OUTDOOR LIVING WALL SYSTEMS EXPLORED
Presentation at the 1st European Green Urban Infrastructure Conference, Vienna, Nov. 2015
by Mechant Els, Goossens Pieter & Gobin Bruno
Six commercial outdoor living wall systems were monitored and evaluated for 2 years to determine their strengths and weaknesses regarding substrate, fertigation, plant selection and maintenance, technical issues and sustainability.
Living wall systems (LWS) have great potential for (re)greening cities. Nevertheless, their implementation is still marginal and not always successful. A practical and objective evaluation of commercially available systems is needed, as knowledge of strengths and weaknesses of each system allow further optimisation and elaborated plant selection.
Therefore, pilot-installations (4m²) of six systems were installed in 2013 at PCS Ornamental Plant Research: PlantDesign, Wallflore-Per-E, 90-Green (Vertical Green Company), LivePanel-Outdoor (Mobilane), Flexipanel (Sempergreen), and Muurtuin.be.
The effect of orientation, plant selection and fertilisation between systems was minimized through the experimental set-up. From start till present, following parameters were monitored and evaluated: water and nutrient consumption, plant growth and maintenance, weediness, water distribution and retention, ornamental value, and technical issues.
The results allow a substantiated choice for the best LWS fitted to each individual green infrastructure project as well as an optimal plant selection for the chosen system. Better choices will result in better performance and, thus, stimulate the implementation of LWS in cities. In addition, discovered weaknesses stimulate producers to optimize their system. During current and future experiments, additional systems will be evaluated, and more plant species will be tested to determine which plants are best fit for each system.
Monitoring and practical evaluation for implementation in cities
By now, everyone is aware of the great importance of (re)greening our cities. More green in the city will first of all temper the urban heat island effect and will create a nicer, cooler microclimate between bricks and walls. In addition, green infrastructure can facilitate the adaptation of cities to other effects of climate change, for example by providing additional water buffering and retention after heavy rains. And last but not least, plants have a beneficial effect on general wellbeing and health. As space is often a limiting factor to introduce more green in the city, outdoor living wall systems (LWS) have a great potential for (re)greening the city and increasing the green area.
Nevertheless, the implementation of outdoor LWS is still marginal and not always successful. Moreover, in Flanders (Belgium) we noticed that some unsuccessful examples of LWS led to a negative public opinion regarding green walls. To counter this bad reputation, PCS Ornamental Plant Research wanted to gain experience in some commercially available systems and learn more about the strengths and weaknesses of the different LWS. More information on substrate, fertigation (i.e. irrigation combined with liquid fertilisation), plant selection, maintenance, technical issues and sustainability would allow us to optimise LWS and the implemented plant assortment.
In spring 2013, we started a pilot-installation on the PCS-site with five outdoor LWS: PlantBox (PlantDesign), 90°Green (Vertical Green Company), LivePanel-Outdoor (Mobilane), Flexipanel (Sempergreen) and Muurtuin (Muurtuin.be) (1). Each LWS covered approximately 4 m². Side-effects were minimised through the parameters orientation, plant selection and positioning, and irrigation and fertilisation:
- All LWS were South-West orientated and equally shaded.
- Selected plant species were: Campanula poscharskyana, Dianthus deltoids, Euonymus ‘Emerald n Gaiety’, Waldsteinia ternata, Bergenia cordifolia and Heuchera. Unfortunately, two LWS had a deviating assortment: Flexipanel was installed with a pre-cultivated plant selection including Euonymus ‘Emerald n Gaiety’, B. cordifolia and Heuchera, whereas the panel of Muurtuin only contained C. poscharskyana, D. deltoides, Euonymus ‘Emerald n Gaiety’ and W. ternata.
- The position of each plant species was scattered throughout the whole panel. However, plant density differed between LWS and was based on the density used during commercial installation.
- All LWS were irrigated as needed using a time-based steering-system that was regularly adapted based on visual control and moisture-measurement of the panels. As some systems work with sensor-based irrigation when they are installed in a commercial set-up, our irrigation-system was considered a weak point in the experimental set-up. Consequently, irrigation of some systems was not optimal.
