Plants on Walls
Walls, and other exposed historic stonework, are often colonised by plants and other wildlife such as birds and insects in a similar way to cliffs and scree. These historic structures are providing additional habitats that are often scarce in the natural environment. They are natural examples of green or living walls.
The long association between these plant species and walls is reflected in names such as wallflower (Erysimum cheiri) or in their scientific names. The ivy-leaved toadflax's scientific name is Cymbalaria muralis. Its species name 'muralis' means 'of a wall'. Natural England's Technical Information Note 0052 'Green walls: specialist and companion plant species' (2009) provides more examples.
Most flowering plants and ferns can establish in the mortar-filled joints of stonework or take advantage of cracks and crannies in old walls. Individual species of lichens and mosses like different types of stone and aspects. Variation in construction materials and orientation can produce conditions suitable for several different species.
Walls provide demanding conditions for plants because of exposure, drought and sparse amounts of soil development. True wall plants are able to withstand extremes of temperature and drought. These specialised conditions allow uncommon plants to grow which would be displaced by more vigorous, common species on less hostile sites. The mortar is usually more alkaline than the stonework so a wall built of acid stone may still support lime-loving plants. Historic sites are especially important because the structure and age of their walls provide a range of opportunities for plants, such as weathered stonework and lime-rich mortar. Building stone that has been brought in from a distance can provide a different growing base for plants to the local stone. For example, granite or sandstone brought into a limestone area.
Control of plant growth on standing historic remains and other walls may be necessary to prevent damage from plants with woody roots, or to clear areas for recording and inspection, or for repair work. However, many plants which grow on walls cause no damage, have considerable ecological value and also add to the visual interest of the walls.
Sometimes wall flora, especially a grassy turf on the top of a ruined wall, can protect the wall from extremes of temperature and heavy rainfall as shown in our soft capping research. More stable conditions within a wall will help conserve the wall itself.
We recommend that before you carry out any plant growth control work you give careful consideration to whether the plants are causing harm. You probably only need to remove plants with woody roots like Buddleia. Wherever possible, remove plants and roots by hand with minimal use of chemicals.
More information is provided in Natural England's Technical Information Note 030 'Green walls: an introduction to the flora and fauna of walls' (2009).