How to Fill a Raised Garden Bed

Fill a raised bed in layers: coarse woody material at the bottom on deep beds, then leafy and bulky organic material, then a bulk soil-and-compost blend, and finally 10 to 12 inches of good growing mix on top. Water each layer as you go so it settles now rather than sinking in June.

1. 1

   Work out the volume before you buy anything

   Length x width x depth, all in feet. Add about 10 percent for settling. Run it through the soil calculator if you would rather not do the arithmetic.

2. 2

   Put cardboard on the ground

   Plain corrugated cardboard, tape and labels removed, overlapped generously. It smothers grass and weeds and then breaks down, leaving the bed connected to the soil below so roots and worms can move between them.

3. 3

   Add a woody base layer, on deep beds only

   On anything over about 15 inches, put 4 to 6 inches of logs, branches and woody prunings at the bottom. They break down slowly, hold water like a sponge and displace expensive fill. Skip this on shallow beds -- you need the depth for roots.

4. 4

   Add bulky organic material

   Leaves, grass clippings, straw, spent potting soil, cardboard. This layer fills volume cheaply and becomes compost in place. Tread it down and water it as you go.

5. 5

   Add a bulk soil and compost blend

   Roughly half topsoil and half compost, brought up to about 12 inches below the rim. Bulk delivery is far cheaper than bags at this volume if you have two or more beds to fill.

6. 6

   Top with 10 to 12 inches of growing mix

   This is the layer that does the work and the only one worth buying a quality product for. See the best raised bed soil for what to use.

7. 7

   Water it in, then mulch

   Water thoroughly so the layers settle, top up any dip, then mulch the surface with 2 to 3 inches of straw or compost. Mulch substantially reduces watering need and steadies soil moisture.

• Gravel or rocks for drainage. This is the most persistent myth in raised bed gardening. A coarse layer under finer soil creates a perched water table -- water will not cross the texture boundary until the soil above is saturated, so the root zone ends up wetter.
• Plastic sheeting or a solid liner. It blocks drainage and cuts the bed off from the soil biology below. The one exception is a bed on a hard surface, which needs drainage holes rather than a sealed base.
• Landscape fabric. It stops worms and roots moving between bed and ground permanently, and it eventually clogs. Cardboard does the weed-smothering job and then disappears.
• Treated timber offcuts or painted wood. You do not want unknown chemistry breaking down under a food bed.

Where the bed sits on a hard surface, the frame depth is all the root depth there is, so build deeper -- 12 inches is a working minimum and more is better. Put a drainage layer of coarse material with a permeable barrier above it, make sure water can actually escape, and expect to water more often because nothing wicks up from below.

After the initial fill, a bed only needs an inch or two of compost each spring. That annual cost is small, and it drops to nothing once you have a compost bin running -- which is why composting is step four in the sequence rather than an afterthought.

If the initial bill is the obstacle, how to fill a raised garden bed cheap is the version written around spending as little as possible.