How to build a wicking bed

How a wicking bed works

The idea was developed by Australian engineer Colin Austin and is now a fixture of community gardens and school gardens around the country. In a normal bed you water from the top. Much of that water evaporates from the surface or drains past the roots. In a wicking bed the water sits in a sealed reservoir below the soil, and the soil draws it upward the way a paper towel draws up a spill. Roots grow down towards the moisture, the surface stays relatively dry, and very little water is lost.

Capillary rise has a limit. In typical garden soil water climbs about 300 mm above a free water surface. Soil deeper than that above the reservoir stays dry at the top, and seedlings struggle. That single number drives the whole design.

Dimensions that work

Designs vary. Some use a shallow 100–150 mm layer of scoria and raise the overflow so the bottom of the soil also stays saturated, with 350–500 mm of soil in total. That holds more water but brings the waterlogged zone closer to the roots. For a first bed, put the overflow at the top of the scoria and keep about 300 mm of soil above it.

Materials list with quantities

For a bed with inside length L, width W, reservoir depth r and soil depth s, all in metres:

The liner covers the base and runs 100 mm up the walls above the water line. The geotextile is turned up 100 mm at the edges so soil cannot slip down past it. For a 2.4 m × 1.2 m bed with a 200 mm reservoir and 300 mm of soil:

For the soil itself, use a free-draining, compost-rich vegetable mix. Sustainable Gardening Australia's design uses half mushroom compost and half organic soil mix. Heavy clay-based soils wick poorly. The raised bed calculator prices it in bags or bulk.

How much water it holds, and how long it lasts

Water fills only the gaps between the stones, roughly a third of the reservoir's volume in scoria or gravel:

The example bed holds 0.576 m³ of scoria, so about 170–230 L of water. A vegetable bed in full sun can use 4–8 L per square metre per day in hot weather, which is 12–23 L a day for 2.9 m². A full reservoir therefore lasts about 7–20 days in summer, and much longer in spring and autumn. A deeper reservoir or a mulched surface stretches that.

Step by step

1. Level the base. Check it with a spirit level in both directions. Remove sharp stones and roots, and lay old carpet, sand or cardboard to protect the liner.
2. Build or place the frame: sleepers, corrugated steel or a large container. The sleeper bed calculator works out the timber.
3. Lay the liner in one piece, folded neatly into the corners and up the walls to at least 100 mm above the planned water line. Staple or clamp it at the top.
4. Set out the pipes. Stand the fill pipe in a corner, connected to the slotted pipe along the base if you are using one.
5. Fill the reservoir with rinsed scoria to its full depth, and level it.
6. Make the overflow: cut a neat hole through the liner and wall, exactly level with the top of the scoria, and seal the fitting.
7. Lay the geotextile over the scoria and turn it up the walls, so no soil can wash into the reservoir.
8. Add the soil to about 300 mm, water it well from the top to settle it and start the wicking, then fill the reservoir through the pipe until water runs from the overflow. Mulch the surface.

Common mistakes

• Soil too deep. Soil more than about 300 mm above the reservoir means a dry top layer and struggling seedlings.
• A base that isn't level. Water pools at the low end. It is the most common failure in beds built on lawns.
• A hole in the liner from a stone, a screw or a garden fork. The reservoir quietly drains away. Protect the liner and never dig deep in a wicking bed.
• Overflow in the wrong place or missing. After heavy rain the soil becomes a swamp and roots rot.
• Salt build-up. Everything you add stays in the bed. Go easy on fertiliser, and every so often water heavily from the top so the excess flushes out through the overflow.
• Mosquitoes. Screen the overflow and cap the fill pipe.

Is a wicking bed worth it?

Yes for thirsty vegetables and herbs in hot, dry places, for gardeners who travel or cannot water daily, and for beds on concrete where roots have nowhere else to go. Probably not for plants that hate wet feet (lavender, rosemary, succulents, many natives), in cool wet climates where the reservoir is rarely needed, or on a balcony or deck. A 2.4 × 1.2 m bed full of wet soil, scoria and water weighs about two tonnes, so check the structure first or use small self-contained wicking boxes.

Questions