Water-sensitive garden design using biofiltration

Source: Department of Water – Water sensitive urban design

Water-sensitive biofilter garden designs help to to reduce water use and filter out pollutants, reduce the urban heat island effect and also provide amenity and biodiversity value to your local area as well.

Creating a water-sensitive garden is a great way to reduce your garden water use and filter pollutants out of storm water to improve the health of the soil and water systems in your local area.
 
What you need to do to achieve this is develop is a biofiltration system which sounds a lot more complex than it is. Basically it is just adding a few other steps into your garden design to improve the natural filtration from your plants and soil.
 
The soils in WA unfortunately have a very poor ability of absorbing water and getting it down to where it needs to be. Making us need to use far more water on our gardens than necessary. That’s where the biofilter design comes in.
 
 

HOW TO CREATE A WATER-SENSITIVE GARDEN WITH BIOFILTRATION

 

Dig your planned garden area down to at least 65 cm and then build your biofilter with the following layers (top to bottom). Note: If you are planning the garden on your verge, check with your local council before you finalise your design.

  • PLANTS – Select a variety of natives that are suitable for your local area (for best filtration, select plants that have many fine roots and high plant mass – e.g. tall sedge (Carex appressa) or grey honey-myrtle (Melaleuca incana).
  • 5cm – STONE MULCH (small pieces of stone, roughly 4-13mm in size) this layer will help prevent weeds and keep moisture down in the lower soil layers.
  • 30cm – FILTER (use a mixture of the following: sandy loam, clay and silt (3%), fine sand (5-30%), medium sand (40-60%), coarse sand (7-10%), gravel (3%), low nutrient organic matter such as compost or cow/sheep manure (3%), mulch (5%), hardwood chips (5%).
  • 10cm – TRANSITION (clean well graded sand (2% fines rating), clean fine gravel or crushed rock. This layer should be saturated in groundwater.
  • 20cm – DRAINAGE (clean gravel)

Once your biofiltration system is set up, ensure you maintain it by pruning plants, replacing dead plants and topping up water during long dry spells.