Clockwise from above: Dwarf peach in a mini wicking box; Happy greens, wicking style; Wicking bed with tomatoes and leafy greens; Overflow Pipe. Photos by VEG
We know plants need water to grow, however understanding when and how much they need can seem like a mystery only ‘green thumbs’ can solve. Enter the wicking bed!
Wicking beds are an innovative vegie bed irrigation solution. They minimise the watering challenge by creating a water reservoir that sits underneath the soil your vegies grow in. A typical wicking bed has a waterproof lining in the bed, on top of which is a layer of small stones. The stones are then covered with water, landscaping fabric is laid on top and the soil goes on top of that. There’s an inlet pipe and a special overflow setup that prevents the water level from rising up into the soil, which would compromise plant health. The water below the soil layer literally wicks up into the soil, maintaining optimal soil moisture levels at all times (so long as you remember to occasionally top up the reservoir).
Wicking beds will typically have less complications than dripline irrigation, will lose less water to evaporation than overhead sprinklers, and take much less time to top up than it would to hand water the same size bed. Knowing when and how much to water is a breeze. All you need to do is simply top up your bed until water flows out of the overflow outlet.
How They Work
The reason wicking beds work is because of a remarkable characteristic of water: it can move against the pull of gravity using capillary action. Capillary action is water passing through the small spaces between other particles.
To see how this works, dip the corner of a dry paper towel in water and watch what happens. The water moves through the towel, even against gravity. This also happens through the wicking bed reservoir material and through our vegie bed soil.
What’s The Catch?
Despite their many benefits, it’s worth knowing that wicking beds use more materials than a standard raised vegie bed, require extra attention to detail when installing and can be damaged (for instance by a tomato stake).
Which Crops Don’t Like Wicking Beds?
Your wicking beds are prime real estate, especially for edibles that like their water. Anything that can handle being in the ground and will still produce well (e.g. garlic, potatoes, pumpkins, Mediterranean herbs) should be planted into the soil, to save your wicking bed for the thirstier members of your edibles (think leafy greens, brassicas, tomatoes, basil, carrots, etc.).
How To Build A Wicking Bed
The key ingredients to a successful wicking bed build are:
- A waterproof container, or a frame that can be made waterproof, ideally using food grade components
- A liner (depending on the type of container)
- An inlet plumbing setup. The VEG wicking bed version we make is: 25 mm poly pipe the height of your bed, a barbed 25 mm poly elbow, a short piece of 25 mm poly pipe (about 20cm) and about 1 metre of 50 mm perforated agricultural pipe (aggie pipe)
- An outlet plumbing setup. The VEG version is: 25 mm tank valve, 25 mm threaded female-male elbow and 25 mm poly riser
- Reservoir media
- Geotextile fabric
- Good quality soil
Step 1: Create Your Raised Bed
The first thing is to create the bed for the water and soil to sit in. In VEG’s case, this is a raised bed 40, 60 or occasionally 80 cm high, made from untreated Golden Cypress (Cupressus macrocarpa) timber sleepers.
All sorts of containers will suit, from corrugated metal beds, wine barrels (with a liner if no longer watertight), polystyrene vegie boxes (although wrap the outside as they decay in UV light) and other food-grade plastic tubs. If your container isn’t waterproof it will need lining.
Step 2: Installing The Liner
It’s worthwhile investing in a good food-grade liner. While builders plastic can work, we steer away from it as it isn’t rated for use with potable water and is too easy to puncture. Food grade plastics are available (we use a high-density polyethylene that is 0.5 mm thick) and while they might cost more, they’re worth it!
If the liner you use is thin, consider two layers. A cushioning layer of sand or similar below can help prevent punctures. Spread the liner over the base of your container and fold up the sides, attaching it to the side of your bed.
Step 3: Creating The Overflow
Once your liner is fitted, you’ll need to choose an overflow style. The simplest is to drill a hole at the level where the reservoir and soil meet and fit a tank valve, or insert a pipe to allow the water to drain out (unperforated aggie pipe can suit, or poly pipe if your container is smaller). See Step 4 to see what height you’ll need.
At VEG we developed our overflow setup which is fitted low in the bed (about 4-5 cm from the base) and we then trim the riser height to match the reservoir-soil interface. The beauty of this setup is it allows you to drain the reservoir whenever needed by rotating the riser and elbow, and allows easy viewing of the water level to know when to top up, as the level in the riser is the same as in the bed. Seal your outlet to the liner with fast-curing silicone (again food-grade if possible). Bear in mind where any water runoff will go and see if you can direct it to other beds or trees.
Site your inlet where you can see the outlet or overflow so when filling you’ll easily see when to stop. Ideally, use potable water grade for your inlet and outlet setups. Avoid using PVC components as they release leachates into the system, especially when exposed to UV light. Place the inlet pipe and snake the aggie pipe along the base of your lined container.
Step 4: Making The Water Reservoir
It’s important to get the reservoir media right. You want an inert substance that will wick well. We use 7 mm bluestone screenings, although most screenings from 7 mm to 15 mm will work too (scoria is also commonly used and works fine). Avoid limestone-based screenings as they’ll affect your pH (towards alkaline).
Add enough screenings to get the reservoir up to your desired height. Our recommended wicking reservoir to soil ratios are below. Anywhere within a 3-5cm range of these ratios should be fine:
- 40 cm high bed = 15 cm reservoir with 25 cm soil
- 60 cm high bed = 25 cm reservoir with 35 cm soil
- 80 cm high bed = 45 cm reservoir with 35 cm soil (or build up the internal base 20 cm to then make it as a 60 cm bed internally).
For smaller containers aim for a reservoir soil ratio. The formula to work out the volume of screenings needed is the container/ beds’ internal length x internal width x reservoir/soil depth.
Now add enough water to almost cover the screenings, leaving the tips clear. Ensure the screenings are all level and there’s no puddling or dry spots. This sets your final reservoir height, so it’s time to trim your overflow riser (or drill the overflow hole). The water level in the riser shows you where to cut.
Step 5: Test For Leaks
Leave overnight to check for any leaks. Emptying a wicking bed is not a fun job so taking care is a very good idea!
Step 6: Insert Your Geotextile Fabric
Geotextile fabric (which is woven PET and stable unless exposed to UV light) is terrific for keeping the soil out of the reservoir, preventing it from filtering down and turning anaerobic. It allows liquids to filter through but is fine enough to keep the soil above the reservoir. Shadecloth is a suitable substitute. While it’s great to use recycled materials, consider if it will release nasties into your food.
Step 7: Add Soil
You want a good sandy loam with healthy levels of organic matter (both when you source it and added over time). Sandy soils have larger particles which means poorer wicking skills, while clay has tiny particles which means awesome wicking skills but it isn’t very good at sharing that with your plants (it ‘holds’ the water tightly). Loams have great wicking skills, aren’t selfish with their moisture retention and support soil life beautifully. Use potting mixes in pots and smaller containers (not soil). Don’t forget to top your bed off with a good mulch such as sugarcane. Pea straw breaks down quickly and feeds the soil life well.
Jeremy Prentice is the manager of VEG Wicking Beds. VEG provide a range of options to help with your wicking bed construction, including online instructions, DIY kits with all the materials required and full installation. www.wickingbeds.com.au