How deep should a raised garden bed be?

6 to 12 inches is the working minimum for a bed sitting on open ground, since roots can push into the soil beneath. Go 12–18 inches for deep-rooted crops like carrots, parsnips, and potatoes, or for any bed built on a hard surface where the soil in the box is all the roots will get.

What soil mix should I use in a raised bed?

Skip bagged 'garden soil' alone — it compacts. Use a blend of roughly 40–50% quality topsoil and 40–50% compost, with a handful of coarse sand or perlite where drainage is poor. Mixing your own from bulk topsoil and compost is usually far cheaper than buying enough bagged mix.

How can I fill a deep raised bed cheaply?

Fill the bottom third to half with coarse organic matter — small logs, branches, chopped leaves, straw — then your soil blend on top. The buried material breaks down and feeds the bed over time while cutting purchased-soil volume. Keep the top 8–10 inches as the good blend, where seeds and most roots live.

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How to Fill a Raised Garden Bed: Soil Depth, Mix, and Saving Money

GardenDraft Team · April 25, 2026 · 7 min read

Part of: Garden Planning Guides · Raised Bed Gardening Guides

A raised bed is only as good as what you fill it with, and this is the step where money is most often wasted: either on bagged "garden soil" that turns to concrete, or on enough premium mix to fill a bed to the brim when half that cost was avoidable. Get the depth, the mix, and the filling order right and the bed will outproduce in-ground soil for years.

How deep does raised bed soil actually need to be?

Most vegetables root in the top 6 to 12 inches, which is the working minimum for a raised bed sitting on open ground, since roots can push into the soil beneath. Go to 12–18 inches for deep-rooted crops (carrots, parsnips, and potatoes) or any bed built on a hard surface like a patio, where the soil in the box is all the roots will ever get.

The mix: skip "garden soil," build a blend

Bagged products labeled "garden soil" or "topsoil" alone compact and drain poorly in a raised bed. You want a blend that is roughly:

40–50% quality topsoil — the mineral body that holds nutrients and moisture.
40–50% compost — the engine of fertility and structure. More than half compost sounds generous but it shrinks and over-feeds.
A handful of coarse sand or perlite where drainage is poor.

Mixing your own from bulk topsoil and compost is usually far cheaper than buying enough bagged "raised bed mix" to fill the whole box, and gives you the same result.

Don't pay to fill the bottom of a deep bed

If your bed is deeper than about 12 inches, the lower third doesn't need premium mix. Fill the bottom ⅓ to ½ with coarse organic matter (small logs, branches, chopped leaves, straw) then your soil blend on top. The buried wood and leaves slowly break down, feeding the bed and improving its structure for years, while cutting the volume of purchased soil substantially. Keep the top 8–10 inches as the good blend, since that's the zone most roots and all your seeds will occupy.

Don't pay to fill the bottom of a deep bed — coarse organic matter below, the good blend where roots and seeds live.

Fill, settle, then plant

Whether you layer the ingredients or mix as you go, both methods work. Mound the bed slightly above the frame — fresh blends settle 1–2 inches in the first weeks as compost compacts and air pockets close. Water it thoroughly and let it sit a few days before sowing; planting into unsettled soil leaves seeds stranded when the surface drops. After the first season, top up with an inch or two of compost each spring rather than rebuilding, and you'll never need to fill the bed from scratch again.

A full bed is a tight bed

Rich, deep raised-bed soil is what makes intensive planting possible — it's the reason a gridded bed can carry plants edge to edge without starving them. Once it's filled and settled, plan the planting on a grid with our square-foot spacing guide, and look up the right sowing dates for your location on the planting calendar.

Frequently asked questions

How deep should a raised garden bed be?: 6 to 12 inches is the working minimum for a bed sitting on open ground, since roots can push into the soil beneath. Go 12–18 inches for deep-rooted crops like carrots, parsnips, and potatoes, or for any bed built on a hard surface where the soil in the box is all the roots will get.
What soil mix should I use in a raised bed?: Skip bagged 'garden soil' alone — it compacts. Use a blend of roughly 40–50% quality topsoil and 40–50% compost, with a handful of coarse sand or perlite where drainage is poor. Mixing your own from bulk topsoil and compost is usually far cheaper than buying enough bagged mix.
How can I fill a deep raised bed cheaply?: Fill the bottom third to half with coarse organic matter — small logs, branches, chopped leaves, straw — then your soil blend on top. The buried material breaks down and feeds the bed over time while cutting purchased-soil volume. Keep the top 8–10 inches as the good blend, where seeds and most roots live.