Native plants key to erosion control

Wild spring weather and hungry possums are just some of the challenges faced by a team of ecologists tackling roadside erosion.

During the past six months, the team have been trialling different ways of using plants to cover exposed soil beside roads around Port Hills and Banks Peninsula.

The Whakaraupō Road Cutting Trial, led by Christchurch City Council and Environment Canterbury, and implemented by EOS Ecology, aims to reduce the amount of sediment getting into the harbour by encouraging vegetation to grow and cover the bare ground and slopes beside the roads.

EOS principal scientist Shelley McMurtrie said the team selected six species of native plant based on their ability to cope with the harsh conditions of life by the roadside.

“The clay-rich soils around the harbour are generally low in nutrients and high in sodium, so the plants we’ve chosen need to be hardy and able to fend for themselves, without the need for irrigation or special care,” McMurtrie said.

Trial site underway at Christchurch Adventure Park

The first of potentially three field trial sites is now underway at the Christchurch Adventure Park, a spot chosen for its hilly and windswept terrain, typical of many Banks Peninsula roads.

This project will help improve the hills and lowlands as part of the Whaka-Ora Healthy Harbour plan.

The team have prepared a 50m horizontal section with a combination of erosion control products and suitable plants, including the New Zealand ice plant, pig fern, bracken fern, cutty grass, Banks Peninsula fescue, and silver tussock.

Plot monitoring of the Cut Slope Soil Erosion Trial in the Christchurch Adventure Park.
Extensive research has gone into looking at the best available plant options to help fight roadside erosion.

Ecologists are now monitoring the progress of the plants to identify which species prove the most enthusiastic to take root and spread across the cut slopes of exposed soil.

While it’s still too early to say what product and plant combinations are proving to be the most successful, Ms McMurtrie said the team had identified that the park’s possum population are particularly partial to the native ice plants.

“Our wildlife cameras caught these hungry critters red-handed, returning night after night to eat any new piece of growth,” she said.

“This resulted in a successful trapping programme, however we’re sad to say the ice plants didn’t make it this time.”

Christchurch City Council principal advisor Peter Kingsbury said the trial results would provide the Council with valuable information on how to better manage roadside cuttings, both at the road design stage and during routine maintenance. 

“A spin-off of the study is that the results will be publicly available, so private landowners with cut-banks will be able to do their bit to reduce soil erosion,” Kingsbury said.

About the trial

  • The Whakaraupō Roadside Cutting Trial is part of the Whaka-Ora Healthy Harbour Plan to restore the cultural and ecological health of Lyttelton Harbour.
  • The trial will run until 2021 and will be expanded to two other sites around Banks Peninsula over the next two years.
  • The project is a collaboration between Christchurch City Council and Environment Canterbury with technical input from EOS Ecology and Landcare Research.