Plants for riparian planting

Carol Bucknell speaks to the experts on how and why to use plants beside waterways, and gives a regional guide of what to plant where.

Expert tips, plus a detailed chart on what to plant region-by-region.

Riparian planting is a term we are becoming more and more familiar with in New Zealand, largely due to increasing revelations about the unhealthy state of our waterways. Planting in riparian zones is now regarded as an important tool in the battle to clean up freshwater streams, rivers and lakes, helping to filter out the excess nutrients and other pollutants in farm and urban runoff, stabilise banks and restore the natural ecology of waterways.

What are riparian zones?

A riparian zone is the area where land and water ecosystems connect, basically the strips of land along the edges of streams, lakes and wetlands. In a natural balanced waterway, these transitional areas act as a buffer, helping to clean and absorb storm water and farm runoff, reduce erosion of stream banks and provide wildlife habitat.

Why are they so important?

It’s no secret that our waterways are in a dire state. Ecologist Dr Mike Joy is among many Kiwi scientists who have been sounding alarm bells for some time, warning in an NZ Herald article that, “New Zealand’s freshwater systems are in awful shape and getting worse fast. Our grandchildren won’t be swimming in our rivers, and there won’t be native fish in them either, unless we make changes.”

A 2019 report from the Ministry for the Environment stated that 65 per cent of New Zealand’s total river length “had modelled nitrogen concentrations above the expected range for natural conditions”; and that 82 per cent of the river length in pastoral farming areas was not suitable for activities such as swimming. The report also noted that nitrogen concentrations were higher in urban areas than in pastoral sites.

The healthiest waterways are those that flow through native forest, but thanks to continual forest clearance and land development, those numbers have reduced significantly. In Auckland for instance, only 19 per cent of the region’s 10,000km of permanently flowing streams now do so through native forest

or scrublands. Riparian planting can help recreate the forest environment that is essential to the health of freshwater streams.

How does it works

Successful riparian planting protects waterways and improves their quality in a number of ways. Filtration of rural and industrial contaminants (for example, nitrogen, phosphorus and faecal pathogens such as E coli) is probably its most significant role. Well-established plants also help to reduce erosion by stabilising stream banks and trapping sediment before it enters waterways.

Riparian vegetation does the same thing when streams flood, acting like a sponge to absorb and hold back excess water flowing into streams.

Trees and taller plants in riparian zones create shade and cool the water keeping it at temperatures which reduce the buildup of algae, ensuring there’s plenty of oxygen for native fish as well as making streams more pleasant to look at and swim in.

On a wider level, trees and other riparian plants also sequester carbon and provide a diversity of food and habitat for native birds and insects.

Don’t forget wetlands

Although 90 per cent of wetlands on pastoral land have been drained over the past 200 years, these areas can be restored and play a significant role in pollutant reduction, according to research on DairyNZ’s website. The chemistry and hydrology of wetlands makes them ideal for treating nitrogen in shallow subsurface and runoff from farms. This denitrification involves bacterial communities converting nitrate into harmless nitrogen gas before it reaches waterway.

DairyNZ’s website also claims that a recent review of scientific studies in New Zealand found wetlands reduced the nitrate entering them by 75 to 98 per cent. Wetlands are also good for trapping sediments and sedimentbound phosphorus.

What to plant

Most ecologists advocate using eco-sourced native plants – these have adapted to local conditions, so need less maintenance. They provide the best habitat for native fish, bird and insect species.

While plant choice does not have to be restricted to native species, care must be taken when planting weedy exotic species as they can become invasive and disperse downstream.

DairyNZ water quality scientists Dr Tom Stephens and Aslan Wright Stow say exotic species can be more beneficial than natives in certain situations. “For nutrient and sediment lost in overland runoff, rank grass can perform equally well at reducing losses to water from paddocks. The science tells us that rank grass (tall grass, not cut or grazed) can generate equivalent or better reductions in these contaminants as native planted margins. Planting natives, sterile willows and poplars (available from most regional councils) is generally

Well-established plants also help to reduce erosion by stabilising stream banks and trapping sediment before it enters waterways.

of greater benefit to reducing erosion if riverbanks are collapsing or for enhanced biodiversity.”

However, Prof Gary Brierley, Chair of Physical Geography at Auckland University’s School of Environment, says care must be taken over species selection. “Breakdown products of wood in rivers can affect the geochemistry of organic matter and water quality, thereby impacting on ecosystem values. In Australia, quite a lot of work has been completed on the negative impacts of willows in this regard. In terms of species choice, consideration is often given to root structure and stem flexibility.

“As environmental scientists, we’re often interested in prospects for ‘recovery’ – what happens when we leave a river alone. Much depends upon the longevity of seed sources and prospects for regeneration.”

In his book Finding the Voice of the River, Gary says a healthy river needs an appropriate level of nutrition to support a “self-sustaining metabolism, enabling the system to cope in times of stress. While empty carbohydrates may taste good and provide instant gratification, excess consumption through nutrient overload engenders secondary health problems that are often expensive to address. Rivers with the capacity to self-heal are able to provide their own healthy meals

(for example floods that deliver water, sediment, organic matter, wood, etc.).”

DairyNZ, in conjunction with regional councils and Landcare Research, has created a series of regional planting guidelines with plant lists and advice that include site preparation, planting techniques, best time of year to plant, how to avoid losing plants in floods and how to identify priority areas. The guides also include planting calendars and weed maintenance schedules. Online riparian planting guides are also available from councils.

Where to plant

In rural areas, the first task is to fence out stock – a considerable investment for land owners that is only part of the cost. As DairyNZ stresses, riparian planting “doesn’t replace, but complements other good practices” such as ensuring sufficient on-farm effluent storage capacity and managing areas prone to leaching.

Within the riparian margins are three key planting areas. The first is a 1m grass strip between the fence and plants – particularly important where electric fences are used, as taller planting close to such fences can cause them to short or plants can be grazed by livestock. The second zone is the upper bank, usually higher ground but it may be partially flooded at times so plant choice must take this into account; a wide range of flaxes, shrubs, grasses and trees are recommended for this zone to provide shade and shelter. The third zone is the lower bank or flood zone; plants must be able to cope with regular waterlogging yet not restrict flood waters by bending with the flow of water. Multi-stemmed grasses, flaxes and shrubs are a good choice in the flood zone.

Planting should be staged, with coloniser species (those that can cope with hotter, exposed conditions – flax, cabbage tree, coprosma and mˉanuka) planted first, and others planted once shade and shelter have been created.

What can you do?

Schools, community and environmental groups are joining forces with farmers, with plants often donated by councils, conservation groups and native plant nurseries. For example, pupils at Pasadena Intermediate in Auckland have been growing and planting native trees along the Waititiko stream that runs through their school grounds as part of the Trees for Survival programme. With support from Rotary Clubs, the programme has organised the planting of over 1.5 million trees and is active in 133 schools throughout the country.

“We now have a planting unit and have thousands of plants potted up,” Pasadena teacher Sue Elliott says.

“For many students, this is the first time they have put a plant in the ground and it is hugely empowering. We consider ourselves to be the kaitiaki of the stream that surrounds our school.”