Our synthesis of German federal states Red Lists and a comprehensive gardening platform estimates that on average 43% of red-listed species may be amenable to gardening. Many of these CG species were classified as highly endangered or even extinct. Despite this high threat level, the majority of CG species (66%) were already available for purchase by a few native seed producers. Beyond its potential to contribute to biodiversity conservation, we found that CG could also help climate adaptation of urban and rural green spaces. In comparison to conventionally used garden plants, CG species more often prefer dry soils, indicating a greater tolerance to increasing droughts and lower water requirements that run in parallel with generally lower nutrient/fertilizer demands. Whilst many of the potential biodiversity benefits of CG remain to be studied, we provide a first crude estimate that large-scale planting of CG plants in private and public green spaces as an extended and participatory form of ex situ conservation could reduce the extinction risk of vascular plants by up to 50% in some German states and by up to 25% in Germany as a whole. In order to harness such a potential, it is critical that policymakers and the gardening industry improve the supply, and encourage the planting of regionally declining native species. In this process, the database we created and made accessible through an R Shiny application is one of many ways to both lower the threshold for gardeners to participate in CG and communicate key data and implementation gaps to policymakers and gardeners.
Whilst we have argued that CG has the potential to initiate transformative change to tackle biodiversity, clearly more research is needed on the conservation potential of gardening to bend the curve of biodiversity loss. In Germany, for example, 14% of the area is covered by settlements and transport infrastructure, and an even smaller fraction constitutes green spaces (Statistisches Bundesamt, 2022). Relatively little area is thus available for CG to reversing biodiversity trends, which raises the question of how useful gardening can really be for biodiversity conservation. Do CG plants actually manage to establish in natural ecosystems from gardens? How often does this occur? Can CG plants establish stable populations in urban areas? How much societal participation is needed for CG to be successful as a conservation tool? Is CG of greater benefit in certain areas or environments? What is the role of genetic diversity of the plant material for CG to sustain viable populations? Although many questions remain unanswered, we would like provide some reasons why CG could be a key player in addressing the biodiversity crisis. 1) Although urban spaces represent relatively little area, 70% of the German population lives in urban areas (World Bank, 2022), constituting a high human power for conservation purposes. Moreover, the human footprint (e.g., mobility infrastructure) extends far beyond settlements opening vast possibilities for secondary dispersal to natural ecosystems (Venter et al., 2016). 2) Humans have been a key dispersal vector for plants throughout history and their importance has increased particularly recently (Auffret, 2011; Hodkinson & Thompson, 1997), with studies suggesting there is an especially high likelihood of human vectored dispersal in and away from urban areas (reviewed in Bullock et al., 2018). Despite the relatively small area, human infrastructure is widely dispersed in Germany. Even a square meter of human-managed vegetation could thus create habitat stepping stones (Rudd et al., 2002; Witt, 2013). 3) The production of native plant species in rural areas constitutes additional agricultural area and is economically profitable whilst helping to restore biodiversity. 4) If there is an economic market for a particular species, i.e., if it is produced and there is demand, it is unlikely that the species will become extinct. 5) Beyond these active contributions to halt biodiversity loss, CG inheres a great educational value, making biodiversity accessible for city dwellers and raising awareness for its loss. This could in turn catalyse greater public demand for other conservation measures (Soga & Gaston, 2016).
