Soils and growing substrates
Plants used for facade greening may be grown in soil in the ground, or in containers filled with a well-designed growing medium. The principles outlined in growing substrates for green roofs is relevant to green facades and is outlined here.
The use of planter boxes, mounted at varying heights above the ground, can allow greater coverage of the facade, where the building is so tall that ground level plants will not reach the top. The advantage of in-ground planting, where soil is of a reasonable quality, is that the plants will have more access to water (the soil will not dry out as quickly as in a container) and will have more space for their root systems to grow.
Container growing media must be designed to support ongoing growth of plant shoots from a limited, contained root volume, and at elevation. Although many climbing plant species have superficial root systems and may thrive in a small volume of substrate, there is a notable link between root volume and sustainable foliage volumes. Climbers required to cover greater areas will require greater substrate volumes. However, weight loading restrictions may limit the depth that can be supported for container systems at elevation.
In-ground plantings will generally outperform container plantings in the long term. Ensure the planting bed soil or growing media has a suitable balance of porosity and water-holding capacity, and an adequate supply of nutrients to ensure optimal growing conditions. In a garden setting with good quality soil and adequate irrigation, there should be little to impede strong plant growth. In city landscapes, with large areas of impervious paved surfaces and soils that may be highly compacted, soil structure may be poor. In such cases, consider installing a ‘structural soil’ that can be compacted to enable footpaths or other hard surfaces to be installed, while still providing adequate porosity to support root growth. In some buildings the foundations sit out from the wall, underneath the ground – these footings should be set deep if a garden bed is planned adjacent to the wall.
Seek the advice of a horticultural consultant to ensure the volume of soil or growing medium will support the desired height and spread for the green facade.