Rain Gardens

Essential Environmental Filters

How WaterUps ensures Rain Gardens work as intended

Rain gardens have been a landscaping tool that councils across Australia have implemented over the past two decades to varying success. It’s a concept developed during the early 1990s in Maryland, a Mid-Atlantic US state that’s defined by abundant waterways and coastlines on the Chesapeake Bay and Atlantic Ocean.

Before the built environment, natural systems filtered rain through soils. The mineral component of soils work as a mechanical filter, while the soil life, such as bacteria, fungi and other microorganisms, consume the pollutants resulting in the removal of biodegradable organic carbon.

The main types of microorganisms that break down pollutants in a biofiltration system are aerobic, requiring oxygen to metabolise compounds (nutrients or pollutants).

This is why a constructed biofiltration system must not be allowed to get waterlogged as the aerobic microorganisms will perish. Adversely, rain gardens must not be allowed to dry out as the soil life will die or hibernate in very low numbers.

In a world that is increasingly challenged with pollutants and water run-off – rain gardens have merit, however, as always, the theory often works better than practice.

On this page we provide an overview of rain gardens:

  • What are they?
  • Why do we have them?
  • How are they meant to work?
  • Our experience in rain gardens
  • How WaterUps is re-engineering rain gardens, so they do in fact work as intended.


What are Rain gardens & why do we have them?

Rain gardens are situated at the point where pollutants can enter waterways. This is typically on roadsides in built up areas where pollutants from vehicles and businesses are most at risk of entering the stormwater system.

Rain gardens are also of great use when installed inline on a buildings stormwater system, as many pollutants gather on roofs and balconies with the biofiltration as the first line of defence when it comes to treating water that washes off hard surfaces during a downpour.


They consist of a filter layer, made up of nutrient poor, free draining sand above a coarser layer of aggregate that lets the treated water move away quickly. This is the mechanical filtration layer, with all of the larger particles being left in the upper part of the sand layer.

Very large pollutants such as drink bottles, cigarette butts and other debris must be removed by hand. Plant selection is important, as the plants (mostly grasses) must have the type of root system (mostly fibrous) to support soil life. It is this soil life, attached to the plant roots, that will break down the soluble organic pollutants.

These plants are typically naturally occurring in waterways native to an area and are chosen for their ability to flourish in wet growing conditions and for their filtering qualities.

These plants clean stormwater before it runs into waterways, removing damaging elements such as chemicals, rubbish and oils.

How are they meant to work?

In Theory

Rain Gardens are designed to drain and filter large amounts of water quickly. The size of the catchment and volume of water expected will dictate the size needed for a rain garden.

The first 5 – 7 mins of a rain event is considered the most polluted water, and is referred to as the first flush. First flush water contains oils and hydrocarbons from roads and high nutrient loads from building roofs. If this water is diverted or treated, the water that follows will be considered clean and can flow into natural waterways without causing too much damage.

The rain garden must be able to absorb and filter the first 5 – 7 mins of heavy rain through the system before it becomes temporarily waterlogged and the ‘clean’ water that follows can be diverted directly into stormwater systems via the ‘high point overflow’.

Here is a photo of a healthy rain garden in the Ku-Ring-Gai LGA, shortly after planting in 2008.

Plant Species

In Sydney (and potentially other areas), plants that are suitable for rain gardens include:

Knotted club rush (Isolepis nodosa).
Basket grass (Lomandra longifolia)
Tropic belle (Lomandra hystrix)
Blady grass (Imperata cylindrical)
Longhair plumegrass (Dichelachne micrantha)
Swamp banksia (Banksia robur)
Gymea lily (Doryanthes excels)
Thyme honey-myrtle (Melaleuca thymifolia)
Lilly pilly (Acmena smithii)
White correa (Correa alba)
Bottlebrush (Callistemon)
Blue flax-lily (Dianella caerulea ‘King Alfred’)
Native rosemary (Westringia fruiticosa).

The microbes consume contaminants such as oil and organic matter, converting them into simple carbon compounds to produce the sugars and fats they need for growth. The byproduct of this is to let off carbon dioxide and water.

If the soil (filter media) stays moist and alive, the carbon dioxide is stored or sequestered in the soil and the water is returned to the greater cycle.

The process uses naturally occurring bacteria, fungi or plants to degrade substances that are hazardous to human health or the environment. That’s the theory.

In Practice

What we find is rain gardens, unfortunately often never reach their potential because they are prone to drying out between rain events causing stress on the plant and letting microbial activity in the soil to go dormant.

When the soil dries out, the intended plants often die as they have been selected from naturally moist environments like riparian banks, and do not have the same resilience when planted next to a hot roadside. As the plants and soil dry out, the carbon being held by the soil is released and the life in the soil goes dormant.

A dry and lifeless soil will now only allow the most opportunistic of plants to repopulate the garden. The plants capable of growing in these conditions are almost always weeds. Even weed species struggle to grow well in these dry baron conditions and are often stunted and are very quick to seed. Rain gardens now become a way for weeds to very quickly distribute their seeds directly into waterways.

Same rain garden in Ku-Ring-Gai experiencing total failure only 3 years later.

WaterUps is re-engineering rain gardens to work as intended

WaterUps has been advancing the concept of modern-day wicking for over seven years through innovative wicking systems built from recycled plastic.

By installing a WaterUps Sub Irrigation Channel (or SIC for short), rain gardens will be kept watered for many weeks between rain events, creating a more resilient garden with healthy plants and an active filter media ready to perform when needed. (See diagram below)

Additionally, councils can, during extreme dry spells, top up the reservoir through an inlet pipe, ensuing the soil biota continues to live and thrive as intended.

Another advantage to a reservoir below the filter media is that water testing can be carried out to gauge the effectiveness of the individual rain garden over time.

By installing a SIC in rain gardens, councils everywhen can take responsibility for surface water before it enters our natural waterways.

Making Rain Gardens Work for you

Property developers as well as councils across Australia have a lot to gain from a WaterUps wicking solution installed in their rain gardens.

Please get in touch if you would like assistance in planning a rain garden.

Eric Sturman, one of our directors and qualified landscape architect, has considerable experience working with council in implementing gardens and related water management solutions.