The natural history of a flint wall

The basic structure of a flint wall

The boundary wall of Heene Cemetery is built substantially of local flint. Flint walls along the coastal strip of West Sussex are generally of three types, using cobbled, knapped, or square-knapped flint. Cobbled flint, which may be seen in our cemetery wall, has the rounded surfaces facing outwards and the flints may be laid randomly or coursed between layers of mortar. Knapped flint, which may also be seen in our cemetery wall, has cleaved surfaces showing, as does square-knapped flint, in which the flints have been cut square for visual effect.

Part of the west wall in Heene Cemetery

The traditional mortar is lime-based; the lime being mixed with volcanic ash (pozzolana) or crushed brick or tile powder. Tallow or beeswax is also sometimes added, and acts like a plasticiser. Cement-based mortar should never be used in flint wall construction or repair as it inhibits evaporation of moisture and creates a damp wall, and one susceptible to cracking after frost.

Wildlife on and in flint walls

Flint wall flora

As the traditional mortar is alkaline lime-loving plants will normally predominate if colonisation proceeds naturally. Weathering of wall constituents will produce cracks and crevices where roots can take hold. However, vertical surfaces can create demanding conditions for plants, because there will be little or no soil, frequent drought on the dry side, and either relentless sun or lack of it in the shade. Wall plants exploit cracks and crevices but generally do not cause them.

A memorial wall plaque with an adjacent Dandelion (Taraxacum agg.), Heene Cemetery, April 2022

Green algae, and indeed non-plant algae, will commonly be found on the shady side of a wall, and also mosses (especially feather and cushion mosses), liverworts, and some ferns. Common wall ferns are hart’s tongue fern, maidenhair spleenwort, wall rue, polypody, and the moisture-loving rusty-back fern. Common flowering plants found in flint walls are cranesbills, the moisture-loving wall pennywort (or navelwort), wall-flower, ivy-leaved toadflax, pellitory-of-the-wall, yellow corydalis, red valerian, fairy foxglove, harebell, biting stonecrop, reflexed stonecrop, and wall-rocket. Three annuals that favour a wall habitat, rue-leaved saxifrage, wall bedstraw, and common whitlowgrass, are of interest in that their lifecycle involves germination in the autumn, growing to full leaf in the winter, and flowering and setting seed in the spring, in order to make use of available rainfall and to avoid summer drought. Shrubby plants like buddleia can damage flint walls so are best removed before they become established.

Common Ivy (Hedera helix), Heene Cemetery, September 2020

The commonest wall plant in Heene Cemetery is ivy. We have both our native common ivy and a large-leaved north American interloper called Atlantic ivy, which has spread all over the country. Whilst our native ivy is a very useful plant in our ecosystem, providing food and nesting and reposing space for many species, we will be seeking advice on whether Atlantic ivy may eliminate it by displacement or hybridisation.

Flint wall fauna

There isn’t space here to list them all but it includes lichens, woodlice, springtails, millipedes, small snails such as the brown-lipped snail, moths, solitary burrowing bees (favouring a sunny, south west aspect), and wasps. The zebra spider and tube-web spider seem to like walls too.

Rough Woodlouse (Porcellio scaber), Heene Cemetery, July 2020

The best way to preserve wall wildlife is to leave the wall alone as much as possible and let nature look after itself. Cleaning a flint wall is not advisable as this constitutes habitat disturbance.

Written by Brian Day