A raised bed is not a box full of bought soil. It is a small, upright compost heap that works for you for years. Build it up properly in autumn and you get warm, loose soil that improves itself. Just fill it with soil and you have paid dearly for a half empty box within three years. The difference lies in four layers that get finer from the bottom to the top.
The best time to build one is autumn, roughly early October to mid November · CW 40 to 46. That is exactly when the material you need turns up: woody prunings, leaves, the last of the shredded clippings, cleared beds. The ground has not frozen yet, and over the winter decomposition already sets in, so the bed is ready for planting by spring.
Why bother layering at all?
The idea behind it is a compost heap turned on its head. Coarse wood sits right at the bottom, holding plenty of air in the bed and rotting down slowly over years. The material gets finer and richer in nutrients towards the top, until the finished soil lies on top, where you sow and plant directly.
Two lovely things happen as it decomposes. First, heat builds up: a freshly layered raised bed is several degrees warmer inside than the garden soil in spring, so you can start earlier. Second, the nutrients slowly move up from below into the root zone. The bed feeds itself for years, instead of you buying in fresh supplies each season.
The price for this: the coarse wood at the bottom needs nitrogen to rot down in the first year. It pulls that out of the soil. That is why heavy feeders go into the bed in the first year, plants that can handle plenty of food, followed by ever more modest plants in the years after. For exactly how these feeding years play out, read the article Planting a raised bed by feeding years.
The four layers from the bottom up
Lay in vole mesh and protection
Before anything goes in, lay a galvanised vole mesh on the ground, otherwise your root vegetables get harvested from below. With a wooden frame, line the inner walls with dimpled membrane. It keeps the damp fill away from the boards and extends the lifespan of the wood considerably. Leave the base itself open so earthworms and water can get through.
Layer 1 · Coarse wood and branches (20 to 30 cm)
The coarsest material goes right at the bottom: tree and shrub prunings, thicker branches, shredded brushwood, and a few pieces of untreated wood are welcome too. This layer is the drainage and aeration heart of the bed. Tread it down roughly, but do not stamp it flat, the air is meant to stay in.
Layer 2 · Turf and coarse material (15 to 20 cm)
On top of that go upturned turf sods (grass facing down), coarser shrub prunings, perennial remains and coarse shredded clippings. Upturned pieces of turf are worth their weight in gold: they bring roots, soil life and structure with them. This layer closes the coarse gaps of the first one without clogging them.
Layer 3 · Coarse compost and leaves (15 to 20 cm)
Now it gets rich in nutrients: half rotted compost, autumn leaves, grass clippings, cleared vegetable plants, well rotted manure if you can get hold of any. This layer is the engine of the decomposition. Mix damp and dry material so that it rots rather than turning slimy.
Layer 4 · Mature soil and compost (20 to 30 cm)
Right on top comes the layer the plants actually root in: mature garden soil mixed with well aged, sieved compost, roughly half and half. This layer should measure at least 20 cm, better 25 to 30. Fine, crumbly, low in weed seeds. This is where you sow and plant in spring.
Add the layers together and you land at around 75 to 90 cm of fill height. If your frame is taller, you make the lower layers thicker; if it is lower, you trim the wood layer down first, never the top soil layer.
A frame of wood, stone or steel
The layering inside always stays the same, whatever your frame is made of. The material only decides the look, the price and the lifespan. Larch and Douglas fir last a good ten years untreated. Corten steel and stone last practically a gardener's lifetime, but cost more and call for quicker watering in the full summer sun.
Choose the frame by location, not by the catalogue photo. A wooden bed blends softly into a natural garden, a steel bed sets a clean edge on the terrace. With wood, insist on untreated material, or wood oiled only with linseed oil. Pressure treated construction timber has no place in a bed where edibles grow.
It settles · count on it firmly
A freshly layered raised bed is on the move in its first year. The coarse layers collapse, the soil life breaks things down, air pockets close. In the first twelve months the bed loses 20 to 30 percent of its height, and with a very coarse wood layer sometimes 30 cm or more. That is not a building fault, but exactly the process that delivers the heat and the nutrients.
