Renaturing the nature strip: Spatial, environmental and social drivers of road verge extent, composition and resident gardening behaviour

Abstract

In this thesis I ask What spatial, environmental and social drivers underpin road verge extent, distribution and vegetation? I investigated road verges across 47 neighbourhoods in Melbourne, Australia, quantifying their extent and distribution and the extent and distribution of the verge gardening undertaken by residents, and I surveyed residents on their beliefs regarding the road verge and verge gardening, and characterised the flora of the road verge understorey.

Road easement green space constituted 7.0% of land cover and a high 36.7% of all public green space. The percentage of the road easement that was green space was positively correlated with date of neighbourhood development, footpath absence, social disadvantage and parcel size. Streets with a greater percentage of road easement green space were associated with residential parcels that had a greater percentage of yard (i.e. garden).

Verge gardening was common, occurring in almost a quarter (22.1%) of verges and in almost every block in every neighbourhood. I investigated two types of verge gardening, resident-planting of understorey and resident-planting of street trees. The absence of footpaths was a major driver of both. Properties with no adjacent footpath were 5.27 times more likely to have understorey verge gardening, and 2.06 times more likely to have resident-planted streets trees, than those with a footpath. Tree cut-outs (also called tree pits) were a second major driver of understorey verge gardening, 1.75 times more likely to be gardened than standard verges. Local roads were 3.74 times more likely to have understorey verge gardening than major roads. Age of street was negatively correlated with understorey verge gardening. Verges without the presence of street trees planted by local government were 1.33 times more likely to have understorey verge gardening than those with local government street trees. Social contagion was also present, with the presence of verge gardening in a neighbouring property increasing the likelihood of verge gardening by 9%.

By surveying residents, I identified cultural background, gardening enthusiasm, sense of community and level of education as significant factors differentiating respondents who planted verge understorey, who planted street trees and who did not verge garden. Normative beliefs were the main cognitive construct affecting verge gardening behaviour, with verge gardeners less likely, compared to residents who didn’t verge garden, to be constrained by others’ perceived disapproval of verge gardening. In particular, residents were constrained by their perceptions of local government attitudes, much more so than their perceptions of neighbours’ attitudes or housemates’ attitudes. Sense of community, beliefs regarding the benefits of verge gardening, and feelings for nature also had significant, but less direct, effects than normative beliefs.

Floral surveys identified 150 species, of which 82.7% were exotic, with native species mostly introduced through verge gardening. Species richness, abundance and composition were mostly driven in part by residents’ verge gardening behaviour, mowing frequency, rainfall, soil compaction and canopy openness, but much variation remained unexplained and was likely to be due to stochastic factors such as degree and frequency of disturbance. Seven vegetation communities were identified, distinguished by the presence of garden plants, rhizomatous turfgrasses, and the relative proportions of three dominant grasses.

The extent of the road verge, combined with its often city-wide distribution, makes the road verge a green space component of fundamental importance to our urban ecosystems. Its varying distribution and extent across neighbourhoods means its significance also varies across the urban area. Verge gardening increased the overall species richness of verges, doubled the number of native species, and introduced structural complexity, suggesting that verge gardening can significantly contribute to quality and complexity of urban greening through the summed effect of the many small acts of citizen greening. Verge gardening promotes further verge gardening in a positive feedback loop. The influence of footpaths, road type and tree cut-outs shows that urban design can encourage this resident greening of public space. Municipal authorities are well-positioned to lead change, through reframing policy and outreach in order to positively frame verge gardening as an acceptable practice, by increasing plantings in the verges they maintain, and by promoting alternative low-mow practices that reduce the normative position of the well-manicured lawn. Planners, landscape architects, urban foresters, engineers and ecologists should work together to reimagine the ecological and greening roles of existing and future road easements. The potential for road easement green space to provide for the biodiversity, ecosystem function and human amenity now being demanded from urban green spaces is much greater than previously thought.