What a fabulous way to have a high garden for grasses, wild flowers etc!
I'd just chuck a few handfuls of soil in, and let nature do the rest!
We don't have these structures around here, but a visit to Durham once, showed us some marvellous walls, and none of them were capped!
Mrs Growster managed to collect just one wild strawberry runner from our brick wall, (left by a blackbird, probably), and within a couple of years we had a field-full!
