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 Ground durable eucalypts for vineyard posts

Paul Millen, New Zealand Tree Grower November 2006.

This article provides an update on research being undertaken in Marlborough to identify eucalypt species suitable for short rotation production of ground durable roundwood – timber which would be suitable for conversion into vineyard posts. See May 2005 Tree Grower for further background information 

Nine million posts and rising

The major and continuing investment in viticulture in the Marlborough region has lead to a rapidly expanding market for vineyard posts. Over the last 18 months new vineyard development has continued, with the total area of vineyards in Marlborough alone now totalling over 17,000 hectares. A minimum of 550 posts per hectare are used in these vineyards, giving an estimated total of over nine million vineyard posts in use. Nearly all of these are CCA treated pine posts.

The average life of a vineyard pine post is about 20 years with replacement generally due to post breakage rather than failure from loss of durability. Vineyard design can also change resulting in a full replacement of the entire vine trellis, including the posts.

The potential market for post replacement offered by the existing vineyards is already significant and new vine plantings are continuing.

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Growers seek alternatives to pine

CCA pine timber posts have always been the main post used in modern day vineyards. However some growers do not consider these posts satisfactory due to high breakage, concerns over possible arsenic leachate and problems with disposing of broken posts in environmentally acceptable ways.

Vineyard companies and private growers are open to alternatives to CCA treated pine posts. This was clearly demonstrated by the big turnout at a focus vineyard field day held at Stembridge, a Marlborough vineyard, where different posts are on trial. This focus vineyard is part of a MAF Sustainable Farming Fund project supporting the development of sustainable winegrowing.

The only timber alterative to pine which has been tried recently is imported teak posts. However, these have little heartwood and will not be durable. Various types of steel posts have been tried in a few vineyards, some reasonably successfully. Recycled plastic posts are now also available, as well as a composite steel and plastic post.

A new concrete post is also due to come on to the market, but the local rumour is that these will crack if hit during harvesting.

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How is natural timber durability classified? 

Definitions 
Natural durability is defined as the inherent resistance of a specific timber to decay and to insect attack. Natural durability classes provide the basis for rating the timber’s performance and longevity in contact with the ground when exposed to average environmental conditions.

Natural durability classes 
The classification system is based on the average life expectancy 
in years for a species.

Using these classifications to rate the timber of different tree species cannot be done with great precision because of the variability of wood properties within species and the wide variety of ground conditions in which it timber be used. Therefore the classifications are only a guide. Other factors that need to be borne in mind include –  

  • The classifications only apply to heartwood, with all sapwood having poor resistance to decay and insect attack
  • The inner core of heartwood around the pith generally has lower durability than the rest of the heartwood
  • Durability is also influenced by the size or diameter of the post, the larger the piece size, the longer it will last.
  • The age of the tree also generally influences natural durability with mature stands producing more durable timber than semi-mature trees.

Interest in eucalypt posts

Discussion with many local vineyard growers has revealed there is considerable interest in the potential to use a post produced from ground durable eucalypt timber. As well being naturally durable, and so requiring no chemical treatment, eucalypt posts are likely to be far tougher than their pine counterparts, and therefore less prone to breakage. However, as there is no local plantation resource of suitable species, a company called  Vineyard Timbers has been established and has focused on investing in species trials and research into a wide selection of ground durable eucalypt species.

Eucalypt trials – progress to date

We planted a series of formal trials in 2003, 2004 and 2005. All of these trials were established with the support of the Marlborough District Council. Locations have included sites within Marlborough Regional Forests as well as on river reserve land alongside the lower Wairau River. The Wairau flows past many vineyards and is the source of irrigation water for the vines.

While the trials at both sites are with similar species, it is the performance of trials on the Wairau River that offer the potential to evaluate which species could be used to grow short rotation round wood for posts and poles. The initial species selected for these trials included a range of eucalypt species with natural durability classified either as class 1 or class 2.

Plants for the trials were grown by Southern Woods Tree Nursery from seed largely sourced from Australia by Proseed NZ, who are also supporting the research programme.

Wairau River trials

The lower Wairau River trials were planted in October 2004. There are two separate river locations some seven kilometres apart. Soils at both sites are light sandy silt with good fertility but prone to summer drought. Prior to planting, the trials were marked out and spot sprayed. A square block design of seven by seven trees at 2.8 metre spacing was used, although for some species there were insufficient trees for a full trial.

E. quadrangulata planted in 2004 and now four metres high

Early survival and growth rates were good for most species, and this continued through their first summer. However in late April 2005 there was an out of season frost, possibly as cold as minus four degrees.

This freeze dried the leaves of many of the eucalypts in the trials. Some species survived with new growth evident in September when a survival count was taken. The 2006 winter has been one of the coldest on record. Again there have been severe frosts with some further losses but a group of species are emerging with good potential.

Promising species emerge

The most promising species identified by the trials to date include Eucalyptus bosistoana, E. quadrangulata and E. microcarpa. These are all species with class 1 durability. The species which show most potential for roundwood production will be investigated further in additional planting trials and possible tree breeding by Vineyard Timbers and Proseed NZ.

In search of naturally durable timber Vineyard Timbers has also been able to source a small quantity of New Zealand grown E. bosistoana sawn timber posts from Jim Rogers of Otorohanga. These were milled from a large old tree and have been sold to three local vine growers keen to test the use of naturally durable sawn timber posts. Any other farm foresters with stands of young, semi mature or mature naturally durable trees can contact paul@vineyardtimbers.co.nz if interested in cutting and selling posts.

Paul Millen is a Marlborough farm forester and consultant, and managing director of Vineyard Timbers.

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