The Gordon Gnohm

Play along at home, read Future Farmer in The Braidwood Bugle every Wednesday. 

Passive Land Hydration Pt 5 Secret Swale

Every time there is a good drop of rain, I look for new opportunities to increase my use of the heavenly bounty before it leaves my property. I first mentioned Basic Passive Land Hydration back in February. So far, the structures have been for use in the broader landscape. This week I will add my name to the list of Australians working to get the most out of the available Skywater, specifically in small spaces. My contribution to Passive Land Hydration is the Secret Swale.

As with previous structures a Secret Swale is a trench on contour, designed to capture runoff and spread it across the garden to soak. The spoils of the trench can be laid downhill as a small mound, or the subsoil can be used elsewhere. 

All the Passive Land Hydration concepts I’ve previously mentioned, are design elements for use where appropriate. As in Nature there is no one solution, but a complex assembly of elements combined to create the basis for a functioning ecosystem. This platform allows the space to flourish and compound itself.

Construction of a Secret Swale

The trench is dug along the chosen contour. The trench is lined with geotextile fabric to prevent erosion of the trench walls. A slotted ag pipe is placed in the bottom of the trench to allow water to flow freely across the level surface. The rest of the trench is filled with 20mm+ pebbles. 

Round river pebbles have more capacity than crushed aggregate. The crushed aggregate can get clogged easily and has less airgaps reducing capacity. The trench is about water holding capacity, so the bigger the air gaps the more water can be held for soakage.

Once the trench is full of pebbles, the geofabric is wrapped around the top and secured, like a big sausage. Soil can be added to the surface to completely hide the structure, and have the topsoil built on top. You can’t plant on it, but you can plant around it. The trick is getting them to flow into the next one downhill discreetly.

Foundations for Success

Without water, your garden will surely die, therefore getting your watering system setup is the first step. The first few years are crucial to the establishment of your garden plants. Watering is key but water loss is just as critical. Mulch is a great way to prevent water loss, as are ground covers, but the king of the garden when it comes to preventing water loss is shade. 

This will be provided by the canopy trees over several years and the pressure on other elements will be lessened. Shrubbery will reduce wind speed across the ground and provide shade in the morning and afternoon. Most vegetables will thrive in some shade especially in the afternoon.

Whats the Secret?

You can build soil directly on top of the Secret Swale, but I generally use the top of the Secret Swale as a path. It is well hidden underfoot and nobody would know it was there if it wasn’t pointed out. But your plants will know. They will have water slowly moving through the soil whenever they need it. Growth rates will be improved and new plants will establish faster than you might expect.

The first few years are crucial to the establishment of trees and shrubs. Deep penetrating roots are the key to long term survival in our Unpredictable Highland Climate. Surface watering as discussed in Hand Watering rarely translates into deep root penetration. 

Best Use

I use Secret Swales extensively in my Ecosystem Designs for suburban homes and around homesteads. I look to capture extra runoff from roofs and hard surfaces to distribute it across the yard for soaking. I’m looking for sheds, lawn lockers and especially the roof of the home. Sometimes these roofs already have tanks, so the overflow of the tank is the source.

Starting at the top of the system I create Secret Swales across the yard, where practical. This is the first element I design, because it is the first opportunity to utilise the water and feed the soil. All the plantings then occur around the watering system. Wet spots and dry spots are created within the ecosystem providing opportunities for different plantings, to feed you, the soil, to play with solar access, to create privacy or produce a specimen tree to marvel at.

Path Toppers

Using the Secret Swale as a path means that it is always visible in the garden. Future placement of new plants will be easier. There are two options for topping the paths for maximum benefit. More pebbles on top allows for better infiltration of falling rain and allows the trench to breathe. Usually, a pebble top is an aesthetic choice. 

For full function, I choose woodchips on top of the Secret Swale. The wood chips are excellent as a path. They steel nitrogen from the surrounding area to start the breakdown of the woodchips. No food for weeds to grow in the path for several years. Once the wood chips start to breakdown they will become habitat for a fungal network. This network is the basis for the food web in your living soil and will exist best where there is moisture. 


There are a couple of things that a Secret Swale can’t do. There is no habitat created in the trench with grasses or reeds and there is no accumulation of nutrients in the trench from leaf fall or windblown debris. This work needs to be done with your mulch and plantings. A Secret Swale is designed primarily to deliver water across the garden, quietly, passively.

Use the best option for your situation, and perhaps combine them into a more complex system. Water is the source of all life so ensure you make the most of it on your patch of earth. 

Stay Awesome.

The Gordon Gnohm

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