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.
How to build a home raingarden
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
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.
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.
- Raingardens should not be fertilised or the soil disturbed once they are planted, as this risks release of nutrients (especially nitrogen) into waterways.
- 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.