This natural system of the Cornish hedge relies on maintaining the moisture of the earth core. The hedge needs to be properly built, resting on the subsoil and using subsoil or similar clay-shale “rab” for its core, as clay and stone are cooling and induce condensation. The correct laying of the stone and proper “batter” shed rainfall, allowing just the right amount to seep into the hedge. The dampness inside the hedge is conserved by the green growth, which should never be removed by trimming in summer. The low fertility of the subsoil core and the tightness of the well-built stones resist invasion by rank weeds which would otherwise destroy the hedge's balanced ecology.
Building and maintaining a Cornish hedge so it will keep itself in this healthy condition is a skilled craft, on which the wildlife depends. These versatile and beautiful hedges were created to suit the prevailing conditions where the soil may be thin and poor and the weather stormy. Cornwall's hedges need to be traditionally cared for so that they continue to be a haven of life. Much damage has been done to their structure and biodiversity by over-zealous trimming with flails, on average two-thirds of the species being eliminated. Trimming in January and February, preferably with a fingerbar trimmer, alternate sides of the hedge in different years, best safeguards the plant and animal life.