A native understory in a concrete jungle

If very small urban plantings are unlikely to provide important habitat, why plant natives at all? Christchurch-based botantist Dr Jaz Morris explores some broad questions of ecological pedantry in small-scale plantings in the urban environment.

Historically, our urban areas are thought to be ‘concrete jungles’ – places where ecology is absent, and where any green spaces or plantings have reflected a colonial approach to nature. They may be characterised by stately English trees and manicured lawns rather than the complex, rich mosaic of New Zealand’s native flora.

More recently, natives are often the centrepiece of urban plantings; and on occasion these plantings are so well designed that they represent almost a perfect ecological snapshot of the past (albeit the trees aren’t so big yet). Sometimes the eyebrows of the most pedantic botanists are raised by rather unusual native plant combinations – that is, unusual when viewed strictly through an ecological lens.

In the photo above, on the left, a recent planting in Ōtautahi / Christchurch contains an attractive but ecologically rich mix of local species representing a range of vegetation tiers (plants that naturally reach different heights). It may take a while to get there, but the lowland tōtara among this mix will eventually reach the height of its ancestors; who, a few hundred years ago, would have sunk roots through the same ground in search of the same water.

In contrast, the two native species present in the image on the right occupy ecologically unfamiliar ground. These strange bedfellows for a roadside raingarden – both coastal species (rengarenga lily and the nationally ‘at risk’ waiu-atua / shore spurge) – have ended up together because they create attractive form, colour, and pattern, rather than being ecologically accurate for the location.

If ‘right’ means genetically wild, in a species’ natural habitat type or propagated from a natural population in a nearby area (‘eco-sourced’), when does it matter to plant only the ‘right’ species? Are there ever advantages in planting an ecologically ‘wrong’ species? And how do those plants fit into the wider ecological landscape?

Of New Zealand’s approximately 2,500 native plant species, around half face extinction based on current trends. Landscape-scale land protection, weed control, and widespread riparian and restoration plantings using eco-sourced species may, in coming decades, turn some of those trends around.

At the smaller urban scale, when planning any kind of planting we can choose from a wide range of  local species to suit the desired form, the habitat, or to take advantage of the opportunity to boost rare plant populations. There’s an appropriate locally common (or rare) grass, herb, shrub or tree for every centre in Aotearoa. If in doubt – ask an ecologist when preparing a planting palette.

But some native plants are difficult to propagate; few have showy flowers, and some take literal ages to get very far off the ground. This is probably why attractive or easily grown native species are typically selected for landscape plantings and therefore are far greater in number and / or are more widely distributed than they were before human arrival. Species like mikoikoi / native iris, rengarenga lily, and horoeka / fierce lancewood come to mind.

However, these species are often planted well outside their natural range. One of the most popular rengarenga lily cultivars descends from seed stock originally taken from the Poor Knights Islands! Location matters. Planting cultivars or non-local native species can compromise the integrity of the natural genetic variation of a species in an area. For example, an unnatural red akeake variety now dominates the wild green form in parts of both Wellington and Christchurch. Other issues are possible: non-local species can introduce disease such as myrtle rust, which may have spread via nurseries; or even become weeds, like karo in Wellington and in the South Island.

It would be presumptuous to insist that ecology always takes front seat when designing planted areas in an urban environment; amenity and functional matters usually come first – planting a species that’s strictly ecologically appropriate for central Christchurch (historically a forested swamp) ignores the modern reality of the changed environment, with modified soils, altered microclimate and abundant artificial surfaces.

It’s entirely fair to plant species based upon appearance and function, particularly in purpose-built habitats like raingardens. And, in ecology, size usually does matter – small areas of a habitat in isolation from nearby counterparts are unlikely to provide enough extent or spread of habitat for the fauna that live among or feed on those plants. Scale works two ways – areas of habitat that are hectares in size can be spaced by kilometres; whereas areas of just metres must be spaced by merely tens or hundreds of metres if they are to provide animals with feeding habitat, shelter and sufficient opportunities to interbreed. Unless similar areas are replicated regularly across the city landscape, a small raingarden or school playground is unlikely to contribute meaningfully in terms of habitat, regardless of how locally appropriate the species are.

If very small urban plantings are unlikely to provide important habitat, why plant natives at all?

The first answer is that they can provide some habitat. A patchwork of harakeke / flax plantings in an urban area may produce enough flowers to sustain a readily mobile tui or korimako / bellbird, though the patches may be too sparse for the less mobile pupurangi / flax snail that feed on their leaves.

Secondly, many urban areas have been almost entirely cleared of their former native vegetation, so that in many gardens, hillsides, and so-called ‘waste’ areas the spread of exotic species is the norm. Maintaining small populations of local species (but not their showy garden cultivars) provides a seed source for the local area as well as a buffer against the extinction of those species in the wild. In fact, many of our most popular species in cultivation, like kopakopa or kowhai ngutu kākā / kakabeak, are seriously threatened or even extinct in the wild. In the same way that no farmed animal species ever really dies out, we can provide some safeguard for the persistence of these species with our urban plantings – especially if we do it in the right places.

Taking this argument as a whole, it should be clear that commissioning a planting palette or a landscape design for an urban project provides opportunities to contribute, even in a small way, to the ecology of our built landscapes. Planting rare species appropriate to the location can boost local populations and even make those species more abundant than in nature. Planting even the smallest patches of local species can provide some habitat for native animals if the patches are closely spaced or connected in the wider landscape. Non-local natives, however beautiful, contribute less ecologically, and in some cases can be detrimental to local plant populations. With hundreds of local species to choose from in every region of Aotearoa, there’s a local native to suit every landscape design – have the rengarenga lilies had their day?

For further information please contact Dr Jaz Morris

20 October 2020