A rain garden is a garden that captures storm water overflows or runoff and slows or holds that water while it infiltrates deeper into the soil or serves to water contained plants. Our blog focuses on home scale setups of up to approximately 10 square metres, If you want more details on designing larger setups, check out the Water Sensitive SA website.
Rain gardens are a great way to use rain water that comes from gutters that are too low to plumb into a rain tank or to use the overflow from the tanks themselves.
There are two broad styles of rain garden. I call them ‘holding‘ and ‘infiltration‘ rain gardens.
‘Holding’ style rain gardens
Holding style rain gardens (also called ‘vegetable rain gardens) retain the water flowing into them via an impermeable membrane such as builders plastic.
The water is directed into a layer of coarse material such as gravel at the bottom and wicks up into the soil above it. In this sense, it is like a rain fed Wicking Bed (for more info on these, see this page).
The gravel and soil layers are separated by a fabric material such as shade cloth or Geotextile which reduces the amount of sediment from the soil settling into the gravel while still allowing the water to pass through.
Holding style rain gardens have some method to allow excess water to overflow whether on to exterior soil or back into the downpipes.
‘Infiltration’ style rain gardens
Infiltrating style rain gardens are designed to capture runoff and slow it down, giving it time to infiltrate into the ground beneath it.
This style of rain gardens generally have a gravel layer that captures the water, above which is layer of soil into which plants are planted. The two layers are generally separated by a permeable fabric such as shade cloth or Geotextile material.
Incoming water is directed into the gravel layer, where its flow is slowed. It waters the plants that are in the rain garden and allows water to slowly infiltrate the ground beneath it.
While its not essential in all cases, there is a permeable liner (we want to let the water infiltrate, not hold it otherwise we’re building a pond) below the gravel.
Infiltration style gardens can be easy made into Reed Beds for filtering the runoff and removing pollutants. They have their own page here (not yet linked)
I’m working on a table about infiltration into soil so that you can size your rain garden – so come back next week and see the finished product!
Planting rain gardens
Rain gardens of all types supply extra water to plants when seasonally available. When there’s no rain, they can’t provide water so they either need to be watered from time to time or the plants that go into them must be able to handle a flood and dry cycle in which the dry time can far exceed the flood time.
Holding style rain gardens are easy to just top up here and there, especially if they are designed as a rain supplemented Wicking Bed.
Infiltration style rain gardens often cover a much wider area and may even consist of patches of different soil types.
In this style of rain garden, the choice of plants is more critical. Drought hardy plants that are able to tolerate inundation seem to be a forte of the Australian biota, so many of the species you see listed will be Australian Native plants.
The plants chosen should be perennial and have fibrous root systems. Having rhizomes is also an advantage. Plants with stronger roots/rhizomes and ‘more rigid forms are best in area nearest the inlet where their strength can keep them in place against the higher water flow. For larger rain gardens, these plants work to screen larger debris from the garden.
Species suited to rain gardens:
Near the inlet
- Sedges (Carex, Cyperus & Ficinia species)
- Rushes (Juncus species)
- Bullrushes (Typha dominigensis)
Away from the inlet
- Black Anther Flax Lily (Dianella revoluta)
- Goodenias (Goodenia species)
- Kidney Weed (Dichondra repens)