Holistic approach creates high performing, low maintenance rain gardens


Holistic approach creates high performing, low maintenance rain gardens

Wholesale Landscapes explains how taking a holistic approach to bioretention media can provide high performing and resilient rain gardens, breaking down the myth of high maintenance and costs.


Our perception on precipitation

One unique aspect of New Zealand is the many microclimates we have. From the rain-drenched West Coast, the drought-prone Canterbury plains to the windy seaside suburbs of our capital, a range of plant varieties will survive – even before we mention hot summers and cold winter weather.


When we put additional stress factors on plants and include functional elements for rain garden use, we are not only more reliant on them to function and thrive but to also live in an environment with even more variables. So based on this, should we also be incorporating this into rain garden design especially regarding the catchment area and rain garden media?


In recent times we have seen rain gardens second-guessed in projects, partly because of their reputation for poor performance and high maintenance costs. But what if we wound back the clock and worked through the requirements of the rain garden for the project location, as opposed to a nationally utilised specification with a wide range of micro-climates? After all, a plant in the Auckland CBD will have different requirements to an equivalent in Dunedin.


Often it will be because of a ‘surrender to simplicity’ approach which streamlines projects, although nature and biology do not work to specifications. Therefore we must learn to adapt to ensure the microbes and plant health that remove the pollutants from stormwater can effectively function. Have we simplified standards too much to the point that now one size fits none?


Poor media in the limelight

When rain gardens do not perform and work as intended everybody loses. From the consultant who specified it to the civil contractor who has the 12-month maintenance clause attached. Lastly, ratepayers and the community must live with the garden so a net positive outcome is a necessity.


One way to reduce risk and maintenance is to look at the micro-climate and plan a mix specific for that region, project and design. We can do this by evaluating past data of rainfall, wind, moisture levels, plants and catchment area to ensure the Bioretention media provides a healthy environment for microbes, plant health and ecological stormwater filtration.


Niwa’s CliFlo database can provide an excellent insight into historic data and although it will not tell us in black and white what the future weather will be, it will certainly help support or indicate what the average climate may be and what influences need to be considered.


This helps set up the rain garden for successful stormwater management in the environment it operates.  It’s taking away the ‘fit the box’ approach and replacing it with a cost-effective, yet intuitive and ‘common sense’ approach without the costs of going outside regulation.




Proven success

One of the recently completed projects west of Richmond, Tasman showed how a successful yet specialised media was used in the rain gardens without negatively impacting on budgets or influencing overall design.


From ongoing droughts to fires and cyclones, the Nelson-Tasman area has had its fair share of unexpected weather events which has affected the civil construction and stormwater sectors significantly.


Wholesale Landscapes was involved in the early discussions with Tasman District Council and Davis Ogilvie which made planning and execution streamlined. The Tasman District Council has a Rain Garden Practice Notice which was a starting point for recipe development. Rather than begin with our own media, we used the TDC specifications as a starting point and staying within their benchmarks ensured Wholesale Landscapes was able to get the media approved quickly whilst still having complete confidence in our product.


Tom Filmer of Wholesale Landscapes commented. “within the TDC rain garden media guidelines there is significant scope to tweak media recipes to ensure they meet the project needs. In addition to this, we utilised in-house laboratory testing verified by Landcare Research to replicate the catchment areas and provide certainty to the council and engineers that this recipe would be an excellent fit for the project without adding costs or delays – bespoke but within budget”


An example of how we adapted the media for this project was being able to cope with floods yet be able to uptake enough for the drier spells. We did this by staying within the 100-300mm hydraulic conductivity (TDC benchmark) but after running tests, we were able to determine that the lower quartile in the range was best suited for the project. We tweaked the recipe and achieved a constant permeability of 130mm HC which provided the plants in the rain garden a flow that allowed them to let go of water quickly during floods but also holds onto water during dry spells.


The constant permeability could be increased or decreased depending on the environment the plants are living in, catchment area and the local climate of the project. This is just one element of rain garden media to ensure rain gardens are successful.


In Summary

The concept of rain gardens is a no-brainer but local government may be hesitant because of costs or unnecessary maintenance if they are installed and specified incorrectly, and rightly so.


Most areas of civil landscaping that involve high levels of customisation will come at the risk of a bespoke price tag but risks should be mitigated. In fact, where materials have a positive influence on the rain garden media and are more cost-effective leads to cost savings. Asking rain garden media suppliers to justify their recipe results and provide a supporting document by an independent laboratory is just one way to ensure the right media is going into the garden without restricting supply or increasing costs.


We as an industry can collectively work together to set rain gardens up as successful components for projects but it does require a collaborative approach from consultant to contractor.


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Recommended depths

Typically you need a finishing layer of screened soil of 100mm when working with an uneven surface. 100mm of soil will allow to even out any hollows and give enough depth to create a flat surface

A general rule for compost is to allow for the depth of the current root structure of your plants with an additional 30-50% to mix in with the base. If you’re planting into holes or filling up a vege garden  you will need to accommodate for this prior to filling up

We recommend 100mm (10cm) deep for new bark gardens and planting into bare areas. For existing gardens with some cover, we recommend 75mm-100mm (7.5cm-10cm)

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This excludes preparation, compaction or drainage gravel underneath. For existing areas 75mm thickness is recommended for areas needing a small top-up.