Growing on raised beds: an ancestral technique for a living, self-sustaining soil

The mound has become the iconic image of permaculture, to the point that it is often built without asking what purpose it serves. Yet it is a situation-specific technique, and above all: there are two opposing forms, one of which is systematically overlooked.

The mound built above the ground drains and warms the soil. The buried mound, on the other hand, retains water. These are two responses to two opposing problems, and this initial choice determines everything else.

The common principle: wood as a sponge

Whichever form is chosen, the core of the method is the same: decomposing wood buried beneath the growing area.

This wood acts like a sponge: it absorbs rainwater and releases it slowly during dry periods. As it decomposes, it gradually releases nutrients and creates lasting porosity, colonized by the fungal network. This is what distinguishes a true mound from a simple raised pile of soil.

What differs between the two forms is where this sponge is located in relation to ground level — and therefore whether it is exposed to evaporation or protected by the soil.

Two forms, two opposing objectives

The raised mound — for drainage

The wood and organic layers are built above the existing soil. The result is a ridge 30 to 60 cm high.

• It drains — the root zone is raised, allowing excess water to drain away. This is the number-one argument on clay soil, where winter root suffocation ruins crops.
• It warms up earlier — a raised mass, exposed on its sides, gains two to three weeks in spring. Decisive in cool regions.
• It increases the cultivable area — about 30% more than a flat bed with the same footprint, thanks to the sides.
• But it dries out — that same exposed surface means equivalent losses through evaporation and wind.

The buried mound — for water retention

The wood is placed below ground level, in a trench, then covered with the excavated soil. The result is level with the ground or slightly rounded — it is no longer really a mound, and that is precisely the idea.

• The sponge is protected — buried in the cool zone of the soil, sheltered from direct evaporation and wind. It recharges with rainfall and releases water deep down, directly into the root zone.
• It reduces watering — this is the main objective in dry climates or on well-drained soil.
• It limits runoff — a slight hollow captures water instead of letting it run off, which matters on sloping ground.
• But it does not drain — and that is its absolute limitation, detailed below.

Choose

Situation	Suitable form	Why
Clayey, heavy, poorly drained soil	Raised	Drainage and warming address the two main flaws of these soils.
Humid climate, cool region	Raised	Earlier growth in spring, less winter waterlogging
High water table, flood-prone land	Raised	Historical use, predating permaculture by a long time
Sandy, free-draining soil	Sunken	It already drains too much; the buried sponge corrects exactly this flaw.
Dry climate, hot summers	Sunken	Maximum water retention, minimal evaporation
Dry, sloping terrain	Sunken, across the slope	Captures runoff instead of suffering from it
Deeply compacted or contaminated soil	Raised	Cultivate above it without disturbing the existing soil.
Already fertile, well-structured soil	None	A flat, mulched bed will produce just as much, with far less work.

Building a raised mound

The wood-core sandwich mound, the most common form. Ideally built in autumn, so winter can initiate decomposition.

Dimensions

• Width: 1.20 m maximum — beyond that, you can no longer reach the center without climbing onto it, and compacting a mound defeats its purpose.
• Height: 30 to 60 cm. It will lose one-third of its height in the first year.
• Preferably orient it north–south, so both sides receive light. Paths should be 50 to 60 cm wide.

The steps

1. Cut down the vegetation without pulling it up, and water the soil thoroughly. The step everyone forgets: dry soil beneath a mound will remain dry for a long time.
2. Lay down plain brown cardboard, unprinted, with tape removed and sheets overlapped by 15 cm. It smothers perennial weeds without herbicide.
3. The wood core — 30 to 40 cm of logs, branches, and twigs, from largest to smallest. Compact it to limit large gaps.
4. A nitrogen-rich layer — fresh grass clippings, manure, nettles. It balances the carbon in the wood.
5. A brown layer — dead leaves, straw, wood chips, 15 to 20 cm deep.
6. Soil — 15 to 20 cm of topsoil mixed with mature compost. This is where you will sow.
7. Mulching — 5 to 10 cm, immediately. A bare mound erodes with the first rain.

Building a sunken mound

The variant for a dry climate and free-draining soil. It requires more earthmoving work but produces an immediately usable result.

The steps

1. Dig a trench 30 to 50 cm deep and 1 to 1.20 m wide. On sloping ground, orient it across the slope so that it intercepts runoff.
2. Separate the soil layers. This is the technical key to the method: set aside the top 20 centimeters — the living topsoil — separately from the poorer subsoil. They will not go back in the same place.
3. Loosen the bottom with a digging fork without turning it over, so that water can continue to drain downward.
4. The wood at the bottom — 20 to 30 cm. Here, favor already dead and partially decomposed wood, spongy and crumbly under your fingernail: it absorbs immediately, whereas fresh wood takes an entire season. Water it thoroughly before covering it — it must be saturated from the start.
5. A nitrogen-rich layer — the same role as in the raised version, and just as essential.
6. The subsoil first, then the topsoil on the surface — you are rebuilding the natural stratification instead of reversing it.
7. Thick mulch, 8 to 10 cm. In dry climates, it is what prevents the sponge from emptying from the top.