- Liquid fertiliser was mixed and applied with irrigation water. We aimed to level the total amount and composition of fertiliser between all LWS.
From spring 2013 until autumn 2015, we measured and evaluated water distribution within the panel (using a mobile W.E.T. sensor calibrated for each substrate), plant growth (measuring height and width of individual plants), weediness and need for maintenance, technical issues and ornamental value. The examined systems and our main conclusions for each, and for LWS in general, are discussed and visualised below.
Figure Pilot-installation of different outdoor living wall systems (LWS)
at PCS Ornamental Plant Research (Destelbergen, Belgium).
PlantBox is a system based on Sphagnum (peat moss) substrate. The front of the panel has no cover material. In the pilot-installation, plant density was 40 plants/m². With 20 to 25 minutes of irrigation every 4 to 9 days, irrigation frequency was rather low. Nevertheless, the water distribution was quite well: mean humidity of the panel was 24.3% and on average there was a difference of 6.4% in mean humidity between top and bottom of the panel (i.e. 2 m height).
2 gives an overview of PlantBox from spring 2013 till summer 2015. The Sphagnum substrate (3) has a natural look and can retain a high volume of water while oxygen stays available for the plant. Consequently, water buffer capacity is good and no frequent irrigation is needed, making this a robust LWS. The downside of the natural look is the sensibility of the panel to weediness: seeds can easily germinate in the substrate and are difficult to remove without simultaneous removal of Sphagnum. We registered a good plant growth on PlantBox (3), even after winter. Unfortunately, the panel got infected with vine weevil (Otiorhynchus sulcatus) during summer 2014, resulting in a considerable plant loss of Waldsteinia ternata, Bergenia cordifolia and, in particular, Heuchera. Despite this fall-out, the remaining plants grew well.
Figure Evolution of LWS PlantBox (PlantDesign). In summer 2014, vine weevil caused great plant loss.
Figure Detail of LWS PlantBox (PlantDesign).
The Sphagnum substrate without cover material gives a natural look to the panel.
Figure Evolution of plant growth on LWS PlantBox (PlantDesign), expressed as mean plant surface area (dm²) based on measurement of length and width of at least 10 plants per species. *: Decreased plant surface area after pruning.
!!: Decreased plant surface area and/or plant death after infection with vine weevil.
90°Green (Vertical Green Company)
The panel of 90°Green consists a thick layer of substrate, based on Fytocell (a biodegradable foam) and Argex (expanded clay grains), covered with canvas. In the pilot-installation, plant density was 43 plants/m². Irrigation frequency varied between 2x5 minutes a day in summer and 1x10 minutes a week in winter. This resulted in a mean humidity of 24.7% and an average difference of 4.9% in mean humidity between top and bottom of the panel (i.e. 2 m height).
5 gives an overview of 90°Green from spring 2013 till summer 2015. The substrate mixture of Fytocell and Agrex results in a very good water retention without oxygen problems. As the substrate layer is thick, water buffer capacity is high and no frequent irrigation is needed. Consequently, 90°Green can be considered as a robust LWS. Although the canvas gives a more or less natural look to the panel (6), the material also allows weed seeds to germinate on the surface. We registered a very good plant growth on 90°Green (7). After winter there were some empty spaces but soon the remaining plants recovered nicely. An infection with vine weevil in the summer of 2014 resulted in growth inhibition of Waldsteinia ternata, Bergenia cordifolia and Heuchera.
Figure Evolution of LWS 90°Green (Vertical Green Company).
Figure Detail of LWS 90°Green (Vertical Green Company).
Figure Evolution of plant growth on LWS 90°Green (Vertical Green Company), expressed as mean plant surface area (dm²) based on measurement of length and width of at least 10 plants per species. *: Decreased plant surface area after pruning. !!: Inhibition of plant growth due to infection with vine weevil.
