Stormwater
A home raingarden
- What is a raingarden
- Frequently asked questions
- How to build a home raingarden - 10 easy steps
- Links to more info
The ARC sponsored raingarden designed by Unitec students, Kirsten Sach, Jamie Douglas and Zoe Carafice for the Ellerslie International Flower Show 2007. It won gold in the outdoor exhibition category
Disclaimer: this information is for home raingardens only - raingardens on other sites, or where part of resource consents usually require specific flow attenuation and conformance to local standards, in Auckland this is Technical Publication10.
What is a 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. The plants and soil absorb water and filter pollutants, particularly metals (zinc and copper from roofs, cladding and car tyres) and sediment. Raingardens can also prevent erosion and flooding by slowing down stormwater runoff.
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Frequently Asked Questions
- What is the difference between a raingarden and a normal garden?
- How much will a raingarden cost?
- What plants are best?
- Can my raingarden be too big? Or too small?
- How long will my raingarden last? How will I know when it needs replacing?
- Can I grow vegetables in my raingarden?
- How do New Zealand home raingardens and guidelines differ from commercial?
Raingardens planted with native rushes and flowering Hebes on the sloping sides where ponding duration is shorter, Waitakere Civic Centre (site in full sunlight).
Answers
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 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 (no annual plants) that filter water and protect the soil surface. Once plants are established there is no bare soil.
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Raingarden often have overflows connected to stormwater pipes (left photo). The round grate at bottom right is the overflow in a raingarden built on the shady side of a building planted mainly with low ferns, Waitakere Civic Centre. Raingardens usually have a dense perennial groundcover (right photo)
- 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. Where the rainwater is from 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 used in raingardens as leaf fall can block outflows. Trees are restricted to larger raingardens.
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- 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 shallow ponding and where the soil is not very permeable. They need several entry points for the stormwater to flow as it is harder to get even ponding 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 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 that treat runoff from roofs and in 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.
This raingarden on sloping ground adjacent to a public road has deep baffles to ensure water ponds evenly and to prevent erosion and a wide concrete apron next to the road. People are separated from the relatively large vertical drop by a fence.
How to build a home rain garden - 10 easy steps
Links to more info
- For website links see related links on the right of this page
Books:
- Nigel Dunnet and Andy Claydon - Raingardens. Managing water sustainably in the garden and designed landscape. 2007. Timber Press. Inspirational ideas for home raingardens, but some of the plants listed are weeds in NZ.
