What You Need to Build a Concrete Block Raised Bed

A raised garden bed made from concrete blocks is straightforward to build because blocks stack without fasteners, sit level on their own, and last for years. You need concrete blocks (also called cinder blocks or foundation blocks), a level, a shovel, soil, and a tape measure. Most people finish one in an afternoon. The blocks come in standard sizes — typically 8 inches wide, 8 inches tall, and 16 inches long — so you can calculate how many you need by deciding your bed's length and width first.

Concrete blocks are hollow, which means they're lighter than solid stone and cheaper than wood. A single block weighs around 35 to 40 pounds. For a 4-by-8-foot bed that's two blocks high, you'll need 12 blocks total (6 per layer). You can buy them at any home center or landscape supply yard, usually for $1 to $3 per block depending on your region and the supplier.

Key Takeaways

  • Concrete blocks stack without screws or mortar, so assembly is just arranging them in a rectangle and checking that corners are square.
  • The ground must be level before you place the first layer, or the bed will rock and blocks will shift as you fill it with soil.
  • Blocks are hollow, so you can plant directly in the cavities or fill them with soil to extend your growing space.
  • A 4-by-8-foot bed two blocks high holds roughly 32 cubic feet of soil, which costs $30 to $60 to fill depending on soil type and delivery.

Choosing Your Location and Size

Pick a spot that gets at least six hours of direct sun if you're growing vegetables, or four hours if you're growing leafy greens and herbs. The ground should be relatively flat — a slope of more than a few inches will make leveling difficult. Avoid spots directly under trees, where roots will compete with your plants and falling leaves will clog the bed.

Decide your bed's footprint before you buy blocks. A 4-by-8-foot bed is common because it's wide enough to reach the center from either side (4 feet) and long enough to fit several rows. A 3-by-6-foot bed works well for smaller spaces. Write down your length and width, then calculate how many blocks you need: multiply your length by 2 and your width by 2, then divide by 1.33 (the length of one block in feet). For a 4-by-8-foot bed, that's (8×2 + 4×2) ÷ 1.33 = 18 block-feet, which means 12 blocks per layer. If you want the bed two blocks high, buy 24 blocks.

Preparing the Ground

Clear the area of grass, weeds, and large rocks. You don't need to remove the topsoil — in fact, leaving it helps drainage — but you do need to level it. Use a shovel to scrape high spots down and fill low spots. A level that's 2 to 3 feet long will show you where the ground is uneven. Work until the ground is flat enough that a level placed across the footprint shows no more than a quarter-inch of tilt across the width.

If the ground is very uneven, you can dig out the high spots and use that soil to fill the low spots. Compact the soil by walking over it or tamping it with the back of a shovel. This prevents the blocks from settling unevenly after you fill the bed. You don't need a power compactor for a small bed — hand tamping is enough.

Laying Out and Stacking the Blocks

Lay out the first layer of blocks in a rectangle. Start at one corner and place blocks end-to-end along the length, then turn and place blocks along the width. The blocks should touch but not overlap. Use a tape measure to check that opposite sides are equal in length — a 4-by-8-foot bed should measure 4 feet on both short sides and 8 feet on both long sides. Check that the corners are square by measuring diagonally from corner to corner; both diagonals should be equal.

Once the first layer is square and level, stack the second layer on top. Offset the blocks so the seams don't line up — place the second layer so that blocks straddle the seams of the first layer. This is called running bond and makes the structure more stable. Check that the second layer is level by placing your level across the top. If a block rocks, remove it and add or remove soil underneath until it sits flat.

Filling the Bed with Soil

Before you add soil, decide whether to line the bottom. A landscape fabric or cardboard layer will slow weed growth from below, but it's not required. If you use cardboard, overlap the pieces and let them break down naturally over the season. Landscape fabric should be permeable so water drains through.

Fill the bed with garden soil or a soil mix. Bagged soil is convenient for small beds but expensive for large ones; bulk soil delivered by truck is cheaper per cubic foot but requires a minimum order. A 4-by-8-foot bed two blocks high holds about 32 cubic feet of soil. Fill to within an inch of the top so soil doesn't spill over when you water. The soil will settle over the first few weeks, so you may need to add more later.

If you want to use the hollow cavities in the blocks as planting pockets, fill them with soil as you go. This gives you extra planting space for shallow-rooted plants like sedums or herbs, though the soil in the cavities dries faster than soil in the main bed.

Securing the Blocks (Optional)

Concrete blocks don't need to be fastened together for a raised bed that's only two blocks high and filled with soil. The weight of the soil holds them in place. However, if you're building a bed taller than two blocks, or if you live in an area with freeze-thaw cycles that can shift blocks, you can add rebar or concrete screws for stability.

To add rebar, drill holes through the blocks at the corners and drive 3-foot lengths of half-inch rebar through them into the ground. Rebar is cheap and effective. Concrete screws are another option — they're designed to bite into concrete without cracking it. Drill pilot holes first, then drive the screws at an angle through one block into the block below. You'll need a drill with a concrete bit and a power driver to make this practical.

Maintenance and Longevity

Concrete blocks last 10 to 15 years in a garden bed before they begin to break down, depending on your climate. In areas with harsh winters, freeze-thaw cycles can crack blocks faster. In dry climates, they last longer. You don't need to seal or treat them — they're designed to weather outdoors.

Check the bed each spring for blocks that have shifted or cracked. A small crack won't affect function, but a badly cracked block can be replaced by removing the soil above it, sliding the damaged block out, and sliding a new one in. If blocks are shifting, add rebar or screws to stabilize them. Refresh the soil every two to three years by adding compost or new soil on top, since plants and water gradually deplete nutrients and organic matter.

Frequently Asked Questions

Can I use concrete blocks that have been painted or sealed?

Yes, but rinse them first to remove loose paint or sealer. Sealed blocks may shed particles into the soil initially, so watering them a few times before planting helps. Avoid blocks that were used to store chemicals or fuel — the residue can leach into your soil and harm plants.

Do I need to put anything on the bottom to keep pests out?

A landscape fabric or cardboard layer slows weeds and burrowing pests, but it's not required. Gophers and moles can tunnel under blocks, but they're more interested in soft soil than in the bed itself. If burrowing pests are a problem in your area, you can line the bottom with half-inch hardware cloth before adding soil.

What if the ground is too uneven to level by hand?

For very uneven ground, you can dig out the high spots and use that soil to fill the low spots, then compact. If the slope is steep, consider building the bed on a terrace — dig into the slope and use the excavated soil to build up the downhill side. This takes more work but creates a level bed on sloped ground.

Can I plant directly against the concrete blocks?

Yes. Concrete blocks don't leach harmful chemicals into soil. Some gardeners worry about concrete, but modern concrete blocks are safe for food gardens. Plant roots won't damage the blocks, and the blocks won't damage roots.

How deep should the bed be for vegetables?

Two blocks high (16 inches) is enough for most vegetables — tomatoes, peppers, lettuce, and beans all do well. Root vegetables like carrots and beets prefer 12 inches or more. Shallow-rooted plants like lettuce and herbs need only 6 to 8 inches. One block high is the minimum for most crops.