Anno 2016, LivePanel-Outdoor is no longer on the market. The system was quiet expensive (approximately 1000 €/m²) and Mobilane replaced it with a cheaper but better system based on cassettes placed in a gutter profile that also serves as a water buffer through capillary water uptake. The LivePanel-Outdoor panel, examined in the pilot-installation, has a rockwool substrate covered with a fire resistant panel. Plant density was 66 plants/m². As the water buffering capacity of rockwool is limited, a frequent irrigation was needed (7x2 to 4x7.5 minutes a day), resulting in a mean humidity of 50.2%. Water distribution within the panel was not optimal (10.5% difference in mean humidity between top and bottom, i.e. 2 m height) and, despite the frequent irrigation and high mean humidity, the lower side was often too dry. This was meanly due to an inadequate steering of the irrigation.
8 gives an overview of LivePanel-Outdoor from spring 2013 till summer 2015. Although the fire resistant panel reduces weediness, the material gives an unnatural look to the system. In addition, plug plants were used which prolonged the time before the LWS was fully grown and visually attractive (9). The water retention of the rockwool layer was very small, making it difficult to optimise irrigation and fertilisation. Consequently, plant growth was rather slow but steady, except for Dianthus deltoides and Campanula poscharskyana (10). D. deltoides was clearly not suited for LWS LivePanel-Outdoor and most plants died within a year (see bottom-left and up-right corners of the panel on 8). The decreased growth of C. poscharskyana during autumn and winter of 2014-2015 could not be explained.
Figure Evolution of LWS LivePanel-Outdoor (Mobilane). After a few months, Dianthus deltoids plants
(left corner below and right corner on top) died without a clear reason.
Figure Detail of LWS LivePanel-Outdoor (Mobilane). The fire resistant panel reduces weediness but as plug plants grow slowly
it takes time before the desired visual effect is reached.
Figure Evolution of plant growth on LWS LivePanel-Outdoor (Mobilane)(planted with plug plants), expressed as mean plant surface area (dm²) based on measurement of length and width of at least 10 plants per species. !!: Decreased plant coverage after inhibited growth in autumn and winter.
Flexipanel comprises a thin layer of rockwool covered with a capillary membrane. The thin rockwool-layer demanded a high irrigation frequency (7-9 times a day for 0.5-3 minutes) because water and nutrient buffering capacity was very low. Consequently, Flexipanel is very sensitive to technical errors and a good follow-up of plant growth and humidity is needed. For this reason, Sempergreen monitors its commercially-installed panels through wireless moist-sensors and offers a maintenance contract after installation of its LWS. During our experiment, we measured a mean humidity of 27.0% and an average difference of 9.2% in mean humidity between top and bottom of the panel (i.e. 2 m height). An advantage of the thin rockwool-layer is its small weight. In the pilot-installation, plant density was 100 plants/m² (i.e. almost twice the density of the other discussed LWS) and plants were pre-cultivated on the panel. In contrast to the LWS discussed above, Flexipanel was planted with a variety of plant species, including Euonymus, B. cordifolia and Heuchera.
11 gives an overview of Flexipanel from spring 2013 till summer 2015. The pre-cultivated plants and high plant density result in an immediate visually attractive green wall (12). We registered a very good plant growth for Bergenia cordifolia and Heuchera. Euonymus, however, was slightly inhibited in its growth, probably due to competition for place and nutrients with the other plants in the assortment (13). During the experiment we noticed some fall-out but the high plant density made this hardly visible.
Figure Evolution of LWS Flexipanel (Sempergreen).
Figure Detail of LWS Flexipanel (Sempergreen).
The pre-cultivated plants and high plant density result in an immediate attractive green wall.
Figure Evolution of LWS Flexipanel (Sempergreen), expressed as mean plant surface area (dm²) based on measurement of length and width of at least 10 plants per species. !!: Decreased plant growth and vitality due to competition with other plant species.
