The NBS guide to façade greening (Part Two)

This time we continue our exploration of systems available and their likely benefits and detractions in more detail.

Previous: Part One: Definitions, benefits and detriments

Green façades

Green façades are achieved with plants that have their roots placed in a conventional topsoil or growing medium and in which the ‘greening’ gains support either directly on the building façade or on a separate framework. The most typical examples of this approach are ivy coverings to traditional buildings or clematis-covered trellis work.

The simplest application of this system approach to wall greening is plant troughs of various shapes and sizes, placed around the base of walls, or sometimes at height on terraces, balconies and façades. These achieve ‘greening’ with grasses, herbs, climbing plants, trailing vines, shrubs and even small trees. Elevated troughs with pre-grown (Hedera helix – Common Ivy or Fagus sylvatica – Common Beech) screens (up to 4.5m high) are commercially available.

Due to the constricted root zone, and risk of drying out, plants in troughs will require maintenance – irrigation and regular nutrient supply. Suspended containers loaded with plants and growing medium will require structural consideration.

The selection of plants and criteria for their survival is a vast subject. There are a range of considerations; the following is a synopsis from Dunnett and Kingsbury (2004):

• Climate and orientation – selection of species suitable to all potential conditions; consider wind exposure, turbulence and chill, resistance to drought and differing wall aspect, particularly with elevated planting. Orientation can affect the variety of suitable plant species; generally sunny façades will provide the best location for the widest range of plants. Mayor of London document ‘Building green’ offers guidance on suitable planting orientation of green façades.
• Soils and moisture – for good establishment, climbers require deep fertile soils (with high organic matter content) which will hold moisture and nutrients. An adequate moisture supply is required; particularly when plants are young. Irrigation may be an option but ground-based systems can be fed by natural ground water levels and as part of landscape maintenance. Hitchmough (2004) states that some large climbers can have the water uptake of a medium sized tree – roots in close proximity to building foundations, particularly on clay ground, can be problematic.
• Supports – size of support spacing must be proportional to plant ‘vigour’. Consider permanent and imposed loads, including those of the planted materials themselves (which can be significant). A plethora of proprietary wood and metal trellis products are available, which can be three dimensional in the form of frames and cages, along with cable, wires and ropes. Consider attachment methods as twining climbers can loosen fixtures.
• Maintenance – pruning, clearing dead growth and leaf litter and cutting back all need to be undertaken on a seasonal basis to prevent self-strangulation and manage undesired plant growth. If neglected, this will interfere with the building structure, and services and clog cavities or gutters. Bi-annual when initially planted then an annual inspection of planting is required, together with checking of supports and fixings at not more than five yearly intervals.
• Impact – climber species can be mixed to create interest and variety in the green facade. Consider evergreen and deciduous species, which provide a varied range of characteristics such as leaf size, shape, foliage colour and flowers.
• Growth behaviour – consider mode of attachment. Self-clinging climbers (e.g. Parthenocissus quinquefolia – Virginia creeper) will attach directly to the building façade with aerial roots (direct greening). Plants with twining stems, tendrils and leaves (e.g. Wisteria spp and Lonicera spp – Honeysuckle) will twist around supports and can be used to climb trellis work or other supports spaced off the façade. Other species include ramblers (e.g. Jasminum nudiflorum – Winter Jasmine and Rosa spp – climbing Roses) they use thorns or spines to attach and need to be trained and managed. Climbing plants are most commonly used (e.g. varieties of Hedera spp – Ivy, Hydrangea spp, Clematis spp). Climbing herbaceous perennials (e.g. hops), vines, wall shrubs and fruit trees are also used.
• Plant size – for large climbers a height of 24m is considered a realistic maximum growth with fast growing varieties covering three to four metres a year; this varies however, even within species. Supports (or the building where direct greening) need to be sized to match maximum growth of supported plants; otherwise overgrowth needs to be managed.
• Cost – these systems tend to be at the lower to middle end of the cost ranges for installing wall greening. Ongoing costs will depend on maintenance and any costs for supplying irrigation, water treatment and power supply.

It is widely recognized that established greening on a façade is a barrier to rain penetration and shades the façade, filtering out around 80% of solar radiation; benefits relating to lessened thermal stress and movement are possible.

Living wall systems

This approach provides instant impact whereas ground and some planter based approaches require time for the climbers to establish themselves. Living walls are vertical structures, which provide a means of containment and physical support for plants, along with their nutritional sustenance through feeding and irrigation mechanisms. Living wall systems are sometimes called ‘green walls’, ‘vertical carpet bedding’, ‘bio-walls’ or ‘vertical gardens’.

