Raised beds can be a money pit. Here’s how to fill one without wasting money.

Raised beds can be surprisingly expensive to fill.

A large bed can swallow hundreds of litres of material, and if your first instinct is to fill the whole thing with bags of compost, the cost quickly adds up.

Fortunately, you don't need premium compost from top to bottom.

In fact, some of the best materials for filling a raised bed may already be sitting around your garden.

Think in layers

The deeper parts of a raised bed don't need to be made from your best growing medium.

Instead, you can use bulky organic material lower down, then gradually move towards finer, more decomposed material as you get closer to the surface.

A simple approach is:

• woody material at the bottom

• coarse organic material above it

• partially decomposed material next

• topsoil and finished compost in the main growing layer

This borrows from the principles of Hugelkultur, a growing method that uses woody material underneath soil and organic matter.

You don't need to build a full Hugelkultur mound to make use of the idea.

Logs and branches

Logs, branches and woody prunings are useful at the bottom of deep raised beds.

They take up a lot of space, which immediately reduces the amount of soil or compost you need to buy.

Over time they slowly break down and become part of the organic matter in the bed.

Use untreated wood only, and keep large woody material well below the main root zone.

Avoid mixing lots of fresh wood chip or sawdust directly into your growing soil, because microbes breaking it down can temporarily use nitrogen that would otherwise be available to plants.

Twiggy prunings

Smaller branches, raspberry canes, old pea sticks and other woody garden prunings work well above larger logs.

They fill the gaps between bigger pieces while still leaving plenty of space for air and water to move through the bed.

This material will break down faster than large logs, so expect some settling over time.

Leaves

Autumn leaves are another excellent raised-bed filler.

They break down into leaf mould, improving soil structure and helping the soil hold moisture.

You can use partially decomposed leaves, finished leaf mould, or even fresh fallen leaves deeper in the bed.

Avoid creating one enormous compacted layer though. Mix or alternate them with other materials so water can still move through the bed.

Grass clippings

Grass clippings contain plenty of nitrogen and decompose quickly.

They can be useful in small amounts within the lower layers of a raised bed, particularly when mixed with carbon-rich material such as leaves or woody prunings.

Don't add a thick layer of fresh grass clippings on its own. It can compact into a wet, airless mat as it decomposes.

Old turf

If you've removed turf to build the raised bed, don't throw it away.

Turn it upside down and use it as one of the lower layers.

The grass will decompose, the soil becomes part of the bed, and you've avoided paying to replace material you already had.

Unfinished compost

Not every bit of compost needs to be perfectly finished before it enters a raised bed.

Partially composted garden material can be useful below the main growing layer, where it can continue decomposing without interfering too much with young plants.

Material containing recognisable stems, leaves and other plant matter is fine lower down.

Save your best finished compost for nearer the surface.

Finished compost

This is where the valuable stuff belongs.

Well-rotted garden compost, composted manure or other mature organic matter should form part of the upper growing layer, mixed with good topsoil.

This is where most of your crop roots will be actively feeding, so there's little benefit in burying your best compost half a metre underground.

Topsoil still matters

Organic matter is important, but a raised bed shouldn't usually be filled entirely with it.

Compost, leaves, wood and other organic materials all continue decomposing. As they do, they lose volume and the bed gradually sinks.

Topsoil provides a more stable mineral component to the bed.

For the main growing layer, aim for a mixture containing plenty of topsoil alongside well-rotted organic matter rather than using compost alone.

Expect the level to drop

Even if you fill the bed right to the top, it probably won't stay there.

Organic materials decompose, gaps disappear and the whole mixture settles.

That's particularly noticeable in a new bed containing lots of woody material.

This isn't a failure. It's part of the process.

Top the bed up with compost as the level falls.

You're also removing nutrients

Every harvest takes something out of the bed.

When you pick tomatoes, courgettes, beans or cabbages and carry them into the kitchen, some of the nutrients that plant absorbed leave the growing system with them.

Those nutrients need to be replenished over time.

Adding a layer of well-rotted compost to the surface each year helps replace nutrients while also maintaining soil organic matter.

If you garden using a no-dig approach, you can simply spread the compost over the surface and allow worms and other soil organisms to incorporate it naturally.

Use what your garden produces

A raised bed doesn't need to start with a delivery of dozens of plastic bags.

Logs from pruning a tree, autumn leaves, old turf, unfinished compost and other suitable garden material can all contribute.

The trick is putting them in roughly the right place.

Keep coarse, slow-to-decompose materials towards the bottom.

Use increasingly decomposed material as you move upwards.

Then reserve your best topsoil and finished compost for the part of the bed where your vegetables will actually grow.

You'll spend less money, waste less material and gradually create a bed rich in organic matter.

And when the level drops next year?

Add some compost and keep growing.