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Bioretention devices include raingardens, swales, filter strips and greenroofs; all use plants and substrate to detain and filter stormwater to reduce the impacts on lakes and streams of excessive runoff from roads, roofs and paved surfaces,and provide opportunities for enhancing natural landscape and biodiversity elements in the urban environment.

Additional benefits of greenroofs include noise and energy insulation.Our research focuses on optimising the design, construction and performance of bioretention devices in New Zealand. We have a particular focus on raingardens and greenroofs. Elements of the work include testing alternative substrates, plants and mulches; evaluating design and construction techniques; and monitoring their hydrological and hydrochemical performance. The latter is largely done in situ through partnerships with property owners and local authorities.


Greenroofs are vegetated roof covers, with growing media and plants taking the place of bare membrane, shingles, tiles or other roofing materials.

The primary reason for greenroofs in cities is for stormwater retention (delay in runoff) and reduction of stormwater volume entering piped drainage infrastructure.

In many cities around the world greenroofs are also used to:

  • improve runoff quality (especially where acid rain, high dust/traffic volumes or Cu/Zn (copper/zinc) roof features)
  • modify city temperatures and reflectance leading to increased energy efficiency of building
  • create or enhance biodiversity: habitat for plants, insects, and birds.

The number of layers and the layer placement vary from system to system and greenroof type. However, all greenroofs at least include a single to multi-ply waterproofing layer, drainage, growing media and the plants. Grasses, trees and shrubs may be used, though various species of sedum are most commonly used in many greenroof applications (note: sedums are weeds in some New Zealand environments). In New Zealand both exotic and native species can be used.

Commonly, the roof function or objective of the roof space determines the design – is it just an ecological cover or is it intended for human recreation, vegetable gardening, etc.? The limiting factors for greenroofs include: the roof loading capacity or maximum dead and live weight loads, determined by a structural engineer; the slope of the roof, and perhaps the client’s budget. 


Raingardens are shallow depressions, typically planted with plants (preferably native), strategically located to collect, infiltrate and filter rain that falls on hard surfaces like roofs, driveways, alleys, or streets to minimise negative impacts of excessive runoff from these surfaces on lakes and streams.

Raingardens are designed to take the place of your stormwater system and soak up and filter the water that comes off your roof and paved areas.

Other terms include mini-wetland, stormwater garden, water quality garden, stormwater marsh, backyard wetland, low swale, wetland biofilter, or bioretention pond. The variables include dimensions, design, engineering components, and plant selection.

Building a raingarden

The design of a raingarden can be varied to accommodate soils, watershed hydrology, existing drainage patterns, aesthetics, microclimate, and purpose.

Basic design components include:

  • Grass filter strip
  • A shallow surface-water ponding area
  • A bioretention planting area
  • A planting soil zone
  • An underdrain system
  • An overflow outlet structure.

There is no standard size for a raingarden. One formula provides that the bioretention area should be 5% to 7% of the drainage area that the raingarden is intended to accommodate. A raingarden should be placed near impervious surfaces so that rainwater will drain into the dip or depression. Locate the garden strategically near impervious surfaces, such as alleys, sidewalks, driveways, and under downspouts or gutters, to capture the rain as close as possible to the point where it falls.

How to build a home raingarden

A raingarden receives rain water that runs off hard surfaces such as roofs and driveways. It passes through plants and soil before being released back to the piped stormwater system. Plants and soil absorb water and filter out pollutants, particularly metals (zinc and copper from roofs, cladding and car tyres) and sediment. Raingardens can also slow down stormwater runoff that cause erosion and flooding.

What not to do! Common mistakes

Because raingardens are quite new devices, most contractors have little experience in constructing them, and may not understand the critical features that make them work. Common mistakes are:

  • Stormwater doesn’t flow into the garden
  • Stormwater enters garden too fast – causing erosion unless a stone mulch is present
  • Over flow set too low, preventing adequate ponding depth. In the worst cases stormwater can flow straight to overflow, bypassing the soil and avoiding treatment.
  • Overflow adjacent to inflow – this tends to allow bypass flow, particularly if the overflow is set too low
  • Poorly drained soils used. This leads to inadequate treatment and long ponding time which can kill the plants and create a mosquito breeding ground.
  • Raingarden filled too high, particularly with mulch. This reduces ponding depth.
  • Floating mulch used (e.g. bark chips). Floating mulches tend to block overflows and/or get washed off-site.
  • Raingarden planted before construction elsewhere is complete – sediment and concrete washing gets into the garden and blocks the surface
  • Not enough plants in the raingarden. Plants are needed to keep the soil surface open, so stormwater enters the soil.
  • Stones used as mulch are too big – preventing plants from fully exploiting the soil surface
  • Garden mix used below 50 cm depth. Most garden mixes are high in compost so settle and release a tea-stained discharge in the short term

Common faults (from left to right): not enough planting and stones too large for plants to be able to push their way through; raingarden over-filled with soil and mulch so stormwater cannot pond and instead runs to the overflow (grate); floating mulch – if the grate was not protected by the concrete blocks, bark would block the grate.