All systems jointly allow the greening of façades in any desired height, width or configuration; subject to limitations, systems include:

• Vegetated mat – these consist of layered felt, fabric matting or horticultural foam. The matting is cut to receive plants; attached felt bags are also common. Plants extend their root systems into this substrate. These systems are hydroponic; they rely on irrigation providing moisture and nutrients soaked into matting, which is permanently wet. While relatively lightweight and simple to install, there can be risks associated with hydroponics used in this way as roots can effectively get trapped in layers of saturated material causing root and crown rot (i.e. fungal diseases such as ‘Pythium’ – induced root rot). Irrigation needs to be aligned to choice of fabric used; some hold water, others shed it. This approach is specialist and requires tailoring of plants to moisture, fertilizer and nutrient levels and frequent monitoring. Water consumption is higher than other systems and interruption to supply can be fatal to plants. Felt systems can generally hold larger plants; according to the University of Staffordshire, this can even include some tree species. They also tend to have a lusher more vibrant look, mostly due to the use of water loving plants.
• Hanging pocket – to resolve the problems attributed to the above; designs are available where continuous felted natural or synthetic material is folded into pleated pockets. Plant roots and their growing medium are ‘wrapped’ to form a root liner; these are inserted into the hanging pockets and can be removed to allow for different arrangements or following plant mortality. The arrangement of contiguous pocket folds is intended to direct moisture down to plants from the front surface – this creates cycles of wet and dry, the intent of which is to limit mould growth. The system is said to be lightweight and is connected to a moisture resistant rigid board substrate and support frame.
• Modular living walls – are available in a range of configurations, usually rigid cellular plastics containers, but also substrate filled cages or solid boxes with pre-cut holes; gabions and rockwool units are also available. These hold the growing medium, topsoil or natural fibre (e.g. coconut coir). The benefit of modular walls is that plants can be pre-grown off-site in the unit under controlled conditions; this permits an immediate established effect when installed. A downside compared with the vegetated mat approach above is that the cellular nature of the modules limits the size and variety of plants. Another disadvantage compared to the above systems is the greater weight.

As with green façades, living walls have a range of requirements for their sustained health:

• Climate and orientation – The Royal Horticultural Society (RHS) suggests that selection should be kept simple, choosing plants with similar light and moisture requirements.
• Growing medium and nutrition:
  • Vegetated mat – purely hydroponic, water mixed with all required nutrients.
  • Hanging pocket – semi-hydroponic, comprising a mixture of compost and inert growing media (perlite, clay aggregate or ceramic media).
  • Modular walls –planting is rooted in a growing medium which is irrigated. The planting medium is a mixture of compost and inert material which, according to Dunnett, needs to be non-biodegradable to ensure its longevity. The medium supports the plant, permitting water, air and nutrient delivery, creating a buffer that reduces the constant management prevalent in hydroponic systems.
  • Irrigation – is mandatory for living walls and moisture levels are often digitally monitored:
    • On hydroponic systems water and nutrients are introduced at the top of the mat or panel which flows through the wall to a drain; on a closed loop system this is recirculated. Irwin (greenroofs.com) suggests that water loving plants are located at the base, which can end up being wetter longer. Multi-zone irrigation is suggested. There is a high degree of complexity in ensuring that emitter/ drip line spacing and flow rates ensure water uniformity across the wall (see Journal of irrigation and drainage engineering, April 2014).
    • With pocketed systems, large vertical panels are sub-divided into roughly 3 metre panels with separate drainage channels. The irrigation system supplies moisture and can also remove excess. As with the above, emitter spacing, water flow rate, application frequency and retention are linked to ensure moisture uniformity; this prevents desiccation.
    • Modular systems use weep hose drip irrigation; emitter pipes with capillary mats are an alternative. Irrigation is usually distributed on a multi-level basis at one metre intervals or split into module heights. Drainage at the base of each panel is also often integral.
    • Plant selection – Plant selection is a complex process requiring specialist horticultural knowledge and/ or experience to ensure species meet the design requirements and establish in the site conditions. The RHS website offers a range of hardy perennials, flowering and bedding plants, grasses, vegetables and fruits, herbs and ferns, which, amongst others, are suitable for green walls in a range of orientations. Plants need to be suitable to the system selected. System manufacturers can provide further guidance on suitable species for proprietary systems.
    • Containment – The design of systems needs to ensure that plants and the growing medium are held firmly in their pockets or sleeves; to resist wind erosion and plant loss. With modular systems this can be achieved with a capillary mat, around which roots are wrapped, to ensure plants are secured and vandal proof.
    • Weight – this information is based on manufacturer’s estimated and should be verified:
      • On hydroponic systems 60kg/ m² when saturated (whole system)
      • On pocketed systems 45kg/ m² when saturated (whole system)
      • On modular systems 72kg/ m² (does not include weight of secondary structure)
      • Cost – these systems are at the medium to high end of the costs range for wall greening.

