Last week the autumn rains started in earnest and Perth had the wettest May day in 9 years. This was just in time to soak the bog garden I planted two weeks ago.
“A bog garden is essentially a pond that has been filled almost entirely with soil or other material, and which retains moisture through all or most of the year.” – Building Frog Friendly Gardens 
The garden bed next to the pond is shaded by a fence. I noticed it retained dew throughout the day in the colder months. This is the perfect site for a bog garden because it won’t dry out as much in summer.
At the end of April I went to the Wildflower Society of Western Australia Northern Suburbs Branch Plant Sale and found lots of bog plants. This Nursery is at Landsdale Farm School and is open every Saturday morning. They sell tube stock native plants and provide advice on what to grow in your garden for a beautiful floral display and to attract wildlife. Bog plants are one of their specialties but they have a wide range of native plants and knowledge about growing them.
To build the bog garden I needed to dig out the garden bed, line the hole with plastic sheeting, and refill with soil. Natural boggy areas have clay or loamy soils which hold the water, but the plastic lining acts as a barrier to stop water draining away, so it doesn’t matter what soil fills the bog garden. It’s important to add some non-phosphorous fertilizer (eg. sheep manure) because the plant roots will be constrained by the plastic and can’t access outside nutrients.
I chose plastic sheeting because I had some to hand. Building Frog Friendly Gardens lists many pond building materials and how to install them. I made my pond with a prefabricated plastic design that has many different levels allowing frogs to easily enter and exit, but these are expensive. Recycled materials like a bathtub can be free and just as serviceable for a pond or bog garden.
The soil I dug out is full of False Onion Weed (Nothoscordum gracile) a horrible weed that’s all through my garden and is the bane of my gardening life. It grows from a bulb which produces bulbils that are easily dislodged when the plant is manually removed. Each bulbil grows into another plant. In the past I’ve dug down to the bulb (which can be 20cm deep) and pulled out the bulb and soil surrounding it to include all the bulbils. The following year they come up again from any bulbils I missed. I didn’t want to return this infested soil to my new bog garden. After digging it out, the soil was piled up and covered in spare plastic sheeting so the sun will heat it over months and hopefully kill all the bulbs and bulbils.
I got a load of Native Soil Mix from Soils Aint Soils, a blend of topsoil, peat and Gin Gin red loam, basically sand which works well for native plants. I needed to add organic matter and minerals because the roots of the bog plants can’t grow past the plastic liner to access nutrients. As I refilled the hole I added the limestone rocks dug out of the original bed (our sandy soils are full of limestone). When planting each bog plant I watered them in with liquid worm castings from my worm farm and I mulched the bed with prunings from the woolly bush in my garden.
The mulch will lessen evaporation from the soil and over time detritus feeders will eat the mulch while their waste adds nutrients to the soil. On days when it didn’t rain since planting, I hand watered with liquid worm castings. In the two weeks since planting, many of the plants have new shoots, so they seem to have enough fertilizer. Depending on how the plants grow, I may add sheep manure in the future.
- Spreading Sword Sedge (Lepidosperma effusum)
- flower of Blue Grass Lily (Agrostocrinum scabrum)
- Goodenia pulchella
- the bog garden after mulching, showing the fence which shades the bed
- Common Buttercup (Ranunculus colonorum) with new shoots
- the bog garden after planting, showing the plastic lining that needs trimming
- native Maidenhair fern (Adiantum aethiopicum)
- Schoenus clandestinus with Goodenia pulchella ssp. Coastal Plain B behind
- bug on Black Bristlerush (Chorizandra enodis)
- new shoots on Blue Grass Lily (Agrostocrinum scabrum)
- Aplin, Piano & Sleep (2002) Building Frog Friendly Gardens. Western Australian Museum: Perth