Can I use untreated pallet wood for building outside garden boxes?
If not what is the best treatment?
Welcome to the Bunnings Workshop community @Mikex. Many thanks for your question, I'd be happy to answer it.
If the pallet wood is untreated pine, then it will have a limited lifespan out in the weather. To extend its lifespan you could use a decking oil, exterior paint or prevent it from being in contact with the soil. I've included below some pictures of my own pallet wood garden bed. You can see that I have lined the bed with plastic to prevent the timber being in constant contact with the moist soil. You will need to allow drainage out the bottom so the bed doesn't fill up with water and I simply cut the plastic for that purpose on mine. I've also included a picture of where the timber meets the concrete and you can see it wicking up moisture. This will likely be the first place to start rotting.
I've deliberately not used any type of preservative to see how long pallet timber lasts in this type of application. I estimate that it will be several years before it needs to be replaced and I will be happy if I get 3 years.
Please let me know if you need further assistance or had other questions.
That would probably be an excellent way to attract termites into your property.