Here’s how to make your own cheap rot-resistant wood for basic gardening raised bed projects.
A friend, who happens to be a technical paint expert and I were chatting over coffee one day about some diy garden construction I was about to take on (building a lot of raised beds) Rather than using chemical laden pressure treated wood, he suggested I use vegetable oil
This made a great deal of sense on the surface as the old farmers used to use linseed oil on wood to preserve it when buried (fence posts etc) but this is so expensive now, you won’t use it in bulk.
To be frank – this is NOT pressure treated. There’s no “pressure” applied to drive the vegetable oil into the wood.
But the title captures the intent of the post – to create a long-lasting wood that doesn’t rot and will last for long enough to be practical.
Any Special Instructions?
- Nope. Use the cheapest vegetable oil you can find and slather it all over the wood. Fill up the cracks and knotholes with it. The oil will penetrate quite well (two coats is about all it will take) and dry in 24-48 hours if you leave it out in the sun.
- Really soak it into the end grains where water can and will enter first and quickest.
- Two coats are better than one but more than that won’t sink into the wood.
- Hard wood such as bamboo (stakes) won’t absorb this oil so don’t bother with those.
How Long Will It Last?
Good question. The honest answer is we don’t know.
Summer 2013 I’m building raised beds now and using this system of diy pressure treated wood for our vegetable garden beds.
Update: Nov 2018 The few beds we use are fine.
Update: Spring 2021 I found the bottom board that had two sides exposed to the soil (side and bottom) tended to rot a bit on the bottom board where the water was the heaviest but was still serviceable. That’s 8-years. (I moved a bed and dismantled it so it was easy to tell.) The other boards with only one side exposed to the soil were in good condition.
Don’t use it in serious construction.
It won’t meet building codes where pressure treated wood is allowed.
So What’s Our Thinking On This?
Our thinking (my friend and I) was that if it lasted only twice as long as untreated wood, we’d get 8-10 years out of each raised garden bed. And the price of rough construction grade wood – and a bit of vegetable oil – is far less than the price of pressure treated wood, cedar or even plastic.
So if we get 8-10 years – the odds are we’ll want to change or move the darn garden. And if we like the layout, it’s no big deal to pull apart a raised bed like this (use good deck screws that don’t rust) and replace it. The soil will mostly stay in one place after that time so it will be like reskinning the bed rather than doing a lot of digging etc.
It’s a simple way to get raised beds.
See updates above for how long will it last
Why Not Just Use Pressure Treated or Cedar?
Pressure treated wood is “supposed” to be safe with the new formulations. But the “old” formula was supposed to be safe too until they took it off the market. Call me skeptical about using pressure treated wood in my raised vegetable garden beds.
I know vegetable oil isn’t going to hurt me nor be a problem if it leaches a bit into the ground.
Using Cedar? Have you checked the price of that stuff? Lovely indeed and I built a deck out of it but it’s out of my budget for the number and size of raised beds I need to build.
Update 2021 re price: Covid has driven the price of all lumber sky-high and this is just another workaround to use the least expensive products for the longest gain.
Some Concerns Answered
It’s not really pressure treated That’s correct. There’s no pressure treatment at all – the oil is not forced into the wood. I used the title to help folks understand what I was trying to do.
“It doesn’t work” Here’s my response. First – oil in one form or other has been used as a wood preservative for some time. Creosote is one example, linseed oil is another – and yes, vegetable oil is a third. Vegetable oil simply happens to be easily obtained and relatively benign (unlike creosote) in the vegetable garden. It’s also the cheapest.
It won’t last for long. Probably not as long as chemically treated wood but I know that going in. I don’t expect it to last forever but I do expect to get a few extra years out of really cheap wood.
It attracts insects. Not so far. Haven’t seen anything out of the unusual or more than expected.
It stinks after a while. Again, not so far.
Borax is better. Borax is a component of boron. A quick search showed two products advertising the use of this material. One was only registered for termite control while the second advertised termites plus wood preservative.
But here’s the thing – boron leaches out into the soil and this is clear on the labels. We can expect homemade borax mixes to do the same thing. The thing about plants is some are quite sensitive to excessive levels of boron and die or grow poorly.
So given this data – borax might indeed act as a wood preservative but it also has the potential to do damage in the garden when it leaches. Given this choice, I prefer the possibly shorter life of the vegetable oil soaked wood rather than the long term problem of possibly killing my plants. But to each their own.