Brown wall systems

This approach is not concerned with creating instant impact; the focus here tuning into naturally prevalent ecologies within an urban setting. In ‘Cities and natural process’ Hough describes the benefits and readiness in which greening occurs naturally, without human intervention, in a city (in North America):

“Nature is vigorous in cities. Despite the fact that cities reduce the amount of vegetation, some plants species (usually early succession stages of ecosystems) have migrated or hybridized for survival in the soil and climatic conditions of the city. In waste lands and old buildings White Poplar, Manitoba Maple, Dandelion, Chickweed and Yarrow can be found. Paved areas can be populated with mosses, grasses, walls and rooftops Dandelions, thistles and docs – particularly gaps, joins and cracks in materials. Ailanthus can grow out of foundations of buildings, around basements areas which provide moisture, warmth and alkalinity of soil.”

In some instances buildings can be overrun as plants find their way deep into fabric. Hough describes the unending struggle to control nature and maintain order – using ‘machines, fertilizers, herbicides and manpower’. Natural diversity is surrendered to landscape improvement whilst abandoned parts of the city fabric are abundance. To Hough urban ecology involves co-operating with natural processes. Further research on urban ecologies by Lundholm, for example in his ‘Urban habitat template’ (see urbanhabitats.org), shows that in an urban setting there are a set of plant species with their origins in rocky terrains (or other harsh habitats) which can be used to colonize the equally impervious territory of the city:

“Viewing building surfaces as potential habitats provides a guiding concept for understanding urban environments…the vegetation that spontaneously colonizes stone walls can be drawn from a variety of habitats but is dominated by cliff and rock outcrop species. The design of walls and other vertical surfaces determines the degree to which plants can grow on them: Building material, degree of shading, aspect, and the presence of microtopography determine the available niche space, much as they do on natural cliffs”.

In his analysis Lundholm cites authors like Rishbeth (1948), Woodell (1979) and Larson et al (2000 and 2004). Larson (et al Cliff ecology 2004) argues that common cliff dwelling species endemic to demanding environments such as Campanula portenschlagiana (Wall Bellflower) are now equally at home in the urban fabric, growing out of cracks in walls. Animals like rats, pigeons and bats have also followed. This, Larson proposes is the ‘urban cliff hypothesis’:

“…the first buildings were simply extensions of rock walls around the mouths of caves in rocky areas. It would have been easy for species originally restricted to rocky environments to opportunistically exploit the expanding rock-wall habitats created by growing human populations that built more of their own optimal habitats (rock shelters) as they moved out of the caves.”

Larson states ‘Cliffs that are made as a result of human activity [i.e. walls] share most if not all of the microclimatic properties of natural cliffs.’ Man-made walls are likened by Larson to isolated stacks and islands, arid and subject to severe temperature variation. Lisci and Picini (1993) illustrate diagrammatically (in Italian towns) on man-made walls that plant microhabitats may be found and growth can occur, the process by which plant germination occurs on buildings along with survival criteria and likely conditions required is defined (see ‘Plants growing on the walls of Italian towns’ (.pdf) ).

The process of colonization usually happens by chance where windblown dust and animal droppings accumulate, with additional moisture and nutrients present in natural stone or other walling; these materials can create sufficient conditions for mosses to grow which preserve a degree of moisture for subsequent plants and trees to gain a foothold – these appear and die creating basic accumulated material for further soil and more growth. Randomly scattered seeds are usually carried by birds and pollinators; once plants are established this attracts yet further wildlife to create arguably an ecology or habitat of sorts.

The author would suggest that there is enough evidence from the above that a third vertical greening option is equally valid. This would be a low tech economic, but no less controlled, approach to populating buildings with plants; and the subsequent insect and animal life that follow – to create the ‘urban cliff’. Such an approach would not require the infrastructure described for some of the other systems above but, obviously, would not generate the instant luscious impact that clients demand either. However, Lundholm suggests that urban settings adapt a more ‘nuanced’ approach to habitat, community, environmental conditions and ecology based at least in the first instance on species that are prevalent to that setting, and working from there. Hough states that:

“Hardy plants adapted to city conditions can survive in hostile environments with very little cost and without adding to the weight of existing or new build structures…the creation of a new and economical landscape…”

Such an approach could be deliberate, created in much the same way as a brown roof is constructed – hence ‘brown wall’.

The next part in this series investigates the performance of the façade greening systems mentioned in previous parts, with reference to a range of recent research projects.