Beside these potentials for biodiversity, CG may also be associated to benefits for people, such as lowering costs and resources to maintain gardens. In Germany, the lion’s share of endangered species of plants comes from nutrient-poor and full-light conditions (e.g., dry calcareous grasslands) (Staude, n.d.). It is therefore to be expected that CG species thrive under different site conditions than conventionally used garden plants. We found that CG species may be, on average, more tolerant to drought and water scarcity. In recent years, Germany has witnessed exceptional droughts (Hari et al., 2020), making it costly and resource intensive to support so called “Wimbledon lawns”. CG species could therefore contribute to climate adaptation in rural and urban spaces. Likewise, lower nutrient requirements of CG plants may reduce effort and costs for gardeners. Fertile soils are currently the paradigm amongst gardeners, with studies finding that 52% of gardeners apply fertilizers at least once a year (Dewaelheyns et al., 2013). We argue that the species that benefit from these activities are exactly those that are already winning in the Anthropocene (Staude et al., 2021). Nutrient-demanding species are increasing inside and outside natural ecosystems alike. Of course, it will be also important in the future to assess the potential of CG plants to become weedy. While the probability of weediness in declining native plants may be generally low, in some regions they may become weedy. With increasingly complete data on species invasiveness (e.g., https://glonaf.org/), it will be possible to estimate this probability. Taking such considerations into account, we suggest CG species could generally help reduce resource inputs and lead to more resilient garden ecosystems, whilst helping to safeguard biodiversity.
Whilst we focus here on declining native plants, we emphasize CG does not and should not be restricted to only those; CG does not suggest that native plants are good and non-native ones bad. In fact, it may even be desirable to have a mixture, for example because neophytes can provide for insects at times where native plants already stopped flowering (Frankie et al., 2019; Tew et al., 2022; but note there is scientific debate on the ecological value of late flowering neophytes, e.g., prairie plants). With the exception of gardens that plant invasive neophytes, most types of gardens already contribute to biodiversity, and CG should not be an exclusive approach to conservation. CG’s focus on declining native plants, is merely a systemic one by which a greater cumulative diversity of plants is gardened across regions, and by which declining plants are put into the spotlight. Moreover, we believe that a widespread shift of the horticultural industry towards predominantly native species could also help address Target 6 of the 2022 Kunming-Montreal Global Biodiversity Framework: “Eliminate, minimize, reduce and or mitigate the impacts of invasive alien species on biodiversity” (https://www.cbd.int/article/cop15-final-text-kunming-montreal-gbf-221222). Given that garden escapes of invasive neophytes are often causative for the spread of invasive plants (Reichard & White, 2001), a predominant focus on declining native plants could eliminate a major causal pathway for species invasions, and instead of being a costly intervention it might generate economic revenue.
Our app may help gardeners to gain an overview, lowering the entry threshold to CG. Nonetheless, our study and app have several shortcomings. Our calculations of how many plants are amenable to CG are currently not verified by practical experience, it only relies on the information provided by one comprehensive gardening platform. Important field trials are still lacking to determine which CG plants are actually easy to grow in gardens (and which are not suitable for amateur gardeners). It would be useful to group plants in this way. By providing recommendations for balcony plants by practitioners (Fig. 5 and Supplementary Table 2), we take a step in this direction, but a broader classification system for all CG plants is desirable. Perhaps some of this expertise already exists among horticultural experts, but is not made readily available for economic reasons. Programs that incentivize such knowledge sharing and mobilization could go a long way toward bridging the gap between science and practice. Furthermore, it is likely that our app still mainly reaches people who are already interested in natural gardening, and more research is needed on how to effectively lower the threshold to CG. For example, in allotment gardens, garden groups could take stewardship for a specific declining species, thereby creating communities that share seedlings, experience and knowledge of their “care plant” within the wider allotment community - a form of block leadership that may be effective in instigating community uptake (Mumaw, 2017; Shaw & Miller, 2016), and producers of seed mixtures could include at least 10% of regionally specific declining native plants. Here, botanical gardens would also be uniquely positioned to help promote CG and provide information on which plants are amenable.
Overall, we show here a workflow that leverages spatially detailed, subnational monitoring data, and integrates these data with gardening and producer platforms to find CG plants. Whilst we focus here on Germany, we hope that our workflow and software facilitate similar undertakings for other regions. We highlight that there is already a high potential for CG to be implemented, and suggest CG plants available for purchase for the balcony as a low entry threshold. Our application may not encompass the definitive list of CG plants and fall short in some key practical advice, but we hope it will be informative to those who want to find a starting point and means to participate in tackling the biodiversity crisis.