Managing Stormwater with Rain Gardens and Bioswales

This resource examines the design, function, and plant selection for rain gardens and bioswales in an Australian context. It is intended to support landscape specifiers in understanding the hydrological, ecological, and design advantages of these systems.

By the end of this module, participants will be able to:

• Define rain gardens and bioswales and describe their distinct functions.
• Compare the hydrology, design, and operational characteristics of rain gardens and bioswales.
• Identify suitable plant species for each system.
• Appreciate the limitations of unsuitable straight species such as Westringia and understand how recent breeding improvements have allowed certain varieties to be worth considering.

Unlike almost all other Westringia, this variety (along with Mundi™ Westringia) can tolerate ex-treme wet as well as dry. If the water drains downhill to the left (out of frame), this is a bioswale, but if it’s just a depressed bowl in the landscape, it’s a rain garden.

Introduction to Stormwater Management

Rain Gardens

• Definition: Shallow, vegetated basins designed to capture and absorb runoff from impervious surfaces (e.g. roofs, pavements).
• Function: Captures water temporarily, allowing infiltration, plant uptake, and evaporation.
• Design Features:
  • Compact, bowl-shaped depressions.
  • Intermittently wet, drying out between rain events.
  • Often underdrained with gravel or sand to enhance infiltration.

Bioswales

• Definition: Linear, sloped channels that convey and filter stormwater.
• Function: Slows water flow, reduces runoff peak, and allows for sediment and pollutant removal through infiltration.
• Design Features:
  • Elongated design with a gentle grade.
  • Variable moisture zones, sometimes permanently moist at the base.
  • May include elements such as check dams or gravel trenches to increase water retention time.

Comparison: Rain Gardens vs. Bioswales

Feature	Rain Garden	Bioswale
Form	Depressed basin	Long, shallow channel
Primary Function	Infiltration and treatment of stormwater	Conveyance, infiltration, and treatment
Water Retention	Temporarily ponded post-rainfall	Primarily channels water while slowing flow
Hydrological Dynamics	More static, with periodic wetting and drying	More dynamic, with continuous water conveyance
Planting Considerations	Suited for species tolerant of both inundation and drought, with more difficult conditions at the bottom of the slope	Suited for species tolerant of both inundation and drought, with more difficult conditions at the bottom of the slope

Both systems utilise vegetation to manage stormwater and require plants that can thrive under fluctuating moisture conditions.

Suitable Plant Selections for Australian Conditions

For optimal performance, the following cultivars are recommended for both rain gardens and bioswales in Australia:

The following native Australian plants are excellent performers in both rain gardens and bioswales:

New Cultivars:

Baby Breeze™ Dianella ‘DCNC3’ PBR Intended	Goldie Box™ Westringia fruticosa ‘WES11’ PBR
Lady Tanika® Lomandra longifolia ‘LM360’ PBR	Shara Blue™ Lomandra fluviatilis ‘LM380’ PBR
Space Saver™ Melaleuca armillaris ‘MELA01’ PBR Intended	White Mundi™ Westringia fruticosa ‘WES09’ PBR

Established Cultivars:

Slim™ Callistemon viminalis ‘CV01’ PBR	Macarthur™ Callistemon viminalis ‘LC01’ PBR
Better John™ Callistemon viminalis ‘LJ1’ PBR	Green John™ Callistemon viminalis ‘LJ23’ PBR
Yalba™ Imperata cylindrica	Isabella® Liriope muscari ‘LIRF’ PBR
Just Right® Liriope muscari ‘LIRJ’ PBR	Amethyst™ Liriope muscari ‘LIRTP’ PBR
Katie Belles™ Lomandra hystrix ‘LHBYF’ PBR	Lucky Stripe™ Lomandra hystrix ‘LMV200’ PBR
Shara™ Lomandra fluviatilis ‘ABU7’ PBR	Evergreen Baby™ Lomandra labill ‘LM600’ PBR
Tropic Cascade™ Lomandra hystrix ‘LHWP’ PBR	Cosmic Pink™ Rhaphiolepis indica ‘RAPH02’ PBR
Cosmic White™ Rhaphiolepis indica ‘RAPH01’ PBR	Grey Box™ Westringia fruticosa ‘WES04’ PBR
Mundi™ Westringia fruticosa ‘WES05’ PBR	Nafray® Pennisetum alopecuroides

Considerations for Westringia spp.

• General Suitability:
  • Traditional varieties of Westringia are generally not well-suited to persist in prolonged wet conditions typical of stormwater systems.
• Modern Breeding Advances:
  • The four varieties listed above are capable of tolerating extreme flooding and drought, including nearby plants infected with root rot Phytophthora.
  • Remember that separate varieties within the same species can behave remarkably differently in the same conditions, and this is why plant breeding exists.

Conclusion and Professional Reflection

Rain gardens and bioswales are vital components of sustainable urban design. The strategic application of these systems not only aids in effective stormwater management but also enhances ecological diversity and urban aesthetics.

The careful selection of suitable plant varieties is imperative, with the toughest varieties positioned at the base, and more sensitive varieties sometimes used toward the top of the slope. All Ozbreed plants recommended within this article have been tested rigorously in wet feet.

You can view our ongoing research methodology here: 

Reflective Questions for Participants

• How might the use of these systems vary between urban and suburban environments in Australia?
• In what ways can the integration of tolerant plant cultivars, such as modern Westringia varieties, expand the design flexibility of stormwater management systems?
• How can the incorporation of these green infrastructure elements help address local water quality concerns?

Learning Outcome:

Participants should now be equipped with a deeper understanding of how to design and maintain functional stormwater systems using rain gardens and bioswales, alongside an informed choice of plant species adapted to Australian environmental conditions.