How do our gardens grow?

Recent large-scale studies have shown that people’s health and wellbeing are positively related to the proximity of urban green spaces and that plants help create psychologically restorative environments that can help to reduce stress and mental fatigue – issues that are a feature of life in modern cities.

Cultivated green spaces are extensive and can comprise half the total urban area, yet we know little about why people choose to cultivate the plants they do and how these decisions combine to create the patterns of distribution and diversity that exist in our city landscapes.

Our project explores the distribution and diversity of cultivated plants in cities at global, local city and individual householder scales and identifies physical, social and psychological factors related to these observed patterns.

At a global level, invasion biology has assumed that people tend to favour and cultivate the same species in cities around the world. These assumptions have some support from evolutionary theories in environmental psychology, which suggest that peoples’ landscape preferences evolved in response to early human habitat and that people around the world have similar preferences for landscapes suggestive of good habitat.

We tested this empirically by comparing the similarity of 71 published lists of cultivated species in gardens, parks and streetscapes from cities and villages around the world. The results show that very different plants are cultivated in different places, and that the distribution of cultivated plants is most strongly related to temperature (which drives plant biogeography generally).

Our results also suggest that even a 2°C change in temperature due to global warming could have a significant effect on species composition. This has significant implications for a city like Melbourne where many species from cooler climates in Europe and North America are being cultivated.

For example, Quercus palustris (Pin Oak) is a widely planted street and park tree in Melbourne, yet in the studies we examined Q. palustris was cultivated in cities with a mean annual temperature almost 4°C cooler than Melbourne (and nowhere warmer than Melbourne).

It is possible that climate change could also have a significant impact on the look of Melbourne’s green spaces, as species that were common in cities that were warmer and drier than Melbourne tended to have finer, lighter coloured foliage and more open canopies than those from cooler cities, where species with coarser dark green foliage and denser canopies were more common.

Several other social and physical factors were also found to be important. Cultural background was the second most significant factor – for example, Melbourne had more species in common from much colder cities in the US and UK than cities in temperate China. Distance and rainfall pattern (having dry summers) were also important, although total rainfall was not significant – perhaps indicating that human behaviour in irrigation is able to overcome this important driver of native plant biogeography.

At a domestic level, urban ecology studies have shown that gardens have very high levels of plant diversity and that this diversity is strongly related to socioeconomic status (in the US at least).

However studies in geography and sociology have related gardening behaviour not to socioeconomics but to individual characteristics such as age, gender, cultural background and personality.

Our study of gardens across Ballarat also found very high levels of overall diversity, and that this was due to the gardens being very different from each other rather than high levels of diversity within gardens.

In contrast with results from the US, we found that diversity in gardens was not related to socioeconomic status. Gardens that had less turf and that were smaller were more diverse than larger gardens with more turf, suggesting that higher housing densities will not necessarily negatively impact plant diversity in gardens; a given area will have more plant diversity if it contains more, smaller gardens with less turf rather than fewer, larger gardens with more turf.

There were no clear relationships between species composition and social or physical variables such as location, socioeconomic status, garden size or house age, although there was some evidence that people ‘copied’ the plants in neighbouring gardens.

Also contrasting with findings from the US, we found that the likelihood of having a street tree was related to socioeconomic status – there were far fewer street trees in disadvantaged areas. Redressing this inequality in the distribution of public urban vegetation in Ballarat provides opportunities for improving ecosystem services such as carbon sequestration and climate moderation, as well as improving health and wellbeing in disadvantaged areas.

At an individual level, we used interviews and a photo-based postal questionnaire to explore why people chose the particular plants they did in their gardens.

Respondents grouped the plants in the photos into four types; traditional garden plants with large flowers and coarse foliage, natives that tended to have finer foliage and smaller flowers, strappy plants and formal (clipped) plants.

A variety of plant characteristics were identified that influenced people’s choices of plants, including factors related to aesthetics (beauty, flowers or foliage), factors related to the site context (drought tolerance, screening, “suits the house” or “fills a space”) and personal factors (childhood memories or nativeness).

The importance of different factors varied for different people; while flower size and colour were the most significant factor overall in people’s plant choices, foliage colour and texture was a more important characteristic for people who preferred native plants.

An interesting finding was that people deliberately chose plants that were different from those that were already in their garden, a practice that clearly results in increased levels of diversity in gardens. At the time of the surveys, Ballarat had been on Stage 4 watering restrictions (no outside watering) for several years, and water use was a crucial issue for many gardeners. The gardening behaviour survey respondents identified with most strongly was conserving water, and many people indicated that the garden plants they liked had changed over the years in response to the hotter and drier conditions we have been experiencing combined with water restrictions.

The choices that people make about which plants to cultivate are complex, and based on the intersection of plant characteristics, site characteristics, cultural traditions and people’s preferences. The results of this research will help us to better understand how the diversity we see in urban green spaces has been created, and perhaps better understand how landscapes that benefit both people and urban ecosystems could be created in the future.