Aquaponics: Edwina Robinson shares her secrets

Seven years ago we met landscape architect Edwina Robinson and wrote about the vegetables she was growing in a raised garden bed in Hackett and a Canberra Organic Growers' plot in Dickson.

In 2010 she purchased a run-down duplex in O'Connor and hankered after a grove of silver birches. Robinson knew the trees suffered from drought in Canberra but decided she could gravity-feed bath water from the second storey of the house if necessary.

Following good soil preparation in the back garden she planted a range of fruit trees and shrubs against an east-facing masonry wall including avocados, a persimmon grown from seed and espaliered, figs, oranges, quince, pomegranate and Meyer lemon.

In colourful plastic tubs she has planted a mulberry, kaffir lime, peacharine, two-way grafted apple and a two-way grafted apricot. She has two edible hedges – a feijoa hedge and smaller hedge of currants. Upstairs in a north-facing window she is nurturing a red papaya.

To screen a chook run are thornless blackberry, boysenberry and raspberries but this now seems to be a bad move as cockatoos are starting to use the top of the run as a perch from which to pick at the raspberry canes.

She is trying to grow a year's supply of garlic, sourced from an organic grower in Braidwood, and over the 2013/14 season she harvested five months' worth. This year she has planted more, some in a raised garden bed, in a patch in the ground, and some in her aquaponics system.

Robinson worked with the ACT government project managing the design and construction of the Banksia Street, O'Connor, wetland and The Valley Ponds in Gungahlin. It was this work, as well as seeing a Gardening Australia program on Joel Malcolm's aquaponics set-ups in Western Australia, that inspired her to start an aquaponics journey. She visited several set-ups in Canberra and participated in a short course at Ricky Somerville's garden in Wanniassa (Kitchen Garden, January 30, 2013).

As a landscape architect, Robinson wanted to design and build an aquaponics system that was not only functional but also looked good. Last year she worked closely with Angus Watson, a cabinetmaker, to design a deck and pergola on the front verandah and the aquaponics were integrated into the deck this year.

The grow bed consists of a bath tub filled with gravel, and a 340-litre pond, from a hardware shop, that is only 300mm deep, the maximum depth allowable in Canberra without fencing. Water is pumped from the pond into the top of the grow bed and distributed by a network of polypipes with holes drilled into the underside.

To start the system, goldfish from an aquarium in Belconnen were introduced and they thrived. A work colleague provided her with brown trout fingerlings; however they have not been successful. The biggest challenge has been getting the PH of the water correct. Tap water has a high PH, over 8, which is too alkaline. She thinks the problem is due to the substrate used in the bath tub, which was river gravel sourced from a landscaper, and she may replace that with clay balls from an aquaponics store.

Silverbeet raised from seed, lemongrass protected with a polycarbonate roof, and Vietnamese mint have survived winter in the grow beds.

A City of Fooduaa.org.au/city-of-food-series

Susan Parsons is a Canberra writer.