How to Build a Raised Garden Bed

What size and height should a raised garden bed be?

A bed width of 1.2 metres lets you reach the centre from either side without stepping on the soil. A wider bed means you will compact the soil when you lean in to weed or harvest, so keep a single accessible bed to that 1.2 m maximum. You can choose the length to suit your site. Most home gardens work well with 2.4 m, the standard timber length from the hardware shop, but anything from 1.8 m to 3.6 m is practical if the site allows it.

Depth depends on what you want to grow. Leafy greens and herbs need only 200 mm of soil, while tomatoes, carrots and most root vegetables do best with 400 mm to 600 mm. A bed 400 mm deep is a comfortable working height and gives enough root space for nearly all common vegies. If you garden from a seat or a wheelchair, set the top of the bed at 600 mm to 700 mm above the ground and keep the width to 600 mm so you can reach across without straining.

Which timber or material is safest and longest lasting?

Untreated Australian hardwood sleepers, such as ironbark or red gum, are the safest choice near vegetables and herbs because they contain no preservatives that can leach into the soil. A well-built hardwood bed will last 15 to 25 years before the boards need replacing, though the upfront cost is higher than softwood options.

ACQ-treated pine (marked H4 for in-ground use) is the most common budget-friendly alternative. The copper and quaternary ammonium preservative is considered safe for edible gardens by Australian standards, unlike the older CCA treatment which is now banned for domestic use. ACQ pine typically lasts 10 to 15 years before rot sets in at the corners.

Corrugated iron beds heat up and cool down quickly, which can stress plant roots on hot days unless you line the inner walls with geotextile fabric or old hessian sacks. Galvanised steel will last 20 years or more, but avoid second-hand sheets with unknown lead-based paint. Recycled brick is virtually permanent and free if you salvage it, though a brick bed is a heavy, immovable structure that needs a mortar or dry-stack skill set.

Common materials compared:

• Untreated hardwood, safest for edibles, no chemicals, lasts 15 to 25 years, higher material cost.
• ACQ-treated pine (H4), budget pick, approved for garden use, lasts 10 to 15 years, check the stamp for the H4 rating.
• Corrugated galvanised iron, long life of 20-plus years, needs inner lining to reduce heat transfer, avoid unknown old paint.
• Recycled brick, permanent and free if salvaged, heavy and permanent once built, requires basic bricklaying.
• Modular steel kits, pre-cut and quick to assemble, powder-coated steel resists rust, confirm the coating is food-safe before purchase.
• Avoid old railway sleepers and CCA-treated timber, both can leach creosote or arsenic residues into garden soil and are not suitable for any bed where you grow food.

How do you prepare the ground and position the bed?

Mark out a level spot that gets at least six hours of direct sun a day, away from large trees whose roots will steal moisture and nutrients. Use a string line and four stakes to set the exact footprint, then check the corners with a builder's square. A north-south orientation gives both sides of the bed even sun as the day moves, which suits most vegetables and herbs.

Remove the turf inside the marked area with a sharp spade, cutting just below the root zone. If the soil underneath is heavy clay, dig it over to a fork's depth to break up any compacted pan. Lay a double thickness of plain brown cardboard directly on the cleared soil, overlapping the edges by at least 150 mm to stop couch and kikuyu runners sneaking through. Wet the cardboard thoroughly before adding soil. Geotextile weed mat is an alternative, but cardboard breaks down over time and lets worms move up into the bed.

Check the ground is truly level with a long spirit level resting on a straight piece of timber across the site. Even a small slope concentrates water at the low end and leaves the high end dry. Shave high spots with the spade rather than building up low spots, because fill settles and the bed will shift later.

Step-by-step assembly of a simple timber bed

Cut your sleepers to the planned length with a circular saw, keeping the factory-cut ends for the visible corners where possible. Lay the first course of sleepers on flat ground and check that all pieces sit flush before you pick up the drill.

Pre-drill clearance holes through the face of each sleeper with a 6 mm timber bit, then drive 100 mm batten screws into the end grain of the adjoining piece. Two screws per corner joint give a solid connection without splitting the wood. Lift the first course onto your prepared base and add the second course, staggering the corner joints like brickwork. Check the diagonals with a tape measure before you tighten everything fully. A square frame makes the rest of the job straightforward.

• Cut all sleepers to length and dry-fit the bottom course on level ground.
• Pre-drill two holes at every corner joint to stop the timber splitting.
• Drive 100 mm batten screws through the face into the end grain of the adjoining sleeper.
• Stack the second course with staggered corners and screw it to the course below every 600 mm.
• Measure both diagonals and adjust the frame until they match, then tighten all screws.
• For beds longer than 1.8 m, fix a centre brace across the middle to stop the sides bowing outward under soil pressure.

What soil mix fills a raised bed without blowing the budget?

A three-layer fill cuts the cost because coarse organic matter fills most of the depth. You only pay for quality soil in the top 30 centimetres, where the roots actually feed.

Start with a base layer of untreated logs, thick branches and dry leaves. This hugelkultur-inspired layer fills the bottom third, breaks down slowly and holds moisture. Next, add a bulk vegetable garden mix, bought by the cubic metre from a landscape yard, to about 15 centimetres below the rim. For the top layer blend that same veggie mix with homemade compost and aged manure, then rake it level. If you order loose bulk material instead of bags, you save more than half the cost on a bed deeper than 40 centimetres.

• Coarse green waste and woody prunings rot over two to three years and feed the soil from below.
• A sandy loam veggie mix gives drainage and enough body for root crops.
• Mature compost lightens clay mixes and adds the microbes vegetables need.
• Aged cow or poultry manure lifts nitrogen gently without burning seedlings.
• One cubic metre of bulk mix fills roughly two standard beds 2.4 metres long, 1.2 metres wide and 30 centimetres deep.

How do you water and mulch a raised bed in Australian summers?

Consistent deep watering early in the morning gives roots the best chance against a hot afternoon. A drip irrigation system or a porous soaker hose laid along the plants delivers water straight to the soil with almost no evaporation loss. Connect the line to a simple battery-operated tap timer and set it to run for 30 to 40 minutes every second day during a dry spell, adjusting after rain.

Spread a 50 mm to 75 mm blanket of pea straw or sugarcane mulch over the entire soil surface, and keep it a finger’s width clear of plant stems. That thickness slashes evaporation, suppresses weeds and stops the top layer from baking hard. Lucerne mulch also works well, as it breaks down into a gentle feed for leafy vegetables over the season. Top up the mulch whenever you see bare patches or when the layer thins to less than 30 mm.

Photo: Greta Hoffman, Pexels (Pexels License)