Jeremy Prentice >
Noelene Pullen Photo

Storm water recycling is at the heart of -

A New Raingarden for the
Royal Botanic Gardens Melbourne

by Jeremy Prentice, RBGM - Click images to enlarge
     

What does a Raingarden have to do with water gardening, you may well ask? Everything, if the water coming out of said Raingarden is feeding the water bodies in which you intend grow your beloved aquatics!

In May 2007 the Royal Botanic Gardens Melbourne (RBGM) was approached by Melbourne Water to discuss the installation of a new Raingarden. As part of the work to improve the water quality of the lake system of the RBG, the Raingarden was approved and construction commenced at rapid rate, as it was hoped to be officially opened on World Environment Day, June 5, 2007 (and it was!).

The selected site was our Canna Bed, which has provided a reliable annual display of colour for many years. The reason for choosing this site was the topography, as the bed sits in line with the storm water drain that runs from a street outside the southern RBG boundary, which in turn feeds into our lake system. Also, the Cannas are a very suitable plant type for this kind of garden as they are voracious feeders and their growth is removed seasonally, thus returning all the nutrients they have stripped from the water back into our soils (via subsequent composting and mulching).

The basic principle of the garden is that as the water moves through the various layers, the filters (including the Cannas) will strip out the pollutants and nitrates as well as the larger organic matter and particulates (i.e. gravel). The water then continues out through a large drain, runs on down through our Fern Gully and into the main lake system. In time, the filter materials in the bed will have to be removed and replaced, probably every five years or so.

The works included removing the existing Cannas and soil, followed by installation of the different layers of the filtration, including course gravel, geotextile fabric and fine grade sand. The garden is terraced to help slow the inflow of water and ensure it penetrates down through the layers, rather than running off into the overflow drain. Once the terracing was completed the Cannas were replanted (with the help of some local school groups) on World Environment Day, in time for their new growing season.


Interestingly, the Raingarden, the Nymphaea Lake (which sits right beside the Raingarden), and the Fern Gully ephemeral creek all form part of what would have been a natural water flow (after significant rainfall) off Birdwood Avenue and down into the lake system. In the late 1800s parts of the RBGM site were still seasonal billabongs and low lying swampy areas. They were connected to the Yarra River which runs through the heart of Melbourne. As part of the efforts to clean up the Yarra there are now plans in motion for large scale water works at the RBG, of which you will hear much more in future issues of WGI Online - especially as it will include putting ornamental aquatics (not just indigenous species) back into our lake system!  


Noelene Pullen Photo

 

Nymphaea Lake

In a nutshell, we will be harvesting more water from the surrounding streets and hard surfaces and channeling it into our lakes. As part of this we will be installing bio-filters in the form of wetlands to both clean up the incoming water and promote a healthier lake system, as we frequently have problems with blue-green algae over the summer period. There will also be designated waterlily plantings to assist with keeping the lake healthy by providing aquatic habitat, reducing the light coming in, etc.


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