Cardboard is useless here: the soil placed on top already smothers the existing vegetation.

The absolute contraindication: never dig in clay soil. A pit in clay behaves like a bathtub — water enters it and does not come back out. The wood then rots anaerobically, producing compounds toxic to roots, and you end up with an impossible-to-remedy, foul-smelling pocket. In clay soil, use the raised version, without exception.

Second cost to accept: by digging, you destroy the existing structure and the area's mycorrhizal network. It is a one-time investment — you will not need to do it again for ten years — but you need to know this, and it is another reason not to dig soil that is already in good condition.

Which woods to use, in both cases

• Excellent — soft hardwoods: poplar, willow, birch, alder, lime. Rapid decomposition.
• Good but slow — oak, beech, fruit trees. A very long-lasting sponge effect.
• Avoid — large quantities of conifers (acidifying and slow to decompose), walnut (inhibitory juglone), black locust and chestnut (rot-resistant, they will never decompose), and any treated, painted, or varnished wood.

Nitrogen hunger: the difficulty no one warns you about

It concerns both forms, and it is the main cause of disappointment in the first year.

Wood is extremely rich in carbon and low in nitrogen. The microorganisms that decompose it need both, so they draw available nitrogen from the surrounding soil to balance their diet. This nitrogen is not lost—it is immobilized in their biomass and will be released later—but in the meantime your crops lack it. Symptom: pale foliage, stalled growth, and stunted plants for no apparent reason.

Three countermeasures:

• The nitrogen layer — this is precisely what it is for. Do not skip it.
• Sufficient soil depth — 15 to 20 cm separates the roots from the wood during the critical period.
• Choosing crops for the first year — legumes, which fix their own nitrogen, and shallow-rooted cucurbits: squash, zucchini, pumpkins, beans, and peas. Avoid heavy-feeding leafy vegetables—cabbages, lettuces, spinach—and long roots.

One advantage of the buried version: because the wood is deeper, the seedling zone is somewhat better insulated from the phenomenon.

The first three years

• Year 1 — around one-third settling for the raised version, possible nitrogen deficiency, and only suitable crops. Replenish the mulch during the season.
• Year 2 — the balance settles in. The wood begins to release nutrients, soil life has become established, and the range of crops expands. Generally the best year for yields.
• Year 3 and beyond — maturity: strong water retention, stable fertility, and greatly reduced watering. Everything becomes suitable for cultivation.

Maintenance: replenish with surface organic matter every autumn. Never turn it over—the layering you built is precisely what makes the whole system work. Allow five to ten years before rebuilding; by then, the core wood will have turned into humus.

Other variations

• Lasagna mound — no wood, only alternating brown and green layers. Faster, lower, and settles more. A good solution if you have no wood.
• Round “keyhole” mound (keyhole) — circular, with an access notch in the center. Excellent ratio of cultivated area to paths, suitable for small spaces.
• Edged mound — contained by boards or logs. Neater, with less erosion, but less side-slope surface area.
• Simple raised bed — 20 cm of improved soil with no wood core. No sponge effect, but immediately ready for cultivation without nitrogen deficiency.

Frequently asked questions

Buried or raised—what if I can’t decide?

Ask yourself just one question: is your problem too much water or too little? If your crops suffer in winter, raise the bed. If they suffer in July, bury it. And if your soil is clay, the answer is raised, whatever the climate.

Do you need to remove the grass before building?

With the raised version, no: mow very short and cover with cardboard; the smothered grass becomes organic matter, and its dead roots leave useful channels. With the buried version, the question does not arise because you strip the area.

Can you build one in spring?

Yes, but the nitrogen shortage will be more pronounced because decomposition will not have had the winter to get started. Increase the nitrogen-rich layer and limit yourself to squash and beans during the first season.

Do beds attract voles?

The gaps in the wood core can provide shelter, especially in the raised version. Two countermeasures: pack the wood properly, and lay small-mesh wire netting on the ground before the cardboard if vole pressure is high.

What is the difference from the Hügelkultur method?

It is the same principle. The German term Hügelkultur refers to a wood-core bed, a practice documented in Central Europe long before permaculture adopted it. Both raised and buried versions already coexist there.

How long does it take to build?

A good half-day for two people to build a 5 m bed, not including material collection. Allow twice as long for the buried version because of the excavation. Start with just one: you will then know whether the method suits your land before building ten.

Can you build a bed on a terrace or slab?

Yes, in a bordered version with a drainage layer at the base. Be careful with the weight: a 5 m² bed saturated with water weighs several tonnes. Best reserved for slabs on the ground.

Key takeaway

The bed is not a symbol; it is a response to a specific problem—and there are two of them. Raise it to improve drainage, bury it to retain water, and do neither if your soil is already doing well. In both cases, plan for the first year: that is what discourages people who have not been warned.

Going further

• The different types of soil: how to recognize and improve them
• Creating a vegetable garden without irrigation: mulch, ground cover, and mulching
• The basics of living soil
• How do you create a permaculture vegetable garden?

For the first year of your bed: our vegetable seeds—squash, zucchini, and beans in particular—and our green manures to cover the sides.