Depth by what you are growing
Root depth is the honest guide. Most of a vegetable's feeding roots live in the top 20–30 cm; only the deep-rooted crops push further, and they only do it if the soil is loose enough. The table gives the minimum depth of good soil the plant wants – if the bed sits on decent garden soil you can count some of that towards the total.
| Crop group | Minimum soil depth | Examples |
|---|---|---|
| Shallow-rooted leafy crops and herbs | 20–25 cm | Lettuce, rocket, spinach, Asian greens, spring onions, basil, coriander, parsley, strawberries |
| Most vegetables | 30 cm | Beans, peas, capsicum, chilli, silverbeet, kale, broccoli, cabbage, zucchini, cucumber |
| Deep-rooted and root crops | 40–45 cm | Tomatoes, carrots, parsnips, beetroot, potatoes, sweet potato, pumpkin, corn, rhubarb, asparagus |
| Dwarf fruit trees, berries and perennials in beds | 50–60 cm | Dwarf citrus, blueberries (acid mix), raspberries, passionfruit, artichoke |
| Beds on concrete, paving or decking | 45–60 cm (any crop) | Roots have nowhere else to go, and shallow beds dry out and overheat |
| Wicking beds | 30–35 cm above the reservoir | Water reaches roots from below; deeper soil stays dry on top. See how to build a wicking bed |
If you cannot decide, build 40 cm. It grows everything on the list except fruit trees, costs a third more soil than 30 cm, and is far more forgiving in a hot, dry summer.
What is underneath matters more than you think
The same 30 cm bed behaves completely differently on different bases:
- On open garden soil. Loosen the existing soil with a fork before you build, remove grass, and roots will happily continue into the ground. A 25–30 cm bed effectively becomes a 50 cm root zone. No drainage layer is needed – a gravel layer here actually slows drainage by creating a perched water table.
- On heavy clay. Water pools at the boundary. Either build 40 cm and mix compost (and gypsum, if the clay disperses) into the top of the clay, or accept the bed will be wetter and choose crops that tolerate it. The clay soil guide shows the simple test.
- On concrete, pavers or a deck. Everything the plant has is inside the box. Go 45–60 cm, line the bottom with a 3–5 cm layer of scoria or 10 mm gravel wrapped in geotextile so drainage holes never clog, and make sure the surface underneath slopes to a drain. Consider a wicking design – it turns the limitation into an advantage.
- Over contaminated or unknown soil (old industrial sites, painted-timber demolition, lead-era inner-city yards). Lay a root barrier or geotextile and build deep enough that edible roots stay in imported soil: 40 cm minimum, 50 cm for root crops.
Depth in Australian and New Zealand conditions
Northern-hemisphere advice tends to recommend shallow beds because deep soil warms slowly in spring. Most of Australia has the opposite problem. A 20 cm bed in Perth, Adelaide or western Sydney in January can dry out between morning and evening, and the soil temperature can exceed what roots tolerate. Extra depth is a water and heat buffer: a 40 cm bed holds twice the water of a 20 cm bed and its lower half stays cool.
In cooler, wetter regions – Tasmania, southern Victoria, most of New Zealand's South Island – drainage matters more than water storage. There a 30 cm bed on well-drained ground is ideal, and going very deep on clay can leave the bottom soggy all winter. Free-draining mixes with more compost and a little coarse sand suit these beds.
Whatever the depth, mulch the surface. A 5 cm layer of straw or sugar cane mulch does more for water retention than an extra 10 cm of soil.
What each depth costs in soil
Depth is the number that drives the bill. For a common 2 m × 1 m bed:
| Depth | Volume | 30 L bags | Bulk m³ | Comment |
|---|---|---|---|---|
| 20 cm | 400 L | 14 | 0.4 | Salad and herb bed |
| 30 cm | 600 L | 20 | 0.6 | Standard veg bed |
| 40 cm | 800 L | 27 | 0.8 | Root crops, tomatoes, hot climates |
| 50 cm | 1,000 L | 34 | 1.0 | Beds on hard surfaces |
| 60 cm | 1,200 L | 40 | 1.2 | Fruit trees, accessible height |
Two ways to cut the bill on a deep bed without cheating the plants: fill the bottom third with cheaper compost or coarse organic matter and put the premium mix on top (the raised bed calculator handles layers), or – on open ground – simply build the bed 10 cm shorter and fork the ground underneath. The "hügelkultur" trick of filling the bottom with logs also works, as long as you are prepared for the bed to sink 10–15 cm in the first year; how to fill a raised bed cheaply covers it and the other options.
When taller beds make sense
Bed height and soil depth are not the same thing. A 75 cm tall bed for gardening without bending does not need 75 cm of soil: many wheelchair-accessible and "no-bend" beds have a false floor or are filled with 30–40 cm of soil over a drained lightweight fill such as scoria. If you do fill a tall bed with soil, remember the weight – a 2 m × 1 m × 0.75 m bed holds about 1.5 m³, roughly two tonnes, which matters on a deck or balcony.
Tall beds also drain faster and dry out sooner. Line the inside walls of timber beds with builder's plastic (sides only, never the base) to slow both timber rot and moisture loss. For widths, lengths and path spacing, see the raised bed sizes guide.
Questions
Is 15 cm deep enough for a raised bed?
Only for lettuce, herbs and strawberries on loosened open soil, and only in a mild climate. On any hard base or in a hot region, 15 cm dries out too fast to be practical.
How deep should a raised bed be for tomatoes?
40 cm on open soil, 50 cm on concrete. Tomatoes root deeply and drink heavily; shallow beds mean blossom-end rot from uneven watering. The planting month depends on your climate: see when to plant tomatoes.
Do I need to line the bottom of a raised bed?
On soil, a layer of cardboard to kill grass is plenty and it rots away. On concrete, use geotextile over a thin drainage layer. Only use a solid liner if you are building a wicking bed or keeping roots out of contaminated ground.
How much soil for a 40 cm deep bed?
Length × width × 0.4 × 1,000 litres. A 2 m × 1 m bed needs 800 L: 27 × 30 L bags or 0.8 m³ bulk, plus 10 % for settling.
Sources
- Root-depth groupings follow the classes used in university extension vegetable guides (shallow < 30 cm, moderate 30–60 cm, deep > 60 cm), adapted to the practical minimums used by Australian nurseries and raised-bed manufacturers.
- ABC Gardening Australia fact sheets on raised beds, wicking beds and soil preparation.
- Perched-water-table behaviour of drainage layers: standard soil-physics texts and container-media research summarised in horticultural extension material.
- Volumes: geometry only; see the raised bed soil calculator.
Last reviewed 23 September 2026 · Report an error