How to line CCA Treated Pine garden beds?

lm4

Hi all,

We have recently built a few garden beds using H4 CCA treated pine sleepers (https://www.bunnings.com.au/200-x-50mm-1-8m-treated-pine-sleeper-h4_p8032701). We’re very happy with how they’ve turned out, although have recently discovered that CCA treatment is less preferred than ACQ for garden beds, due to the possibility of chemicals leeching. 

To protect against leeching, we’ve lined the garden beds with 70um builders plastic and made sure the soil is only touching this and not the wood. We’ve also planted the herbs/veggies with plenty of space to the bed wall. 

I’m after recommendations as to (1) whether the above mitigations are appropriate when using CCA treated pine and (2) if there is any layer/paint/seal we could apply to the external sides of the pine to prevent contact with CCA.

Would also appreciate if there is any other authority I can reach out to regarding CCA garden beds as the advice I’ve been able to find online has been fairly ambiguous.

Thanks!

MitchellMc

Hi @lm4,

You've taken the right steps to mitigate any chance of the CCA treatment entering the soil, and installing plastic is what the manufacturer recommends. However, and this is only precautionary, I'd recommend you paint the exposed timber portion above the soil level with an exterior paint like Dulux Weathersheild. This paint can also be used on the exterior of the garden beds to prevent contact with the timber.

There are limited sources of information on the leeching of CCA into soil and the uptake of it from plants. The CSIRO has a report that found that leeching of CCA was not detectable in the soil more than 200mm from the timber and that above-ground food crops do not absorb CCA. Root vegetables that were grown up against the timber had a slight increase in arsenic content. They advise providing 100mm of space between crops and timber. However, no leeching can occur through the plastic you've installed, so there isn't a concern here. Here's that report for you: The facts about CCA-treated timber.

Please let me know if you have any questions.

Mitchell