Urban Nature Planting Guide research and development
The Urban Nature Planting Guide was initially developed by ecologists and horticulturalists from the University of Melbourne in response to the lack of public information on the performance of many native understorey species in urban environments and the consequent limited use of these species in public open spaces by landscape architects. The Urban Nature Planting Guide was intended for landscape architects and other urban design professionals. Its evidence-based recommendations can assist the design and plant selection of future streetscape renewal and similar projects to improve biodiversity and ecosystem services while being successful and cost effective.
Plant selection was based on a series of criteria that included: 1) ability to perform well in the streetscape, 2) horticultural attributes and 3) provision of important resources for target fauna. Streetscapes are hostile environments for plant growth. Altered soil structure and chemistry, reduced water availability and increased solar radiation (Volder 2010), coupled with the horticultural challenges of many Australian species, makes recreating the indigenous vegetation communities typical of local remnant habitats on streets impossible (Williams et al. 2009; Threlfall et al. 2015). Instead, a fundamental requirement for inclusion of plant species in the Urban Nature Planting Guide was tolerance of the environmental constraints of streetscapes. Plants were also selected based on having growth forms and lifecycles suitable for streetscapes, and for provision of important biodiversity resources for target fauna; this contrasts with many approaches to plant selection where ornamental and visual traits are of higher priority. Consequently, the majority of plants listed in the Urban Nature Planting Guide were perennial herbs, grasses and low-growing shrubs and most, but not all, were indigenous to the greater Melbourne area. The ability of native species to establish self-sustaining populations was also highly desirable as this could reduce maintenance costs and increase the likelihood of these plants spreading beyond the streetscape.
The City of Melbourne identified birds, bees and butterflies as priority target fauna because they were expected to receive greatest benefit from the biodiverse streetscape plantings and are of interest to the public. Plants were selected based on documented scientific evidence that they could provide important resources for target fauna, or alternatively, the expert opinion of researchers. Resources included nectar and pollen for pollinating insects (Latty and Threlfall 2016) and birds (Low 2002); larval food plants for local butterflies (Kurylo et al. 2020); nesting material for birds (Pellissier et al. 2012) and solitary bees (Threlfall et al. 2015; Latty and Threlfall 2016); seed and fruit for birds; and protection for small birds in the form of dense, spiny shrubs (Adams 2015). The horticultural attributes and tolerances were obtained from the Burnley Plant Guide (University of Melbourne 2016) and the expert opinion of researchers involved in the project. A preliminary list of plant species was peer reviewed by internal and external expert horticulturalists. To promote biodiversity friendly urban plantings, the Urban Nature Planting Guide was then developed into a publicly-accessible online database (melbourne.vic.gov.au/plantingguide) with over 100 species, and including plant selection filters (e.g. based on growing conditions, plant characteristics and biodiversity attributes) to assist users to choose suitable species.
Design And Construction Of The Ecological Design Experiment
The ecological impact of planting species from the Urban Nature Planting Guide on streets was tested using a Before-After-Control-Impact (BACI) experiment (Eberhardt 1976). The diversity and abundance of target fauna were surveyed before and after planting species from the Urban Nature Planting Guide at four biodiversity ‘impact’ sites (Fig. 2), and at three similar ‘control’ sites selected for each impact site (n = 12). Impact sites were selected by balancing the concerns of scientists (e.g. that plantings should be close to existing habitat to increase the likelihood of colonisation), with the budget and resources available, and the existing schedule of streetscape upgrades. Control sites were as similar as possible to impact sites, but to better account for the variable and dynamic nature of the urban landscape, rather than the standard single paired control site we used three for each impact site. Plant performance assessments (i.e. health and appearance) were undertaken immediately after planting, and at 12-week intervals thereafter over a 12 month period.
Landscape architects from the City of Melbourne used the Urban Nature Planting Guide to prepare a planting plan for each streetscape. Rather than a prescriptive site-specific planting design and schedule outlining the location of every individual plant on site, these indicated generalized arrangements of plants using ‘drifts’ or groupings based on life form and growth characteristics, with varying densities and proportions (e.g. 15% medium shrubs, 15% small shrubs, 15% grasses/tussocks, 25% ground covers and 30% wildflowers). This planting strategy was adopted to minimise maintenance requirements (e.g. plants with comparable growth habits typically require similar maintenance inputs) and to place species together that provide similar resources. It also provided contractors the flexibility to respond to specific site conditions and plant availability.
Because the project was novel, complex and evolving, the construction of the streetscape plantings was supervised by a senior project manager with knowledge in horticulture, landscape management and streetscape projects. Site preparation differed from standard streetscape planting specifications and protocols. Sites were prepared to ensure that the soils favoured native plants, and one site also had irrigation installed. For example, at the Clowes Street site the existing bitumen footpath was dug up and new topsoil imported (Fig. 2B). A 20 mm stone aggregate mulch to a depth of 50 mm was applied over the topsoil and herbicide spraying was used to eradicate weeds prior to planting.
The species planted were influenced by market availability. Consequently, they may have not been of local provenance even though that would have been desirable. Not all species selected from the Urban Nature Planting Guide for inclusion were available from suppliers at the time, and substitutions were made by the planting contractors without consultation with the internal project manager. In some cases, these substitutions were unsuitable for use or of poor quality, with many dying post-planting. This highlighted four key lessons: 1) species that are uncommon in cultivation, as many were, or are needed in large quantities and require longer production times, are best delivered through contract arrangements with nursery suppliers to avoid planting delays; 2) specialist contractors with horticultural knowledge and experience with native or indigenous species should be engaged by the project manager; 3) proposed substitutions by planting contractors need to be communicated to, and be approved by, those delivering the outcomes of the project; and 4) species identity and plant quality should be inspected and verified prior to installation. It is essential that the last two key lessons are clearly defined in the contractual agreement with the planting contractor.
