Best Rot-resistant wood/finish combo for commercial raised bed venture.

I am in the planning stages of a luxury garden installation business and am looking at materials and methods for building and finishing the raised beds that is aesthetic, non-toxic and long-lived for the customer and economic and practical for the business. I am targeting some of the more well-to-do neighborhoods in my area, so I am not afraid to spend and charge for high-quality materials, however this needs to be a repeatable, 1-2-3 process that balances idealism and efficiency. I also want to be able to give customers a reasonable expectation for the life-span of these boxes.
I have read some of the excellent advice already given on Yakisugi, wood oil etc. and want to run a few ideas by the brain trust here on Permies.

Keep in mind that these beds will rest on some kind of gravel pad so they will not be resting directly on the ground.
The materials:
1. either cedar or black locust 4"x8" sleepers
2. tung oil or Land Ark (as mentioned by Jay White Cloud in another topic) or other food-grade oil
3. mix oil with salt? Can anyone provide a reference to more information about how this is best done?
4. HDPE Plastic liner?

The Method:
1.Before assembly, char material using weed torch (settling on the monetary expense of propane vs the time expense of traditional wood stove method unless someone changes my mind)
2.Wire brush to remove excess char
3. Assemble
4. Apply Oil (repeat application if necessary)

Some questions and considerations:
1. Should I line the box with some sort of HDPE dimpled plastic moisture barrier (microplastic vs constant contact with moisture/soil)?
2. One of the services we will provide would be to re-oil the wood every year. Obviously, when filled with soil, this can only be done to the outside of the box. Thoughts on efficacy of this and how this is best done?
3. Does anyone have experience using Yakisugi method on black locust rather than cedar?
4. If using black locust, does the wood need to be treated at all? Is there a special saw blade available for cutting black locust?
5. Given these materials and proposed methods, how long could such a box be expected to last?
5. What have I not considered in my process?

Cheers and thanks so much to anyone who can help.
Chris

Hi Chris,
I think that is a great business idea. I think you covered all the ideas I would have for wood preservation.

I do think that people who are interested in gardening would probably also appreciate an all natural method... in other words, maybe not adding the liner you were talking about.

Another angle would be to use stone or brick. It is natural, doesn't rot, doesn't need treatment... and if its dry stacked, can be moved somewhere else if needed.

How tall are these raised beds?

For 1-2 foot tall beds I've had good success building planters with corrugated metal roofing like this:



This style is nice because it keeps the wood largely out of contact with the earth. If you framed a planter like this with cedar it should last a really long time. I use pine because it's cheaper and stain it with linseed oil tinted with burnt umber primarily to protect the wood from the sun.

I've used logs to created terraces and retaining walls. To create a rectangular low raised bed I would cut half lap joints on the ends. If you used rot resistant wood this type of bed could last 7+ years.  

A few options for wood preservation...

First of all, I suspect for black locust it may not be necessary at all, based on what I've heard.

Raw linseed oil is great. You can also char the wood. Not sure if you can combine both.

You can also put the boxes on gravel and maybe avoid the liner.

I think this is a great idea, though!

Thanks, all for the input!

My idea, Aaron, was that I would use 4”x8” lumber stacked 3 high, thus making the boxes 2 feet tall. I would bury some old logs (hugelkultur-style) to serve as filler and a nutrient-sink.

Those beds you posted are really sharp. I considered metal as an option but I think the aesthetic is more of a farm-vibe than what I am picturing.

I suppose I could still line the inside with metal sheets but I’m concerned about trapping water between the wood and the metal. This, along with micro-plastics, is also my concern of using and HDPE liner.

Hi Chris,
If the only liner you use is the metal sheets, and it is installed vertically... the water should drip out the bottom and shouldn't ever get trapped between the metal and wood.

You mentioned a plastic liner.  How will you be using this liner?

Anne Miller wrote:You mentioned a plastic liner.  How will you be using this liner?

I would run it along all interior walls as a moisture barrier. It would be invisible once filled with soil.

HDPE plastic is allegedly “food-grade.”

Thanks, Chris

That sounds good as long as it is completely covered with dirt so the sun cannot degrade the plastic.

We bought a property with old garden beds that were lined with black plastic, I assume HDPE. They have not aged well at all.

Replacing wood every so often isn't the biggest deal. Sometimes a 20 year product is a better option environmentally than a 30 year product. Especially with cedar or black locust, you should do pretty well.

Almost everything good for the environment will rust/rot away at some point.

Black Locust is some really durable stuff, it just does not like to cooperate when you mill it, I'm told.

What I would really, REALLY, like to have is a raised garden bed that is 'rebuildable'. I use 2" rough pine walls with no treatment for my first iteration and they have been holding up pretty well on their fourth year. I'm not just starting to see fruiting bodies of fungus coming through. If I went with a style that used a galvanized side framed out with wood, I would want to be able to swap the wood pieces out as needed. We get rain/snow and all sorts of weather factors so it isn't if but when things will need to be fixed.

Ideally the bottoms would be rot resistant black locust, maybe the top and sides could be cedar?

I would want no plastic liner in my setup personally.