You draw two consequences from this. First: fill the bed to the brim and slightly mounded, so heap the top soil into a shallow mound rather than levelling it off flat. Second: do not be alarmed when the soil sits ten centimetres below the rim in March. That is part of the deal and gets topped up.
In its first year a raised bed is not a finished piece of furniture, but a process. It sinks, it rots, it warms up. The settling is proof that it is alive.
The Gartenkern editors
Top it up every spring
Topping up is the small, yearly bit of care that turns a raised bed into a long lasting one. Here is how you go about it:
- Top up in spring
As soon as the soil has sunk in March, you add 5 to 10 cm of mature soil with compost on top and rake it in. That replaces what rotted and shrank over the winter, and starts the season at full height.
- Mulch in autumn
After the harvest, again around CW 40 to 46, you cover the bed with leaves, grass clippings or half ripe compost. This cover protects the soil life over the winter and is already half rotted by spring.
- Add nutrients
In the later feeding years, when the decomposition at the bottom is spent, you work mature compost or organic fertiliser in during spring. From around the third year the bed needs active feeding again.
- Rebuild after five to seven years
At some point the wood at the bottom has turned completely to soil and the bed is just a box of fine, sinking earth. Then you empty it in autumn, set the valuable top soil aside and layer from scratch. The old fill is the best compost for the beds all around.
The squash is the classic resident for the first year. As a vigorous heavy feeder it takes up the full nutrient load of the fresh decomposition well and shades the still open, settling soil with its large leaves. Courgette, cucumber and tomato suit the first year too; they are followed in the second year by moderate feeders such as cabbage or carrot, and in the third by the modest herbs and beans.
Common mistakes when layering
Three things go wrong again and again, and all three are easy to avoid. Too little top soil: put only ten centimetres on top and the roots starve as soon as the decomposition below sinks away. Keep the top layer at at least 20 cm. Only wood and green waste, no compost: without the nutrient rich third layer the decomposition barely gets going and the bed stays cold. Everything stamped too firm: a raised bed thrives on air. Press the layers flat and you take the oxygen from the decomposition, then it turns slimy instead of rotting.
Häufige Fragen
What order do the layers go in a raised bed?
Finer from the bottom to the top: first coarse wood and branches for drainage, then upturned turf and shrub prunings, then coarse compost with leaves and grass clippings, and a thick layer of mature soil with compost right on top. The coarse layers provide air and slow decomposition, and the fine soil on top is the actual planting zone.
How thick should the individual layers in a raised bed be?
As a rule of thumb: wood layer 20 to 30 cm, turf and coarse material 15 to 20 cm, compost and leaves 15 to 20 cm, top soil layer 20 to 30 cm. Together around 75 to 90 cm. If the frame is lower, you trim the wood layer first, never the top soil, because that is where the plants root.
When is the best time to build a raised bed?
In autumn, roughly CW 40 to 46, so from early October to mid November. That is when woody prunings and leaves come up anyway, the ground is still open, and decomposition begins over the winter. The bed is then warm and ready for planting in spring. Building in spring works too, but then you miss the head start on decomposition from the winter.
Why is my raised bed settling so much?
Because the coarse lower layers collapse and rot down in the first year. A loss of 20 to 30 percent of the height is completely normal and even desirable, because it is exactly this process that generates heat and nutrients. That is why you fill to the brim and slightly mounded when building, and add 5 to 10 cm of mature soil every spring.
How often do you have to renew the soil in a raised bed?
Top it up every spring with 5 to 10 cm of compost soil. From the third year add organic fertiliser too, because the decomposition at the bottom is spent. After around five to seven years the wood has turned completely to soil and the structure is gone. Then you empty the bed completely and layer anew; the old fill is excellent compost for the remaining beds.
Spotted a mistake?