Ultimately, construction across all sites was delayed by approximately one year. This was due to multiple factors, including ordering and administration, poor site preparation including inadequate herbicide application, planting season challenges, and uncertainty around budget cycles and project management. These challenges highlight the complexities of these types of projects and the need for an adequate project planning phase and careful project management, including having a project manager on board early to ensure that major delays are avoided.
The Importance Of Maintenance
A clear, affordable, long-term (5–10 years) maintenance plan that ensures delivery and persistence of the intended planting design and that outlines ecologically sensitive maintenance techniques, their frequency and timing, is required for success. Because of the experimental nature of the plantings, we accepted a higher risk of plant failures but intentionally integrated more ‘risky’ plants into the least publicly prominent sites. Approximately 16 hours of maintenance per month occurs across the four biodiverse streetscape sites combined. Maintenance is undertaken by an externally contracted horticultural operations team. Tasks include
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temporary retention of weeds (i.e. defined as ‘unintentional plantings’ for this project) that are considered highly beneficial to insects;
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fostering natural recruitment of plantings whenever possible through appropriate weed and pest management practices;
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reducing the frequency of pruning in order to retain habitat;
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spot spraying weeds only in open areas and edges to avoid off-target damage to native plants and insects;
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minimising soil disturbance;
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removing leaf litter only when it is clearly having a detrimental effect to plants;
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manual hand weeding; and
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deadheading of weeds to reduce weed-seed load.
Because the horticultural operations team is maintaining native vegetation for biodiversity outcomes, adequate plant identification skills and relevant horticultural knowledge is critical. Staff contracted to manage standard streetscapes and parks typically do not have the skillset or experience required to effectively manage diverse native plantings in urban settings. Overcoming this problem, for example through upskilling staff, would result in more effective maintenance and better, more cost-efficient project outcomes. Moreover, it is important to regularly share knowledge between horticultural maintenance teams and the ecologists monitoring the biodiversity values of the sites in order to adapt management as required.
Good soil preparation is essential to minimising the long-term costs of weed management. In some cases, retrofitting a streetscape site for biodiversity planting may be impractical due to high weed loads in the existing soil and the high cost of excavation and disposal. Herbicide spraying was used to eradicate weeds prior to planting but efforts were only partially successful due to persistent weed seedbanks in some retained soil and weed tubers in some imported soil. More effective weed management practices (e.g. repeated herbicide spraying commencing earlier on) implemented during site preparation would have substantially reduced overall costs. A critical evaluation of the weed flora present will help determine the weed treatments that will be most successful (e.g. scalping, herbicide applications or hand removal). Addressing these issues early will be valuable for informing stakeholders from other disciplines involved later throughout the project.
Location can also influence maintenance costs and ease. For instance, the stricter safety and traffic control requirements at the Arden Street site located on a median strip in the centre of a busy road (Fig. 2A), required additional maintenance resources. Traffic Management Plans need to be undertaken and the safety of maintenance staff needs to be considered early in the planning phase, and may limit site selection.
Analysis of total project costs over a five year period from project inception found that maintenance was the greatest cost (City of Melbourne 2020). The costs of maintaining urban planting projects are often underestimated. Ongoing maintenance forms a significant proportion of project costs and this needs to be given as much consideration as project construction costs when allocating budgets. This is particularly relevant when establishing diverse plantings which are typically more costly to maintain, especially when a variety of life forms and growth habits are used.
The Need For Clear And Frequent Project Communications
Planting diverse, native understorey in streetscapes was not a typical practice for City of Melbourne staff, or one that residents were familiar with. Consequently, clear and frequent communication between internal and external stakeholders across multiple disciplines and professions was necessary for project success and to mitigate potential risks. While there are inherent risks with novel projects, support from ‘internal champions’ who are willing to take those risks, and the acknowledgement that failure forms part of the learning process, are important to pursuing these projects in a local government context.
Communication with the community was also essential and efforts to engage the community are highly valuable. Signage was installed during the construction phase to keep the community informed of what was occurring and the plantings have been largely well-received. The clever and innovative native planting designs across the four sites have provoked curiosity from residents, many of whom ask the horticultural maintenance teams for advice or the names of plant species. Permanent plant labels may be a useful additional communication tool.
Communication, through sharing research data and evidence-based recommendations, is fundamental to strengthening collaborative networks, increasing capacity and reducing risks for local government authorities to implement and maintain these types of projects and, more broadly, to encourage and inspire others to devote efforts towards conserving urban biodiversity. To assist other cities both locally and internationally to undertake similar projects, this project was developed into a publicly accessible case study shared on the ICLEI’s Cities with Nature platform (https://citieswithnature.org/city-profiles/Melbourne%20City%20Council). To date, the project and its results have been shared at local and international conferences and will also be detailed in a future scientific paper. Internally, the Urban Nature Planting Guide has been used by landscape architects on Council capital works planting projects but not with an experimental design intent. The Urban Nature Planting Guide has also been promoted to private landholders to increase biodiverse plantings in residential spaces.