So I've build a 2m by 2m herb spiral and filled with soil. In the process of planting it. However. I used reclaimed materials for the walls - bricks, concrete blocks, chunks of granite, stones. This has given it a very rough look and some of the stones are loose. I am wanting to render/clad the walls to strengthen it and neaten it up. My thoughts were to wrap it in chicken wire or mesh and then clad with a mud/clay mixture. do it on a sunny weekend and allow it to bake dry. Obviously whatever I use has to be reasonably weatherproof (I live in midland England so can be rainy) but it's also important to me that whatever I use does not add toxicity to the soil or leech anything harmful (or even drastically change the p.h). Could anyone advise a solution? Thanks - Samuel
Regarding toxicity stuff, I think that depends on what flavor of "clean and healthy" you're after and to what extent you're willing to pursue it. If you're going for Paul Wheaton level of decreasing toxicity, then, using concrete is not good (some things get a little sad from concrete).
I think most natural cladding (like cobb) needs good "hats and boots"--a roof to keep it dry from rain, and a rock foundation to keep it from contact with wet earth.
My herb spiral (you can see the link in my signature) is made in a mish-mashed way like yours, with bits of concrete, bricks and rocks. Now, 4 years later, the plants have over grown many of the edges so that you can't even see the bricks. I personally don't think it needs any cladding, but maybe there is something that might work. If so, I'd love to know about it, because I'd really love to build with a natural finish that doesn't disolve in our rain!
I recently built a similar herb spiral with whatever rocks and bricks and stuff I've been gathering. Some bits don't fit together all that tightly but it's being held together fine just by the soil and it seems to be becoming increasingly stable as it settles and the plants grow.