

Dimensional stability and density

All indications are that New Zealand redwood is a very stable timber. Several studies have found New Zealand grown timber with low shrinkage and good dimensional stability.

The average basic wood density from redwood sampled from 107 trees from throughout New Zealand is 336 kilograms per cubic metre. The research indicates that wood density is relatively uniform within the stem. All these wood properties are similar to that reported for Californian young- growth redwood.

Durability

Redwood meets the durability requirements of the standard NZS 3602:2003 for weatherboards and many local councils will accept it for this use. The Australian Standard AS 5604−2005 categorises redwood natural durability as class 2, suggesting a probable life expectancy of 15 to 25 years in ground contact and 15 to 40 years above the ground. New Zealand grown heart redwood is therefore more than adequate for the expected uses of redwood including weatherboards and decking.

Genetics is likely to be a factor in redwood durability but the degree of influence is unknown. Current research has indicated that it is probable we can improve durability by clonal selections. However, genetics may play a lesser role than site or silviculture. A current Future Forests Research study investigating the effects of genetics and of siting has indicated that whatever the origin of the provenance, the more productive the site is. The faster the trees grow, the less dense the wood becomes. Research on the effects of genetics and site on wood properties are progressing.

Redwood durability for most clones and seedlots will improve with age. Redwood logs will be more valuable if grown on a longer rotation. Older trees usually have higher heartwood content.

What does this mean?

New Zealand grown heart redwood is more than adequate for the expected uses of redwood including weatherboards and decking. We can see that redwood resistance to fungal attack improves with age and that redwood logs will be more valuable if grown on a longer rotation. Older trees also have a higher heartwood content.

The best way we can improve the performance of heart redwood is to segregate the juvenile core at processing and use it for purposes which are not subject to the weather. We can improve durability by screening clonal selections. Growers can also improve the quality of their redwood stands by growing them for at least 35 years and preferably longer.

Patrick Milne is the Diverse SpeciesTheme Leader for Future Forest Research Ltd. Contributors also include Dean Meason from Scion, Paul Silcock of NZ Forestry and Simon Rapley of NZ Redwood Company.


One post

Post from Dean Satchell on September 22, 2018 at 7:32pm

There are some inaccurate statements made in this article.

Firstly, because NZS 3602 is cited in clause B2 of the building code as an Acceptable Solution, local councils MUST accept redwood heartwood for weatherboards.

Secondly, redwood is classified as durability class 3 from tests in New Zealand* in accordance with the Australian standard AS 5604−2003. This is 5-15 years in the ground for 50 mm stakes and 10-20 years above the ground and exposed to weather.

Thirdly, the suitability of redwood for decking would require evidence proving a minimum 15 year durability performance in New Zealand and I'm not aware of evidence or research showing that the durability of redwood is indeed "more than adequate" for this application.

*Durability of New Zealand grown timbers (2014) Page, D, Singh, T. NZ Journal of Forestry, February 2014, Vol. 58, No. 4.

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