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Raised bed construction -wood type and assembly

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George the Pigman:
I would be interested to know what sort of wood people are using for their raised beds and also how they attach them. Looking at the RHS website they suggest using pressure treated gravel boards nailed or screwed  to 18inch, 3inch by 3 inch posts set about 1ft in the ground at the corners.
 However I am interested in any alternatives people have found successful.

In our Belgian garden we used a wall of railway sleepers  to give us a level surface on a slope behind the house.  We lined them with plastic to protect the wood from damp soil and the soil from any leaching from the sleepers.  Once the ground was level we laid out beds just using 8" pine planks painted with Cuprinol and fixed to 2 x 2 posts banged in the ground.    Those planks lasted a good 10 years but were eventually replaced with thicker roofing planks which were pressure treated.   Still going strong 12 years later when we moved.

Here we've just used treated wood sold for garden use and pegged to 2 x 1 posts but for more recent beds we are now recycling oak beams that came down when our barns lost their heads when a tornado freaked thru our plot in Storm Alex, October 2020.  They are heavy enough just to sit there with no support needed.

Tee Gee:
Over30 years ago, I managed to acquire a number of wood railway sleepers when BR was replacing these with the more modern concrete sleepers.

Similarly, in those days, people were changing to divan beds and getting rid of their bed spring beds with the angle iron.

I found that if I cut the angle irons into 2x 2ft lengths and drove them back to back into the soil at the point where two sleepers joined, I had the perfect support for my sleepers. (alternatively, I could have used Tee bar to get the same effect.)

At the end of my career, I worked for a fencing company and I got a load of chipped/cracked concrete gravel board sections, as in the attached photo, and they were brilliant.

If I were starting again, this is the route I would go! (see attached pic and imagine it minus the post)

n.b. As I recall the concrete gravel boards were around the the same price as treated timber sleepers but lasted longer!

As usual I'm looking at this question backwards, and as usual Murphy's law has put me in a  position where I can't implement my best idea.

Anyway I've been watching my raised beds rot away for decades, and its quite clear that all wooden boards rot most where the soil rests against them (at significant pressure in winter, but reducing as the soil dries out in spring - a gap may even appear on clay soils). They rot even more quickly if any part of them has two surfaces against the soil - eg. the plank edge that rests on the unraised soil.

Obviously some woods rot faster than others - my chances of getting sweet-chestnut boards at a reasonable price are extremely slim (and my chances of getting Osage Orange are zero in this country).

It is noticeable that hardwoods are better than softwoods, and I would also say that wood that has weathered outside (even without getting wet) succumbs much quicker than wood that has seen no harsh weather at all.

The obvious solution is plastic - though insufficient UV inhibitor in cheap versions may mean they become brittle quickly and may not last even a single season more than wood.

Please bear in mind that you can still pay through the nose for what is actually cheap plastic - there's no such thing as a reliable supplier these days - I agree that there's no such thing as a free lunch, but extending that to "you get what you pay for" is utter and total cr@p.

Fo years now I have been cutting uPVC doorframes into 4 giving me ~7m of sturdy 10x10cm. Using this as the first 'plank' means the wooden planks above them last at least an extra season or two.

I've been thinking (at least a year) about how I can possibly keep damp soil away from the inside surfaces of the bed planks - any plastic barrier directly against the wood is utterly useless - if anything it stops the planks drying out - keeping the planks wetter for longer so they actually rot sooner.

The answer was staring me in the face and I ignored it - because it is a material that I despise - it is a total waste of money for its designated purpose - corrugated PVC roofing.

I needed an angle grinder to cut out the last of it from an old shed last spring - and I was so relieved to see the back of it that it took a month before I realised I'd made a huge error.

Those short pieces of the long panels are the perfect way to keep soil away from wooden planks. Set with the corrugations facing upwards and down they are strong enough that they don't flatten against the planks, even though they aren't strong enough to hold back the soil without resting against them.

However the corrugations mean airgaps between the plastic and 90% of the plank - so it dries out 10x faster. And the plank means the PVC 'sees' very little UV.

I have just enough left to make a 1m demo which I will photograph and post as soon as it stops raining...


PS. as I implied earlier, the best way to cut a straight edge across the corrugations (especially on old recovered stuff) is to use an angle grinder with a slim disc - I use a 1mm steel cutter that works a treat (minimises the smell of burnt plastic).

Tiny Clanger:
On our site, there is a guy who gets rectangular box form type things from work.  The sides are about 6 in 3 feet long and have some sort of galvanised hinge on the corners.  This allows them to be laid flat in the shed at the end of the season.  We grow strawberries in a "one high" box and for carrots go as high as 4. One on top of the other.  Sorry I dont know if there is a formal name for these things, but I think they may be allied to packaging in the Engineering industry


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