• Clive Pates

Organic Raised Beds, part 1: Choosing the right wood and preservative.

Updated: Mar 12, 2019

Having just moved house, I had the chance to create a new vegetable garden. Raised beds seemed to be an obvious choice to cut down on weeding time, and give some kind of order to our unruly hilly yard. The first task was to choose the wood, but I was concerned about the copper-based pesticides used to preserve most of it. I wanted to do my own wood preservation, so did my best to research the a source of untreated timber. In the end Home Depot 4 in. x 4 in. x 8 ft. Untreated Kiln-Dried Douglas Fir seemed to be a readily available and economical solution.

Douglas Fir before treatment with Ferrous sulfate.

The hard part was finding a wood preservation process that was both organic and would hopefully protect the wood for many years. Readily available commercial wood preservatives were definitely not organic and were relatively expensive. A natural oil based wood coating looked like the best way forward. There were also a number of ecological wood treatments available in packets as a powder to dilute in water. Again, these looked promising, but did not list the ingredients and were also expensive for the coverage. All these proprietary blends seemed to give a gray patina to the treated wood, so after some research I'm now reasonably convinced that the main powder ingredient is Ferrous Sulfate. Other ingredients could be tannin extract and borax?

Douglas Fir treated with Ferrous Sulfate
Douglas Fir treated with Ferrous Sulfate, pictured the next day.

In the end I decided a good way forward would be to coat the wood with a Ferrous Sulfate solution and then paint on a layer of oil. The next step was to research these ingredients, find the best natural oil and work out exactly what the Ferrous Sulfate is doing to the wood.

It seemed the best available oil was linseed oil, again commercially available and with good all round preservation properties. There are two types of linseed oil available, boiled linseed and raw. Boiled linseed oil used to be heated with lead to produce its drying properties, now raw linseed oil is most likely used as a base and mixed with metallic dryers (possibly cobalt) to achieve the same result. It is worth buying the unadulterated raw linseed oil to avoid any extra ingredients. The raw oil does not have the drying properties of the boiled oil, but in my experience the raw linseed readily soaked into the dry wood with little residue.

Ferrous sulfate powder with garden spray.
Ferrous sulfate powder with garden spray.

Ferrous Sulfate reacts with tannins in the wood to make Ferrous Tannate, this is also a process used to make Iron Gall Ink (where certain tree galls are used because of their high levels of tannin). Ferrous (Fe2+) Tannate when exposed to air turns into Ferric (Fe3+) Tannate which is not water soluble. When this reaction occurs in wood fiber it should act as a permanent water barrier and preservative. I found several web sources mentioning antimicrobial and also hardening properties of Ferrous Sulfate. I could not find any definitive research to back this up, but would be interested to hear of any research that supports this? The addition of tannin extract might also strengthen the reaction?

I started by spraying a Ferrous Sulfate solution onto the wood finding recommendations from 1 to 4 Tbsp of powder per gallon, I used three and that seemed perfectly adequate. I also tried painting on the solution, the brush technique seemed to give a greater degree of saturation to the wood. After the solution had dried overnight, I painted a good coat of raw linseed oil that seemed to easily absorb into the Douglas Fir. Here's a pic. of the wood a year later, the surface of the wood has continued to darken and age over the course of the year giving a great patina to the wood. If you are interested in treating you own raised beds, please do you own research and follow the manufactures guidelines when it comes to health and safety (here is a materials data sheet on Ferrous Sulfate).

Hope this article helps anyone looking for a similar solution...


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