Green walls and biodiversity: investigating green façades for invertebrate abundance and diversity

RHS project team

Andrew Salisbury, Tijana Blanusa, Helen Bostock

Partners

Joe Perry (consultant statistician)

Start date

03/01/2016 00:00:00

Keywords

Green walls, Biodiversity, species richness, invertebrate abundance, Greening Grey Britain

The problem

Green façades (the simplest form of green wall, in which vegetation grows on or against vertical surfaces) can be a valuable component of urban green infrastructure, which also includes public green space, allotments, green corridors, street trees, urban forests, and private gardens.

Green infrastructure can provide a number of benefits to residents and the wider environment. These include temperature regulation, reducing the likelihood of localised flooding, trapping of gaseous and particulate pollution, providing habitat and resources for wildlife, and benefits to human wellbeing. Green façades are a valuable part of this resource; achieving greening in areas where ground space is at premium – it is known that green façades trap pollutant particles on urban streets and can protect the walls they cover.

The potential for green façades to support biodiversity is considerable; for example by providing feeding, breeding and resting sites. However, whilst the value of gardens and other urban green spaces has been repeatedly demonstrated, the value of green façades for biodiversity is much less studied.

Project summary
Plants growing against walls (green façades) are an important part of urban greening. This study used a set of replicated mini model buildings to measure and compare the potential biodiversity benefits of three plant species that are commonly grown as green façades in temperate climates. The abundance of invertebrates on each of these was assessed over two growing seasons.

Invertebrate abundance was found to increase with wall vegetation depth and cover, and considerably more invertebrates were collected from vigorous/deeper leaf wall cover by ivy (Hedera helix) compared to the other treatments. A combination of two ivy taxa showed a higher invertebrate abundance compared to H. helix alone.

This experiment demonstrated that green façades on buildings support more invertebrates than bare walls, and that the more vegetation there is, and the more varied it is, the more invertebrates are supported. Of the plant species studied, the green façade that supported the greatest abundance of invertebrates was a combination of Hedera helix (common ivy) and H. helix ‘Glacier’.

“Greening walls with climbers and other plants creates places for a wide range of invertebrates to live and feed. In urban areas, where many people have little space to garden, they can bring huge benefits”
– Andrew Salisbury

Approach

In order to assess the abundance and diversity of invertebrates on green façades, an experimental approach was used to compare the value of three frequently used green façade plants; one of which was included as two different cultivars. Invertebrate abundance was measured on existing plots at the University of Reading using a Vortis suction sampler.

The plots were vegetated with four plant treatments, consisting of three species (with two cultivars of one of the species) arranged in a randomised design. There were four replicate buildings (60x50x50cm,) for each treatment. The treatments were (1) Hedera helix (common ivy); (2) a 50%−50% mix of H. helix and H. helix ‘Glacier’, (3) Parthenocissus tricuspidata ‘Veitchii’ (Virginia creeper); and (4) Pileostegia viburnoides (climbing hydrangea); along with control ‘buildings’ that featured no plants.



In order to obtain a set of data representative throughout the growing season, invertebrates were sampled between April and October on ten occasions between July 2016 and July 2018. Invertebrates were sampled using a Vortis suction sampler, with the nozzle held steady within the foliage in the centre of the ‘building’ wall for 10 seconds on each side (i.e. 40 seconds per building). On each occasion, all buildings were sampled on the same morning in a random order, by the same operator.  Invertebrates were then sorted and identified in the laboratory. Identification was to at least functional group level (e.g. predator, herbivore, detritivore, omnivore). All specimens were stored, enabling further identification and investigation into species richness if required. 

In addition to invertebrate collection the depth of vegetation was recorded as an additional explanatory variable (co-variate).
 

(Click to enlarge)

Aims

This project aimed to investigate whether there are differences in invertebrate abundance between different species (and cultivars) used as green façades.
 

Benefits to gardeners

Taking up very little ground space, green façades are an easy way to provide for wildlife and reap a range of other benefits in almost any outdoor space. We can now provide evidence-based information on the planting and management of green façades to encourage biodiversity. A few key tips are below:
 

• Any vegetation on a wall is beneficial to invertebrates compared to a bare wall.
• The more vegetation cover you have, and the deeper it is, the more wildlife you can support.
• Using a combination of climbers may provide greater benefits than one type of climber alone.
• Ivy (Hedera helix) is a good choice for a green façade in terms of supporting invertebrates. Growing a combination of ivy cultivars is even better.

Previous research by the University of Reading and the RHS has also shown that ivy can keep buildings cooler in summer and reduce damp in winter (Thomsit-Ireland et al. 2020).

Project lead Andrew Salisbury, RHS Head of Plant Health, says: “Greening walls with climbers and other plants creates places for a wide range of invertebrates, including beetles, springtails (which recycle decomposing matter) and a variety of predatory invertebrates, to live and to feed. In urban areas where many people have little space to garden they can bring huge benefits – not only supporting invertebrates, but providing shelter and food for birds and other wildlife too.”
 

Summary of results

The experiment has shown that green façades can provide a resource for plant-inhabiting invertebrates from a range of primary functional groups, including herbivores, predators and detritovores. This indicates that this type of greening can support a good ecological balance of invertebrates, and therefore the function of these green façades; for example predators keeping herbivores in check and detritovores in nutrient recycling and avoiding the build-up of dead material. Invertebrate abundance will also support other parts of the food web such as birds.

In addition, the greater the amount of vegetation, the higher the abundance of all the invertebrate functional groups, regardless of plant species. Invertebrate abundance is a good representation of diversity, reflecting the value of green façades for biodiversity. This will help private and public building owners maximize their space for wildlife as well as designers and developers to meet requirements such as those for the UK’s biodiversity net gain strategy and local planning level to green neighbourhoods.

Plant selection has an effect on the amount of biodiversity that is supported. Whilst this study only compared three plant types, it was clear that common ivy (Hedera) supported higher invertebrate abundance than Parthenocissus tricuspidata and Pileostegia viburnoides. Additionally, ivy grown as a mix of two cultivars supported a higher abundance of invertebrates, indicating that plant mixes may provide even greater benefits to biodiversity.

The results of this experiment were published in Urban Forestry & Urban Greening Volume 89, November 2023 (128118): Careful plant choice can deliver more biodiverse vertical greening (green façades). The full article can be accessed here.

We are using our new knowledge to update the advice we provide to gardeners and managers of green façades.
 

Further information

• Salibury A, Blanusa T, Bostock H and Perry JN (2023): Careful plant choice can deliver more biodiverse vertical greening (green façades).Urban Forestry & Urban Greening 89 (128118)
• Thomsit-Ireland F, Essah EA, Hadley P and Blanusa T (2020): The impact of green facades and vegetative cover on the temperature and relative humidity within model buildings. Building and Environment 181 (107009)
• Green walls – RHS advice
• Climbers and wall shrubs  – RHS advice
• Greening Grey Britain
• University of Reading School of Agriculture, Policy and Development