Often the first response I get when people see the beds, is “Why didn`t you build the walls with wood ?”
It was years of fustration, disappointment, difficulties and problems with Wooden Walls that made me and my friends look at concrete in the first place.
To begin with, if you use untreated wood the walls will rot and start falling down in a few years. Even H3 timber treatment is not ground durable and will rot after several years and on top of this it will leach out copper, chrome and arsenic into the environment. This is a problem in the garden because you don`t want heavy metals getting into your food !
So unless you can find some naturally ground durable wood eg jarrah from railway sleepers, then if you decide to use tanalised timber , it must be H4 or H5 and this starts to get a bit expensive and involves even greater quantities of heavy metals going into your garden.
Many people have made do with offcuts from Timber Yards and have wacked up walls with any odd bit of wood they`ve been able to scrounge. Their idea is, that this is cheap and easy and if it all falls down in a few years then we`ll rebuild it. To their surprise a few years comes round surprisingly fast and they have to go through the whole process again, but in the meantime they have had to put up with flimsy, ugly, slug and snail infested walls.
Wooden walls contain gaps and holes that allow weeds such has Kikuyu to penetrate and provide habitat for slugs and snails. These gaps in the walls also make it easier for moisture to be lost from the raised beds and in the summer a major problem with the beds is stopping them from getting too dry.
Concrete walls are “seamless” and provide no habitat for slugs and snails, and it works even better, if the paths are weeded and heavily mulched with sawdust, which slugs and snails don`t like.
The thickness of the walls helps preserve the soil moisture and in my experience does not act like a wick, but quite the opposite, the concrete actually holds the moisture.
Another great advantage is being able to sit and work on the walls, and the height of these one foot concrete walls are just ideal for this, as they are not too low which would make it difficult to get up from, and it is not too high, which would make the raised beds expensive to build, needing a lot of soil to fill up and be difficult to keep wet in summer. In my view Waist-High raised bed gardens are a waste of money and are dysfunctional !
Not only can you sit and work from the walls, you can walk and work from the walls. I find that I do a lot of my work on the beds standing on the walls, there seems to be many advantages to standing at the same height as the soil you are working on.
I would recommend you study the work of Austrian Permaculturalist Sepp Holzer and especially his use of Stone Mulches to create Warm Microclimates, because concrete walls have huge thermal mass which is radiated out at night raising the ambient temperature around the whole garden. This can be enhanced by planting low shelterbelts or putting up fences. The bottom of the walls imitates a stone mulch in another way too, it seals in soil moisture. In the winter you can put up cloche frames over the beds to grow beautiful winter greens and salad veges and by purchasing 2.5m wide clear polythene sheets, there is enough overlap with the walls, that the edges of the plastic can fold over them and then be weighted down with lovely round river rocks. This firmly secures the polythene to the raised beds, prevents wind from getting under the cloches and helps trap the heat emanating from the walls at night.
Some people don`t like the grey concrete appearance of the walls and would prefer them to look more adobe like. I originally tried to build the walls with adobe like mixes of clay, gravel, sand and minimum cement, but the clay really gunked up my concrete mixer and stuck to the sides and was very difficult to trowel and work with, and on top of all this the walls were much weaker. So I gave up on this and went to a straight concrete mix.
Probably the most attractive feature of the walls is the corners or shall we say, the lack of corners !
This is a very Anthroposophical thing to do, as people who follow the teachings of Rudolf Steiner always try to remove corners off windows and doors. The first beds I made had rectangular corners but after a few weeks of looking at them and walking around them, I went and got a sledge hammer and knocked them off. This is a major advantage of this system and I have never seen a wooden sided raised bed garden where someone has gone to the trouble of removing the corners. It would be difficult to do in wood.