Safely Treating Wood for My Garden (Article taken from the Spring 2013 newsletter)

Barbara Pearson

My number having come up, I’m now the proud permittee of a double plot. My new garden is going to have a wood retaining wall to hold in the manure and compost that has been and will be added to the plot. So I was looking for a way to safely give the wood some kind of protection against rotting, knowing perfectly well that I can only delay the wood’s demise, not prevent it.

Pressure treated wood is no longer treated with  arsenic and chromium and so is now considered environmentally safe, but the specifications for it say it’s only for use above ground; some would say the retaining wall is above ground but I disagree since it will be partially buried. The cost of treated wood is about one and a half times that of untreated wood.

Cedar, costing nearly triple the price of the more readily available Douglas fir, is naturally resistant to rot, which really only means that it takes longer before it does. Since I couldn’t find any convincing information about how much longer I could expect cedar to last, paying that much more for it was an option that I decided early on wasn’t worth it.

I considered using oil-based paint which is often made using a vegetable oil base, but that made me think “Why can’t I use just any old cooking oil?” Well, because nothing is ever that simple. Oil used on wood should be a drying oil – that is, one that will harden to form a protective coating. Why I should care that a non-drying oil will never dry to the point of not feeling gooey, I couldn’t tell you – after all, the wood is going to be used in a garden, not for furniture, and it’ll be mostly covered with soil anyway. Maybe it doesn’t matter, I just couldn’t find a single source that would tell me that.

In the long list of drying oils there’s linseed oil,  which has been used to treat wood for a long time. It’s not easy to find “raw” linseed oil because boiled linseed oil is better at penetrating the wood and dries faster so there’s more demand for it. But these days, “boiled” on the label usually doesn’t mean that the oil was heated to the point of smoking. What it most often means now is that the “boiling” was replaced by the addition of chemicals that make the linseed oil act the way heating it to its smoking point would have (a truly perfect example of taking something that wasn’t broken and “fixing” it).

It was, of course, a gardening forum post that gave me the answer. Soybean oil! It’s a drying oil, readily available, and compared to other drying oils like walnut or safflower oil, it’s fairly inexpensive – $10 for a gallon of house brand, as little as half that when on sale. The same quantity of boiled linseed oil, with its nasty chemicals, costs over $20; raw linseed oil a/k/a flaxseed oil, likely only available at health food store prices, would cost over $50. Most generically labeled vegetable oil is soybean oil (check the label though); one gallon is all that’s needed to generously coat eight 2”x12”x12’ planks.

According to my resident expert – my husband Jim – it’s most important to treat the rough, cut ends of the wood, so if you’re short on time or motivation, doing only that is a lot better than doing nothing at all. I treated the whole of every plank using an old brush that was one use shy of the garbage.

I did the work in the garage and it took about two days per side to dry; that’s a long time but it was cold in there (I did this in February) and the lower the temperature, the longer it takes for oil paint or varnish or boiled soybean oil to dry. If done outside on a warm, sunny day the planks would probably be dry enough to work with in a few hours.

There are safety concerns with doing this. Oil  heated significantly past its smoking point will reach its flash point and can ignite; watch it carefully – the smoking can be subtle. While you probably can use it right away, smoked (boiled) oil is extremely hot, much hotter than boiling water; let it cool. The drying of the oil releases heat, which is not a problem as the oil dries on the wood, but if you apply the oil with a rag and then leave the rag crumpled up somewhere, it could spontaneously burst into flames. My feeling is just use a cheap brush to apply the oil and avoid the issue altogether.

Treating the wood with boiled soybean oil, which gave it a nice warm color and some protection from rot, made me feel better in the here and now but I won’t know for at least five years if it made a worthwhile difference. Check back with me then.