Putting Down Roots

The putaputāwētā (marble leaf) tree is a beautiful member of Aotearoa’s native flora. Thriving in wet conditions, the species is named after wētā that live in holes in its trunk. Come autumn, its masses of white flowers and small chewy berries attract flocks of insects and birds.

New Zealand | Tuesday, 11 July 2023

Since Aotearoa separated from the ancient supercontinent of Gondwana, the native plant species found here have had millions of years to adapt to local rainfall patterns, humidity, temperature, and soil type – along with developing a symbiotic relationship to our unique fauna. As a result, we boast a dazzling array of indigenous plants from towering trees to small shrubs, climbers, and grasses.

Native plants such as putaputāwētā play an important role in flood mitigation and stormwater management. As water soaks into the ground, some is taken up by the plants and some evaporates out of the soil. At the same time, leaves emit some of the water they’ve picked up.

This process is performed by all plants, but due to their superior adaptation to local conditions, native plants are generally more effective at using water. In places with thick canopies and lush undergrowth, they’re also better at deflecting water - slowing it down before it hits the ground.

A study from Scion reveals the incredible sponginess of Aotearoa’s planted forests. Data from Mahurangi Forest over Auckland Anniversary weekend and during Cyclone Gabrielle shows 60 percent of that intense rainfall was held within the forest, preventing excess runoff. Elsewhere, native planting along streams and rivers in Marlborough has been effective at mitigating flooding in catchment areas.

Inspiring Urban Transformation with Native Planting

While we can't turn towns and cities into native forests, we can incorporate more natural systems and processes into our urban areas. Integrating well-designed (and properly cared for) native planting is an effective and cost-efficient way of bridging the gap between our built and natural environments.

Reducing impervious surfaces and increasing permeable areas of planting is a simple first step. This approach is often overlooked as a tool in the climate fight, due to competing space demands from other infrastructure. It also has significant benefits for biodiversity and potential for ‘cultural harvest,’ and can reduce urban heat islands.

Over the past few decades, attitudes towards native planting have changed for the better. There’s now greater recognition of its cultural and social values, and biodiversity benefits. In contemporary Aotearoa, decorative exotic planting has lost much of its significance - especially in public spaces. In fact, most local authorities now prioritise the use of native species in public projects.

Take a stroll along Wellington’s waterfront and see the beauty of such nature-based approaches. Developed in 2006, Waitangi Park uses water-loving natives to help manage the flow of stormwater. The entire area used to be a wetland fed by the Waitangi Stream before urbanisation. It’s wonderful to see this part restored to a truer version of its former self.

Further south in Christchurch is the Dudley Creek waterway. An award-winning flood remediation project run by WSP, Beca and Eos saw flooding in the creek reduced to pre-earthquake levels. Along with waterway widening and infrastructure improvements, riparian and native tree planting was a strong feature of the project, helping inhibit flood capacity at times of peak flow - also providing an inviting and accessible corridor for locals and visitors. Collaboration between technical specialists, council organisations and community was a big part of the project's success.

But it is not all plain sailing

There’s an inherent risk in propagating any plants – let alone natives. After all, they’re living organisms that survive or die. For this reason, it’s important to prepare plant supply contracts well in advance – especially if ‘eco-sourcing’ plants, where seed is gathered from local, naturally occurring populations. This too can be tricky when you consider some projects require tens of thousands of natives and have complex construction contract arrangements.

Thanks to climate change, plants of all varieties are at risk of being exposed to more pathogens. We’re already seeing this, with myrtle rust and Kauri dieback diseases running rampant. Warm, moist environments are part and parcel of climate change, and we need to prepare ourselves for the arrival of more such diseases.

To address these challenges, it’s important that urban planners and landscape architects continue to work closely with the nursery industry to keep genetic plant material diverse, and with the science community to identify and use resilient local varieties that are better able to take the knocks.

Author

Heather Wilkins

Technical Principal – Landscape Architecture

New Zealand

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Careful Planning Needed

Landscape architects are fond of saying ‘right plant, right place’. Much of that is about understanding the regional significance of a plant species, its relationship to other species, and what it needs to succeed.

Walk down a major throughfare and it’s likely you’ll see the consequences of poor spatial planning, where the built environment and infrastructure has been prioritised over natural systems and processes. All too often, natives in urban public areas are confined to narrow strips along the perimeters of new developments – usually as an afterthought to meet minimum requirements.

It’s far better to take a cue from what natives are already thriving in an area and how they function in the water cycle. These days, landscape architects are also benefiting from scientific research into naturally occurring ecosystems and what would once have been the dominant land cover.

Using more native plants in urban areas is a powerful way to create a greener and more sustainable future. So, let's prioritise the right plant for the right place and embrace the beauty and benefits of naturally occurring ecosystems, while also making things a little easier when the heavens open!