Factors influencing naturalisation success in horticultural species: a case study using planting records from the inception of a planned city

Worldwide, many invasive plant species are garden escapees. While weed risk assessment can identify new plant introductions with weedy potential, it does not address the large number of non-native plant species already present in many regions, the majority of which are horticultural species. Here we evaluate the drivers of plant naturalisation success using historical data on the horticultural woody species planted in Canberra, Australia. Canberra provides a unique opportunity to study plant naturalisation as it is a planned city with extensive horticultural plantings originating from government nurseries that kept extensive records documenting the planting efforts from the city’s inception. We identified factors linked to naturalisation success in 1439 horticultural, woody, non-native species planted in Canberra over 150 years by fitting univariate and multivariate regression models, and identified both direct and indirect effects using path analysis in a Bayesian framework. We found species were more likely to naturalise with greater planting effort, longer residence time, smaller seeds and dispersal mechanisms linked to wind and animal vectors. Cold-hardy and tall plants were also more likely to naturalise, although cold hardiness and height mostly affected naturalisation success indirectly via planting effort. These findings can aid in generating quantitative risk assessment models to predict woody garden species that would naturalise and pose the greatest risk of becoming invasive in the future.