Stunning results in native plant trials in Port Hills

5 April 2018

Native direct seeding methods trialled and developed in the Port Hills Residential Red Zone could have huge implications for native plant re-vegetation and erosion control on steep sites around the country.

Land Information New Zealand (LINZ) has been working closely with contractor Red Tree Environmental Solutions to develop rapid and cost-effective methods to grow native plants on steep terrains.

Red Tree specialises in ecological restoration and approached LINZ to use a number of Residential Red Zone sites in the Port Hills to trial a variety of direct seeding methods across multiple sites with different soil profiles and geographical orientations.

"We have seeded a variety of native plants species over the past few years looking for the ideal blend. The goal was to achieve rapid plant establishment whilst reducing weed competition." says Red Tree project manager Andrew Cummins.

"The trials have come up with a really potent method that combines compost applied pneumatically as a blanket, which is direct seeded.

Compost is one of the best ways to filter sediment in run off and plant root growth is the best way to stabilise these hill sites." says Mr Cummins.

Over 450 properties in the Port Hills have been cleared since the Canterbury earthquakes and LINZ has been responsible for replanting and maintaining the sites where houses can no longer be built due to rock fall and cliff collapse.

Matt Bradley, acting group manager Ports Hills, says that the partnership with Red Tree has created some great outcomes within the first 18 months and may have more positive knock-on effects.

"These trials have given us effective new eco-friendly methods to regenerate the vegetation on the hill sites and reduce the amount of maintenance required," says Mr Bradley.

"The outcomes are really exciting for the environment, the community, for LINZ and, of course, for the taxpayers."

The clover trials on particularly steep sites has been so successful in suppressing weed growth that LINZ has asked the company to introduce clover across most of its planted Port Hills sites.

Mr Bradley says that one of the biggest challenges of regenerating the Port Hills red zone properties has been the condition of the soil following demolitions, which was different in each site, and often dominated by loess clay, which gets boggy in winter and dry in summer.

Red Tree’s solution to this challenge was the compost blanket.

"It provides an ideal growing medium, much like the forest floor. It is a cost effective alternative to top soil that is pasteurised to render it free of weeds and pathogens," says Mr Cummins.

"And of course it is a wonderful recycling story to see the content of Christchurch green bins providing the key ingredient for returning native plants to Christchurch hills."

The impact of the compost blanket is easy to see, even for the uninitiated, on the final trial site on Brenchley Road in Lyttelton.

On one side, the traditional revegetation approach on a clay site shows plants struggling to establish, despite the abundance of rain and sun, while the compost blanket is covered in a lush carpet of native plants, all within the space of nine months.

Intriguingly, one unorthodox crop turned out to provide the final piece of the puzzle.

"Oats have been used to stabilise the compost blanket until the native plants germinate, this was a tip from a veteran gardener," smiles Cummins. "I was looking for a plant with an open root system that was quick to establish, he suggested oats."

The oats spring up rapidly to bind the compost in place and do not adversely compete with the natives. Once the natives are established, the oats are removed and used as mulch between the plants.

Mr Cummins is clearly proud of his findings, which he thinks could have nationwide benefits in land stabilisation and revegetation projects.

"Using the compost blankets allows us to grow natives faster even on really difficult sites, it helps to stabilise hill sites, manage sediment run off and control the weeds which can reduce the cost of maintenance by at least 25%."

View a video about the plant trials

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