Nature on our doorsteps: The wonder of walls

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into nature on our doorsteps and the beautiful biodiversity of its plants and wildlife.

Old walls can offer many different opportunities for biodiversity to take hold and thrive.

In times past, boundary walls or roadside walls were mostly constructed with stone that was common to the locality.

Walls between fields were usually built with stones removed from the fields themselves.

These were carefully placed in patterned layers on top of one another to form a dry-stone wall, where mortar was not used to hold the lose stones together.

Dry-stones walls create many little spaces between the individual stones where small mammals and birds can shelter.

They are also great for creatures such as lizards who like to bask on the dry, sun-warmed, stones.

Where more stable structures were required, mortar made from crushed lime would have been used to hold the stones together and strengthen the walls.

Mosses and ferns will quickly colonise the cracks and crevices in an old wall.

As these walls aged, many little spores of mosses and ferns drifted in on the wind and grew in the tiny spaces between the stones and the mortar.

These plants need very little soil in which to grow.

They can also survive periods of dry weather, bouncing back again when the rain returns.

As the mosses continue to grow and decompose over time, and as the mortar begins to crumble into grains of sand, a thin layer of poor soil begins to gather between the stones.

Seeds of small fast-growing plants with very shallow roots can now grow.

These will flower and set seeds in early spring when there is plenty of rain to provide moisture.

Old walls can also be very important places of refuge for a wide range of insects, spiders, and snails.  Certain solitary bee species specifically search for cracks and tiny tunnels in old walls where they will make nests for their young.

If left to grow undisturbed, sometimes old walls with their layers of mosses, ferns and flowering plants might be the only place left in a developing area where a little piece of nature can continue to survive.