Stone walls are a big feature in our little town in Northamptonshire. The local limestone is used extensively for building and with the soft mortar that often goes with it, the result is a great habitat for plants.

A year or two back I mentioned the fern growing in a wall behind the supermarket, but walking round town before I returned to the US turned up quite a few plants taking advantage of this habitat.

The mahonia growing through the low wall (above, click to enlarge) demonstrates the power of its creeping roots as it penetrates through 15in/38cm of limestone wall, and in this case the wall is built with very hard mortar. Planted in the border on the inside of the wall, it’s made it way straight through the wall and is now flourishing on the outside.

Aubrieta (right, click to enlarge) is an old limestone wall favourite and here two seedlings have germinated together and are flowering in slightly different colours. In turn these have seeded and young plants are now becoming established in the soil at the base.

Just around the corner from the aubrieta I spotted a lovely flowering plant of white comfrey, Symphytum orientale, growing in a wall, about 5ft/1.5m above the ground. But when I returned with the camera, someone had torn it out.

Then another fern, growing just under the gutter about 15ft/4.5m above ground. Where two lengths of gutter join there’s just enough of a leak to moisten the mortar at the top of the wall and allow the maidenhair spleenwort, Asplenium trichomanes, to settle in.

Other plants I've spotted growing in walls around town include Helleborus foetidus, with the seed secreted away in the cracks by ants; wallflowers in various reds and yellows turn up in quite a few places; California poppies, Eschscholzia, in orange and cream; Campanula persicifolia, in blue and in white. Just to mention a few...