eager to see how it works!
If the wooden surfaces dry out too quickly, you could try adding a non-toxic paint to slow evaporation - oiling with any sort of vegetable oil
should work, but drying oils like linseed or poppyseed oils will eventually harden into a paint-like skin. You could extend them with a non-toxic mineral pigment such as chalk or iron ochre (red or yellow ochre): white (chalk) if you want to reflect more visible light to the plants, and red if you want to absorb it and convert it to heat. Black soot could also work, but is likely to make the paint remain tacky (as it would anyway, if you use a cooking oil that doesn't polymerize as easily).
But if you're watering the whole wall, I imagine the evaporation from each planter-tray is just keeping the soil relatively cool.
Is this
fence also providing a windbreak for a conventional garden?
-Erica