The long-standing debate over the role of native and exotic (introduced) species continues, with an unfortunate tendency toward adopting entrenched positions in some countries (such as the USA) or arousing little interest in others (Japan). The key issue is the role of plants in gardens and designed landscapes to contribute to biodiversity by supporting food webs of insects, birds and other wild animals. It is worth noting the positions adopted by leading practitioners working in planting design. There does seem to be a consensus that using only native species is entirely appropriate in certain environments—chiefly rural ones or where the conservation of local and indigenous biodiversity is a priority. In many other situations, planting in the past would have used mostly or entirely non-native species, but now involves a larger proportion of natives.
The strictly nativist lobby, which believes in the exclusive use of regional natives, is not one which has arisen from within the garden and landscape communities, but rather has been imported from outside, from the world of environmentalist politics, where ecology is a word which can often weave dangerously between the evidence-based and scientific and the emotive and ideological. Unfortunately, the native plant lobby has on occasion acquired sufficient political support in some communities that the use of native plants has been mandated for landscaping projects, compromising their visual effectiveness and therefore the public support they receive.
Piet Oudolf’s work at the Lurie Garden in Chicago and the New York High Line provides an example of synthesis which many in the profession would agree with. He chooses plants because they perform a function and meet certain visual criteria. In these two projects over half the plants used are natives of their region. One of the criteria for both commissions was that the planting should reflect something of the regional natural environment: in the case of the High Line, this being the plant community which had established on the elevated rail line prior to restoration. A large proportion of natives was therefore essential. Crucially, though, the plants selected highlight just how good in design terms many of the plants in these regional floras are—a great number of species with garden and landscape potential have been largely ignored up until now. This is perhaps the most crucial point here – that prior to the current wave of interest in native plants, the nursery industry produced and sold what was beginning to look like a global flora of easy to use, easy to propagate plants. In the case of plants used by the landscape industry, they may have been different from one climate to another, but the effect was the same—too often both architecture and planting could be anywhere. The use of a proportion of locally native plants can do much to add a distinct signature to projects.
The native/exotic debate is complex, and here we will consider a range of points, some of which are more relevant to some localities than others.
• The issues are different from place to place, often very different. Take two island groups: the British Isles and New Zealand. The former has a very limited flora, with whatever managed to get across after the last ice age before the land bridge with the mainland was flooded by rising sea levels; the plant-eating invertebrate fauna at the base of the food web is largely generalist, and relatively few are totally dependent on native plant species. The local grass flora has a tendency to dominate—hugely reducing the ability of introduced species to spread. New Zealand also has a limited flora and fauna but one which has evolved in isolation and is extremely vulnerable to introduced plants, many of which have become invasive (the native flora possibly lacking its own vigorous pioneer species).
• Native plants are often a hugely underexploited design resource. The globalized garden and landscape flora alluded to above offers limited options. A walk in any natural habitat will reveal plants with ornamental or amenity potential, but they need to be brought into cultivation and evaluated for their visual and commercial potential, which takes time and effort. Increasingly, gardeners, nurseries and designers are realizing this, and undertaking this ‘close to home’ plant hunting. Even in Britain, with its restricted flora and long garden history, we are still learning to exploit the ornamental potential of our wildflowers—look at Stachys officinalis, almost unknown as an ornamental twenty years ago.
• Native plants enhance a sense of regional distinctiveness. Cultures sometimes want to emphasize their superiority by using ‘civilized’ plants. Hence the scandal when, in one of the rare iconic single actions in garden and landscape history, Roberto Burle Marx planted local plants in a public square in Recife, Brazil, in 1945. Nowadays the zeitgeist has moved toward celebration of the local and the regionally distinctive, about what sets a place apart and gives it its identity in the world. Locally native plants clearly have an important role to play here.
• The idea that exotic equals potentially invasive has no factual basis. What makes a plant an invasive alien is a question which ecologists continue to argue about. The reality is that of all the plants introduced into one country from another, only a few go feral and spread. The more dogmatic plant-nativists tend to tar all exotics with the same brush. Having said this, the nursery trade does have a responsibility to assess new plants for their ability to spread.
• Some native plants play a crucial role in local biodiversity webs, but this does not mean that exotic plants do not have any wildlife value. The food web which supports larger animals (mostly birds) is founded on invertebrates, chiefly insects. In many regions these are predominantly specialists—their larvae will only eat particular plants; a garden of introduced species will therefore support very few of them and thus greatly impoverish the food web. However, most other animals are more generalist: nectar-drinking insects such as bees are not tied to native species in this way, and neither are berry-eating birds. Adding species to a locality through planting may actually improve the richness of the food web by providing nectar sources for bees at a time when the native flora has little to offer.
• Planting design is at least as important in supporting biodiversity as the species chosen. Research work undertaken in Britain at the BUGS project (Biodiversity in Urban Gardens) has shown that the most important factors in improving biodiversity are not the species used but the diversity of habitat. Trees, some shrubs, perennials, ground cover, and connectivity between different plant layers are what matter. And, a diversity of species.
• Planting design is, fundamentally, for people. In urban areas, planting in private gardens or public spaces is about providing a habitat for people. Anything which fails to interest or please them will lose support, as local governments that have created untidy ‘wildlife areas’ in parks have found out to their cost. In regions where poisonous spiders and snakes are common, there may be good reasons to actually fear such places! Areas for nature have to be seen to be attractive or in some way valued by people; only then will they gain political support. Using introduced species to provide interest for human users of landscape is one way to do this; very often non-gardening users of public spaces will find it easier to ‘read’ a planting if some familiar cultivated plants are included.
• There is plenty of room for both natives and exotics. Gardens, parks and areas of land around offices, malls, airports and road systems take up a lot of space on our crowded planet. Add up all the space which is currently mown, and which does not actually need to be 20 millimeter high grass, and the total will be huge. Globally it would probably be the equivalent of a modest-sized European country. There is plenty of space for both natives and exotics.
Text and photographs by Noel Kingsbury and Piet Oudolf
“Oudolf is eager to teach the rest of us how to recreate the effect in our home gardens. Of particular interest are the book’s detailed planting plans, which show how Oudolf…deliberately creates a ‘layered look’ that looks unstudied and provides year-round interest.” —Gardenista
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