Green Walls and Biodiversity: Investigating green walls for invertebrate abundance and diversity

RHS project team
Andrew Salisbury, Tijana Blanusa, Rosalyn Marshall, Fiona Davison, Tony Dickerson
Joe Perry (Consultant statistician)
Start date
01/03/2016 00:00:00
End date
01/12/2018 00:00:00
Green walls, Biodiversity, species richness, invertebrate abundance, Greening Grey Britain
The problem
Green walls (vegetation growing on or against vertical surfaces) form part of urban green infrastructure (which also includes public green space, allotments, green corridors, street trees, urban forests, private gardens and green walls).

This green infrastructure can provide a number of benefits to residents and are key to the RHS Greening Grey Britain Campaign. Benefits include regulation of temperatures, reducing the likelihood of localised flooding, trapping of gaseous and particulate pollution, to name just a few. Green walls are a valuable part of this resource, they achieve greening in areas where ground space is at premium; it is known that they offer particle pollutant sequestration on urban streets and can protect the walls they cover. The potential for green walls 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 demonstrated the value of green walls for biodiversity remains understudied.

In 2016 a pilot study sampled invertebrates on existing experimental model building plots at the University of Reading. The pilot provided provisional data comparing the abundance of invertebrates on three frequently used green wall (façade) climbing plant species and between two cultivars of one species. The results indicated that green walls support a wide range of invertebrates and that the more plant material present the more invertebrates are supported; there were also indications of differences between the different plant species and cultivars. However, the pilot study did not provide enough data to make robust conclusions.
In order to assess the abundance and diversity of invertebrates on green walls an experimental approach will be used comparing the value of three frequently used green wall plants.

Invertebrate abundance will be measured on existing plots at the University of Reading using a Vortis suction sampler.

The plots are at the glasshouse grounds complex at the School of Agriculture, Policy and Development, University of Reading. The plots are vegetated with four plant treatments consisting of three species (with two cultivars of one of the species) arranged in a randomised design. There are four replicate buildings (60x60x60 cm,) for each treatment. The treatments are No plant, Hedera helix L., H. helix ‘Glacier’, Parthenocissus tricuspidata ‘Veitchii’, Pileostegia viburnoides Hook. f. & Thomson.

In situ green walls growing up buildings and walls suitable for sampling on the University of Reading campus will also be identified to provide a comparison with data from the plots.  Protocols for sampling will be developed once suitable sampling sites have been identified.

In order to obtain a set of data representative throughout the growing season recording will take place on five occasions at approximately six week intervals.

Invertebrate recording will be done using a Vortis suction sampler. 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). All buildings would be sampled in a day. Invertebrates would be sorted and identified in the laboratory. Identification will be to at least level that allows allocation of a functional group (e.g. predator, herbivore, detritivore, omnivore). All specimens will be stored enabling further identification and investigation into species richness if required. 
In addition to invertebrate collection several plant parameters will also be recorded as potential additional explanatory variables (co-variates). These will include; depth of leaf cover, visual estimate of leaf cover and availability of floral resources (a visual estimate of the number of flowering units). 
(Click to enlarge)

Project limitations and future work.
This project will identify priorities for future work investigating the value for biodiversity of green walls. The designed experiment part of this project tests plants used as green wall (façade) which are self-supporting. There are several other species of plant which are commonly grown as green walls and this project does not compare mixtures. A wide diversity of cultivars, with an array of characters (e.g. leaf shape and size), are available to grow and may have different wildlife values. However, it will allow an initial contrast and enable comparisons of plant structure.  There are also other types of green wall, including those where plants grow from within the wall or in structures attached to walls. To enable insights into some of these questions sampling real green walls (at least at the University of Reading) will be carried out.
This project will indicate if there are differences in invertebrate abundance between different species (and cultivars) used as green walls.
Benefits to gardeners
The RHS will be able to provide evidence-based information on the planting and management of green walls to encourage biodiversity.
Summary of results
We will publish the results of this experiment in the scientific literature and write interpretive pieces accessible to the gardener in The Garden and the wider gardening press. There will also be  displays in the RHS Wisley Garden Library which will share our results and point visitors to other sources of information on green walls. We will use our new knowledge to update the advice we provide to gardeners and managers of green walls.
Further information
Climbers and wall shrubs  – RHS advice
Green walls - RHS advice
Greening Grey Britain
University of Reading School of Agriculture, Policy and Development

Staff profiles

Andrew Salisbury
Tijana Blanusa

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