Where to exercise you imagination - negotiables and non-negotiables

Negotiables are areas where you can exercise your imagination. Individual flair can be shown in: 

  • Plant selection (as long as they tolerate temporary ponding and dry periods)
  • Raingarden shape (as long as water flows evenly across the raingarden)
  • How runoff moves to and from the raingarden: inflows lend themselves to rainy-day water features

Non-negotiables are things that a raingarden must have, or it will not work properly, or may not work at all. The non-negotiables are:

  • Water must flow into the raingarden
  • Water must flow evenly through the raingarden and pond to a relatively even depth. The sides of the raingarden may be gently sloping, however, the remainder of the garden should be fairly level
  • The soil must be well-drained. Raingardens treat water by passing it through the soil – poorly-drained soils can only treat small volumes of runoff.
  • Overflow must flow to a stormwater drain or protected overland flow path

Three innovative ways to move rainwater from roof or downpipe to raingarden, and create a feature. From left: wooden gutters; from pipe to corrugated glass sheet and basin of river-rocks; down a water chain of angled glass discs. All photos from displays at the 2006 and 2007 Ellerslie International Flower Shows.

Frequently Asked Questions

What is the difference between a raingarden and a normal garden?

A raingarden is a simple stormwater management system designed to treat and minimize runoff from hard surfaces such as roofs, driveways and roads. Raingardens receive more runoff than normal gardens, are usually lower than the surrounding surface, and often have imported or sand-amended soils so they drain well. They also have underdrains, and usually overflows that are connected to stormwater pipes. They need far less watering than normal gardens and should need no fertilising. Most raingardens also have a dense groundcover of perennial plants that filter water and protect the soil surface. There is no bare soil once plants are established and no annual plants.

How much will a raingarden cost?

Raingardens are slightly more expensive than standard gardens. Costs depend on the extent of soil excavation, removal, and replacement, and the length of pipes needed.

What plants are best?

The best raingarden plants form a dense, weed-suppressing cover and tolerate dry conditions as well as short-term flooding – these are plants typically found on the edges of wetlands that dry out in summer.

This is why native rushes (Juncus and Apodasmia/Leptocarpus species), sedges (Baumea and Carex species) and flaxes are commonly used in raingardens.

Generally the plants will have most of their foliage above the maximum height water will pond, however, where the rainwater is clear (e.g. roof runoff), shorter groundcovers may be suitable, e.g., Selliera, Acaena and Leptinella species).

These plants can also be used on the gently sloping edges of raingardens where the water ponds for very short periods – these edges are also suitable for plants less tolerant of ‘wet feet’ (e.g., Hebe and Muelenbeckia species). Deciduous plants are not generally used in raingardens as leaf fall can block outflows. Trees are generally restricted to larger raingardens, and are either naturally cast a light shade or are pruned (lifted and thinned) to ensure the groundcover plants get enough light to maintain a dense growth.

Can my raingarden be too big? Or too small?

Large raingardens tend to dry out faster, so may need more drought-tolerant plant species. Larger raingardens are suited to a shallow ponding depth and where the soil is not very permeable. It is best to have several entry points for stormwater into a large raingarden as it is harder to get even ponding over a large raingarden if water only enters in one end.

Small raingardens struggle to detain and treat runoff from heavy rainfall. They overflow more often than properly sized raingardens. The overflows therefore may need to be larger than for standard-sized raingardens and a small raingarden will not be as effective at reducing downstream flooding.

How long will my raingarden last? How will I know when my raingarden needs replacing?

Raingardens need replacing if the soil or drains block up, or when the soil cannot hold any more pollutants. Blocked soils are indicated by runoff ponding for more than 1 to 2 days. Remedies depend on what is blocking the soil, and include: surface forking (to break up surface compaction or a surface crust), removing soil or leaves that have been washed into the raingarden, or clearing-out the under-drain (if accessible). Raingardens treating runoff from roofs and areas not trafficked by cars are unlikely to need replacing for decades.

Can I grow vegetables in my raingarden?

Raingardens are not designed for growing plants for eating for two reasons.

  1. Raingardens should not be fertilised or the soil disturbed once they are planted, as this risks release of nutrients (especially nitrogen) into waterways.
  2. Raingardens are designed to filter contaminants from stormwater runoff – these contaminants include metals and other substances that could be harmful.

How do New Zealand home raingardens and guidelines differ from commercial raingardens?

Raingardens receiving runoff from busy roads accumulate contaminants relatively quickly and need replacing more often than home raingardens. Raingardens in streetscapes may also be more engineered (more concrete) as adjacent ground needs to support heavy vehicles. The plantings are often much simpler, using only a few species, as these suit public spaces and are easier to maintain.

Are there rules about raingardens?

If your raingarden is smaller than 1000 sq m (e.g. 20 m long by 5 m wide) it is a Permitted Activity and does not require a resource consent. However it is best to check with local council before commencing excavation work.

Vegetated Swales

Swales are vegetated areas used in place of curbs or paved gutters to transport stormwater runoff. They also can temporarily hold small quantities of runoff and allow it to infiltrate into the soil.

A vegetated swale may also be known as a grassed channel, dry swale, wet swale or biofilter.

Vegetated swales can serve as part of a stormwater drainage system and can replace kerbs, gutters and storm sewer systems. Swales are best suited for residential, industrial, and commercial areas with low flow and smaller populations.

Swales can have significant environmental benefits but they do have limitations. Swales can reduce peak flows, remove pollutants, and promote runoff infiltration, and they tend to have lower capital costs. However, vegetated swales are typically ineffective in, and vulnerable to, large storms, because high-velocity flows can erode the vegetated cover.

While swales are generally used as a stand-alone stormwater Best Management Practice (BMP), they are most effective when used in conjunction with other BMPs, such as wet ponds, infiltration strips, and wetlands.