The secret’s out – if you want to grow lush, resilient crops without the need for time-consuming watering, then swales are the way to go. If your section is very wet, swales can be used to move water around; for normal-to-dry properties, swales have the ability to hold water for longer, keeping the ground moist, cool and resilient. So, what is a swale and how can we engineer one on our own property?
A swale is simply a ditch; engineered to enable it to capture water, retain it for longer, move it around if need be, and slowly release it into the surrounding soil. How much water is released and how quickly depends on a number of factors including soil type, slope and how the swale is engineered.
Water use is highly efficient when using swales. Imagine a sloping section without a swale. As it rains, water quickly flows down the section and away from your property. Over time this can create run-off, cause loss of soil nutrients, and degrade topsoil. Add a well-dug swale to the equation, and the movement of water is slowed down; increasing water efficiency, reducing run-off and degradation of topsoil, and assisting with plant growth. Plants have better access to water when they need it which equates to a less stressed, healthier and more robust garden.
There are a number of ways to build swales – it all depends on your particular situation. Some people like to dig a swale and retain the ‘ditch’ or even grow water-loving plants in it. Others prefer to fill the swale with mulch, such as woodchip, so they can continue to walk over the swale. By filling the swale with mulch you’re increasing its capacity to hold water for longer, reducing evaporation, and helping to build on the soil’s microbial life (for instance, leaf matter and woodchip both help promote fungi).
If you have a wet property and your aim is to move water around the property, then your swale can be dug on a very slight slope, with materials (bricks for example) placed at each end, to retain water. When the water reaches a certain level (the top of the brick) it will flow on to the next part of the swale, or a drain (as illustrated in issue 441, in Brian Stephen’s South Canterbury garden).
If you have a normal-to-dry property (or one in which you wish to retain water for longer periods of time), then your swale should be dug on a level contour. It is very important to get it ‘level’, here’s how you do it.
Build an A-frame for a level contour
Digging swales on a level contour enables water to be held in the swale for longer.
A simple way to ensuring your contour is level is to build an A-frame, then use that A-frame to mark your contour out before digging it. To make an A-frame:
Cut two identical pieces of wood (say 1.3m each) and join them together to form the ‘legs’.
Attach a horizontal brace across the middle of the two ‘legs’ so the frame looks like a capital ‘A’.
Tape a level to the centre of the horizontal brace, ensuring the level indicator ‘bubble’ is dead centre.
Map and mark
Using your A-frame, begin at one side of your property. Stand beside your A-frame, and, moving your A-frame around, find the contour that is ‘level’. When you find it, mark the opposite side of the A-frame with a flag or stick, then move the A-frame along to mark out the next area of contour (don’t switch the A-frame around, keep it facing the same way). Continue doing this, finding the area that is level, marking it with a flag or stick, then moving the A-frame onto the next part to find another level area. When your swale contour is all marked out across your property, you can use a piece of string, or turf paint, to mark the swale area to be dug – tracing back your steps from the beginning, marking a line along each flag you have used to mark the contour.
Dig your swale. A general guide is to dig at least a spade depth, placing the soil you have dug at the front of the swale (the sloping side). This is where you will plant moisture-loving trees, shrubs, and lower-growing plants. Larger swales can be dug using a digger. Before starting be sure this won’t affect the stability of the land if the section is on a steep slope.
Once the swale has been dug, check the level again using the A-frame, and any areas not level can be dug out a little more.
Fill your swale with water and test whether the water is held level. If the swale has been dug on a slope the water will run to one side. This isn’t ideal, you want the water to remain level. If the water is running to one end you will need to do more digging to make
Once you’re happy with how your swale operates, you can then fill it with mulch (if you choose), and plant around it. Remember to plant water-loving plants down the slope, where the water moves naturally with gravity.
Swales are not only a tool for backyard gardeners, they can be used on large-scale farming operations, with great success.