Green walls, i.e. vegetation growing on or against vertical surfaces, can be found outdoors or indoors on any type of vertical surface, from building façades to boundary demarcation or even free-standing support. They can be incorporated into new construction or easily retrofitted to existing buildings. Green walls can vary considerably in construction; they can be rooted in or off the ground (i.e. soilless), in several kind of growing media (mineral or organic), or in an inert medium acting only as a rooting element. According to the type of structure, the system can be either ‘completely natural’ or hydroponic. When required, irrigation can be manual or automatic.
Green walls have been divided into two main categories: green façades and living walls; however the concept of the green wall can be extended to a wider range of systems: urban hedges, stone walls, green screens, live curtains and living walls.
Green façades are made of climbing plants growing on a wall either with no additional infrastructure, or with the use of stainless steel or wooden trellis, meshwork, or cabling, as plant support. They are historically set outdoors, rooted in the ground and don’t require additional irrigation. They can also be off the ground and erected indoors, usually free-standing with irrigation.
The great diversity of climbing plants, in terms of flower and foliage colours, flowering season, profile, etc. make them attractive for humans. They can be evergreen or deciduous and are usually woody and perennial, although some can be herbaceous or/and annual. As they use different ways of adhering to a surface, they need different kinds of support either vertical and/or horizontal, or no support at all in the case of self-adhering climbers such as Hedera helix (common ivy), Parthenocissus sp. (Boston ivy) or Wisteria sp. (Virginia creeper).
Plants that can be trained against the wall or in espalier (e.g. Camellia sp., Ceanothus sp., Chaenomeles sp. (“flowering quince”), Coronilla valentine (scorpion vetch), Garrya sp., Fuchsia sp., Magnolia grandiflora, Pyracantha sp.) referred to as ‘wall shrubs’ can be included in the term ‘green façade’.
Living walls are recently developed, completely artificial systems, using continuous or modular, planted-up, units. Continuous living wall systems can be made of felt-layers or be a block of concrete. Modular panels are using modules of sphagnum, substrate filled metallic cage, gabions, preformed plastic modules or rockwool units. Plants are rooted directly in the structure (in the case of felt layers or sphagnum units) or in growth medium, beforehand added to the structure (for concrete block, rockwool, plastic preformed module or gabion panel). The growing media can be organic materials such as coconut coir (Cocos nucifera L.), peat, tree bark, or inorganic materials such as expended clay pebbles, gravel, perlite, mineral soil, mineral wool, sand, vermiculite; although different components are often used in mixes. The system is usually hydroponic (i.e. the mineral nutrients are brought to the plant as inorganic ions in water).
Any plant species can be grown on a living wall system. Typically, the only constraint is the weight of the mature plant; some felt layers systems have been shown to support tree species. Indoor walls are usually planted with tropical species due to the constant mild temperature and the lack of light; while outdoor walls are more restricted to rustic plants. Living walls are sometimes referred to as “vertical gardens” when they are used to grow herbs and/or plants producing vegetables or fruits. When growing herbs, the green wall is usually called a “herb wall”.
Depending on the system and the manufacturer, units are either pre-grown in greenhouse (vertically or not) prior to installation or planted on site once installed.
Intermediate green walls
Green Façades and Living Walls are the opposites, in terms of complexity of structure and man-made features, of the large panel of the green wall concept. Between these two can be found features like Green screens, made of a climbing plant (typically Hedera sp.), pre-grown on a freestanding, galvanized steel framework, and established as an instant hedge. They are usually included in the concept of green façades. However, the facts that they are commercially pre-grown in nursery, completely free-standing when historically green façades are against walls, and usually installed with automatic irrigation, may set them apart from green façades.
Live curtains combine the features of green façades and living walls. Like green façades, this system is made of plants climbing on a structure, but rooted off the ground, in small planter boxes, as hydroponic systems like living walls.
Urban hedges can be considered as part of the green wall concept as they are interchangeable with green façades or living walls for some of their features and ecosystem services.
Stone walls are horizontal structures of overlapping stones build upwards, with successive rows of stones overlapping each other. The space between two rows is filled with smaller stones and sometimes with capping stones bridging the top. A distinction is made between dry stone walls and mortared walls that are usually more shaped with regularly rectangular stones held together by mortar. While dry stone walls are usually only freestanding demarcation walls, mortared walls can be used as boundary walls and retaining walls. Stone walls can be colonized by vegetation, usually following a natural process rather than being deliberately man-made.
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