Rooting through some boxes (and failing to find where I have securely stored my weathering powders), I unearthed some tongue depressors I bought from Hobbycraft a while ago. They're 6" (15cm) in length, and 3/4" (18mm) across, so they're a great size for all sorts of things. I used them recently as the bases for 28mm walls and fences. Clearly, 6mm terrain doesn't need to be so tall as 28mm or even 15mm, so I just did the simplest thing possible. I applied UHU to the wood to prevent warping, then used PVA atop that to glue and secure sand. I painted over that with dark brown paint, then gave it a light drybrush of a coffee colour, then a lighter one yet of white. Then I used the UHU again to glue three colours of clump foliage onto the top. I tried to mix them up and keep them irregular in appearance. Simple as anything, and now I have 6' of hedgerows for 6mm.
