About tanalised timber in the veg garden…

Our bean frame - it's hard to avoid tanalised pine in the veg garden

Our bean frame – it’s hard to avoid tanalised pine in the veg garden

Living rurally, the stream of free farming newspapers descending into our letterbox seems never ending. I cannot say we read them from cover to cover, but it is surprising how much interesting material is in these publications. It was ‘Straight Furrow’ that yielded this week’s most interesting article. It was about a project looking for hard wood alternatives to tanalised pine.

The timber production of this country, based as it is on Pinus radiata, derived from a forestry project started way back in the 1870s, trialling various options. Clearly the pine was the stand out performer and work has been ongoing on selecting the best characteristics for plants which are to be commercially cropped. Modern plantation pine is a somewhat different tree to the gnarly old pines sourced originally from Californian and then Australian seed sources. The use of pine as a building material is a boon for what little is left of our native forests, though we are guilty of plundering the hard woods of Asia instead – especially when it comes to outdoor furniture and more upmarket decking.

The pine we use is grown very quickly and is soft as a result. To get any durability outdoors, it is treated by tanalising it in a solution that contains chrome, copper and arsenic. There is now a quest to find quick growing hardwoods as an alternative and some of the eucalypts look extremely promising.

What does this have to do with gardening? The impetus for this research was, apparently, the discovery of arsenic in the Marlborough aquifer back in 2003. In this case, it was not a cause for panic. That arsenic was occurring naturally and not a danger to health. But, as an aside to that particular study, elevated arsenic levels were found in the soils. These were linked to the use of tanalised posts. When you think about it, we use gazillions of tanalised posts in this country, many of them treated to an advanced level so that they are durable in the ground. There is a scary thought.

It appears that those readers who ask whether it is safe to use tanalised timber in the vegetable garden are indeed asking a valid question. It is pretty difficult to avoid when you think about it. The planks many people use to avoid compacting soil when walking between rows are likely to tanalised. The waratahs often used to stake tomatoes, the poles to build the bean frame… but especially the sides of raised vegetable beds. Yes, apparently these do leach toxic chemicals into the surrounding soil. And plants are very good at absorbing heavy metals, especially leafy greens like vegetables.

It appears that some within the wine and kiwifruit industries are very keen on non-tanalised hardwoods as an alternative. Both these producers have extensive need for wooden growing frames. Organic farmers and growers are also anxious.

I have not read that it has been proven beyond any doubt that anyone has suffered from ill health or death as a result of secondary contamination from tanalised timber. It is your call entirely as to whether you consider it is a risk worth taking. It appears that different people respond differently to chemical poisoning. An anecdote about somebody who lived to a ripe old age despite an extremely careless attitude to sprays is not proof of safety because it may be that the next person suffering multiple allergies is a result of very little exposure.

Presumably the leaching of chemicals from tanalised timber slows down as it ages, so if you are using some near edible crops, look for the old stuffveg.

Heart macrocarpa is probably the most suitable alternative to tanalised pine at this stage, though it will rot eventually. The eucalypt alternatives being researched are offering maybe 25 years without being treated but the research project has a way to go yet.

For the record, the article referred to is in the October 15 issue of Straight Furrow, page 11, headed “Hardwood project promises billions”, written by Jon Morgan. The sad point of that story is that they just missed out on $3 million dollars of government funding because they were not able to guarantee that the project would earn $200 million dollars in exports by 2030. It is a fairly sad comment on the nature of government funding for research. It seems an exorbitantly high bar to set for a modest contribution, but the project is continuing. Imagine some of our pine plantations replaced with eucalypts. That would change the landscape.

First published in the Waikato Times and reprinted here with their permission.

2 thoughts on “About tanalised timber in the veg garden…

  1. Katie Cole

    Abbie, I enjoy your thoughtful discussions….however, my recent experience of mature eucalypts (E.nitens) on a peaty/boggy Canterbury plains site during the recent winds is not a good one. Many have been uprooted largely because of their top-heavy nature & superficial root system-our water tables are high despite the abundance of dairying! The only ones to remain are on clay planted in West-East running shelter belts. However, I notice that others still fell over on neighbour’s properties despite this. At least, there is a market for pine logs but none for the excess amount of firewood this recent storm has generated!

    1. Abbie Jury Post author

      Thanks for your comments, Katie. I know nothing at all about the root systems of eucalypts so I have no idea if different species have different rooting habits. Nor do I know offhand what the main species being trialled are, but I would guess the ability to stay upright in the fairly extreme conditions we can get in this country is part of the study.

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