Muurtuin is a system where plants are planted in canvas pockets (15). In and behind the pockets is a substrate layer of peat mixed with lava granules, and an additional layer of rockwool. Plant density was very low, only 25 plants/m², and the assortment only contained four species (Campanula poscharskyana, Dianthus deltoides, Euonymus ‘Emerald n Gaiety’ and Waldsteinia ternata). The irrigation frequency varied between 2x7 minutes a day in summer and 4 minutes every 4 days in winter. This resulted in a mean humidity of 36.5% and a difference of 28.0% in mean humidity between top and bottom (i.e. 2 m height). The latter indicates that water distribution within the panel was poor and water accumulated on the lower end of the panel.
14 gives an overview of Muurtuin from spring 2013 till summer 2015. The canvas gives a natural look to the system and, despite the low plant density, the panel was almost fully covered after a few months. Analysis of drainage water from the panel indicated that some materials can cause problems in relation to the re-use of drainage water or its discharge to surface water: the iron frame used to support the system released zinc in the drainage water. The combination of organic substrate (peat and lava granules) and additional rockwool-layer resulted in a good water buffer and nutrient retention. Although water sometimes accumulated on the lower end of the panel, the lava granules guaranteed sufficient oxygen at all times. Consequently, plant growth was very good and maybe even ‘too good’ (16). As discussed above, the total amount and composition of fertiliser was similar for all examined LWS. Because nutrients were better available for the plants in the Muurtuin-substrate, fertilisation in this system was actually too high, resulting in a very good plant growth (especially Campanula poscharskyana), a lot of plant competition and an increased need for maintenance and pruning.
Figure Evolution of LWS Muurtuin (Muurtuin.be).
Figure Detail of LWS Muurtuin (Muurtuin.be). Plants are planted in individual canvas pockets.
Figure Evolution of plant growth on LWS Muurtuin (Muurtuin.be), expressed as mean plant surface area (dm²) based on measurement of length
and width of at least 10 plants per species. *: Decreased plant surface area after pruning.
When comparing our experience with the five LWS discussed above, we can conclude that substrate plays a key-role within the system. More ‘organic’ substrates (as used in PlantBox, 90°Green and Muurtuin) give a natural look to the system and, most importantly, provide a good water and nutrient retention and buffer. This results in a more robust LWS that has a lower demand for monitoring of plant growth and irrigation-steering. On the other hand, ‘organic’ substrates tend to be more sensitive to weediness and degradation of the used materials. Inert substrates like rockwool (as used in LivePanel-Outdoor and Flexipanel) create a fully controlled environment were plants are actually grown in a hydroculture-system. The water and nutrient buffer in inert substrate is very low. Consequently, a good steering of fertigation is important and the sensitivity to technical errors is high. In addition, periods of drought might result in loss of water retention capacity of rockwool. When investing in LWS or trying to optimise LWS, one has to make the choice between an optimal substrate (i.e. high water and nutrient buffer) with a simple and cheap fertigation-system, or a basic inert substrate (i.e. hydroculture) with a high-tech sensor-based but expensive fertigation-system. While the first option will be ideal for smaller LWS-installations or installations maintained by the owner, the latter option will be suited for large LWS-installations monitored and maintained through a maintenance-contract. Indeed, at present the high-tech fertigation-systems are too expensive to be cost-effective in small LWS-installations.
The future still brings a lot of opportunities for research and optimisation of LWS, including:
- development of new substrates or substrate-additives that enhance water and nutrient retention;
- fine-tuning of sensor-based irrigation to enhance water and nutrient uptake and to reduce the amount of drainage water to zero;
- screening plant assortment for each LWS (as some plant species are not suited for each system, cf. Dianthus deltoides on LivePanel-Outdoor);
- optimisation of fertilisation to compact plant growth resulting in less pruning and decreased plant competition;
- avoiding weediness (e.g. inert cover on natural material);
- gathering more data on the effect of frost and ways to cope with it;
- reducing cost for irrigation by optimising water use efficiency by the plants, minimising drain and/or facilitating recirculation (if necessary after water treatment or disinfection);
- gathering more data on the preservation of substrate characteristics (e.g. water retention of rockwool after drought, effect of frost on Fytocell, degradation of organic materials …);
- developing clear guidelines and legislative framework for installation and maintenance (e.g. fire resistance …).