Landscapers are coming under increased pressure to consider the environmental implications of their work – whether that’s from regulation or clients. In this article, Jon Muller considers how the landscaping industry can make a meaningful contribution when planting gardens. We also explore how Wellington City Council uses rain gardens to make urban areas more sustainable
How can we make our gardens and planting more sustainable? Before we answer that question, we need to consider the simple definition of sustainability. It’s “meeting the needs of the present without compromising the ability of future generations to meet their own needs”.
Recently, I have been involved in looking at the fast-track legislation proposed by the current government. A lot of it considers balancing the care of our living environment versus the needs of rebuilding of our economy. As landscapers, we want to create better environments, but we also need an economy that is robust enough to allow this.
Plants are an integral part of the living environment, as is soil and air. They are all connected, so it’s hard to isolate one part of the living environment from another.
Plants can stabilise the soil and create a living ecosystem which purifies the air and soil. If your aim is to make your gardens more sustainable, you should ask yourself the following questions – otherwise, you may be making yourself feel better about your efforts, but you won’t attain your goals of sustainability.
Solutions start with questions
When we use plants, are we thinking of the long-term effect of our living landscape? How will the plants look in 20 years and will they provide a suitable living environment? In time, will the plants blend in with the environs and not create issues with height, views, or light and must be removed? Are the plants suitable for the environment so needing minimal soil and climatic adjustments?
Is it better to plant native plants alongside rivers to combat erosion or easier to plant willows to get a quick effect but which could be a source of a weed problem later?
Is it better to use artificial lawns rather than natural ones which need more energy in the form of chemicals and fuel to maintain?
Even if you are using natives, is it better to eco-source plant material from local areas rather than from further away? Will these plants be hardier and blend in better with the local species?
Filtering pollutants through planting
In Wellington, we have two gardens that are designed with sustainability in mind: Waitangi Park and the inner-city rain gardens. Both purify water – the rain gardens purify run-off from streets and Waitangi Park purifies water in Waitangi Park stream before it enters the harbour.
The soil and plants filter out any pollutants, so there is an interaction between plants, soil, and water to maintain a cleaner environment. Some plants, like some of our native grasses, are very good at filtering out pollutants and the specialised soil mixes used in rain gardens are designed to filter out pollutants. All the plants are also eco sourced locally.
You can usually find information on rain gardens from your local council. Plants that filter pollutants and are suitable in wet areas such as streams include:
Apodasmia similis, jointed wire rush
Astelia grandis, swamp astelia
Carex comans, longwood tussock
Machaerina sinclarii, tuhara
Uncinia uncinata, hook grass
Rain gardens explained
Wellington City Council introduced rain gardens on Swan Lane and Garret Street as a natural way of removing pollution and stormwater run-off before it enters streams and reaches the harbour.
At first glance, a rain garden may look like a regular garden-bed, but it’s actually a complex system that requires technical planning to have a positive impact on our stream health, explains Wellington City Council Senior Urban Designer Stephanie O’Shea.
“First of all, the surrounding areas are designed to channel stormwater run-off from the street, which will have a bunch of pollutants in it because it's been capturing the muck on the road from things like vehicles, into the rain garden.
“As the water goes in, there's a specific soil material layering within the garden, which absorbs pollutants as the water filters through it, cleansing it before it enters the stormwater system.”
The rain garden was installed by Natural Habitats, who used a mix of Purikireki (Carex appressa), Hinarepe (Poa billardierei), Wiwi (Ficinia nodosa) and Creeping pihuehue (Muehlenbeckia axillaris). The rain gardens are set 300mm below edge level, and include a drainage layer, geotextile transition layer and a geotextile filter media.
The filtration system is designed to let water sit on the surface of the rain garden for 24-48 hours before slowly filtering down.
As the official pilot for rain gardens in the capital, these gardens are not only special because they’re the first-of-their-kind, but also because of the cultural significance of the Waimapihi Stream that they flow into, and the life that exists within it, says Stephanie.
The name Waimapihi refers to the waters of Mapihi, a rangatira of Ngai Tara and Ngati Mamoe. It’s known that this Māori chieftainess used to bathe in the waters of the stream, further up the catchment. This stream has seen banded kōkopu, kōura and even tuna living within its culverted walls today.
The Public Space Design Team worked in collaboration with Stu Farrent, a Water Sensitive Design Specialist from Morphum Environmental Ltd, to create the rain gardens.
“Locally eco-sourced plant species support treatment through root growth to support a range of biological processes to take up nutrients, metals and hydrocarbons, which would otherwise flow to Waimapihi Stream and Te Whanganui-a-Tara. These solutions provide resilient water quality improvements, while adding to the biodiversity of the inner city.”
This re-imagined public space has helped us move towards an eco-friendly future that we want for our city, Stu says.
“These rain gardens will demonstrate how we can work within the constraints of the inner city to still achieve environmental outcomes and therefore meet the aspirations of the community.
"The redevelopment of Swan Lane and Garret Street provides a timely opportunity to explore how we can better shape a future city which protects and enhances our urban water systems and the biodiversity they support.”
Jon Muller has owned Wellington Gardens Ltd for 24 years. He practices landscape design and construction, with an emphasis on planting design.
He has taught at polytechnics and university, mostly in soft landscape subjects. Wellington City Council also contributed towards this piece via the article ‘Rain Gardens Explained’, to which NZ Landscaper Magazine added some technical detail.