How To Build Concrete-Block Raised Planting Beds

Gardeners love raised planting beds for many reasons, from improved soil control to easier maintenance. Though there are several ways to make these elevated gardens, concrete blocks are a durable and affordable option. In this guide, we’ll walk you through building your own concrete-block raised planting beds.

Planning Your Raised Planting Bed Project

Plan carefully before you start building. This involves choosing the right location and determining the right size and height.

Choosing the Right Location

Consider these factors when picking a location for your raised garden bed:

• Accessibility: Pick a location that’s easy to access for planting, watering, and harvesting.
• Drainage: Choose a level area with good drainage to prevent waterlogging. Avoid low-lying areas where water tends to collect.
• Proximity to a water source: Position the bed close to a water source for easier irrigation.
• Sunlight: Most vegetables and flowers require at least six to eight hours of direct sunlight daily. Watch your yard throughout the day to identify areas that get enough sunlight.

Determining Size and Height

The size and height of your raised bed will depend on your available space, gardening needs, and personal preferences. Consider these guidelines:

• Height: Two or three concrete blocks (16-24 inches high) are comfortable for most gardeners and don’t require excessive bending. You may want to build the bed higher if you have mobility issues.
• Length: The length can vary based on your available space and needs. Eight feet is a common length.
• Width: We recommend a width of four feet, as it allows you to reach the center of the bed from either side without stepping on the soil.

If you’re planning multiple beds, leave enough space between them for easy access and maintenance (typically two or three feet).

Materials Needed for Concrete-Block Raised Beds

Here’s what you’ll need to build your concrete-block raised planting beds.

Essential Building Supplies

Gather these materials before you begin. The amount you need will depend on the size of your garden bed:

• Concrete blocks: 4-by-8-by-16-inch hollow blocks for the main structure
• Concrete mix: For the footing and filling block voids
• Half-inch rebar: For reinforcing the structure
• L-shaped corner blocks: For creating stable corners
• Landscape fabric: To line the bottom of the bed
• Mortar: For setting the blocks
• Smooth capstones: Two inches thick, for topping the walls
• Soil mix: High-quality soil for filling the bed

Tools Required

We recommend having these tools on hand to help with construction:

• Level
• Measuring tape
• Rubber mallet
• Safety gear (gloves, safety glasses, dust mask)
• Shovel and spade
• String line
• Trowel
• Wheelbarrow

Steps for Building Raised Planting Beds With Concrete Blocks

Follow these step-by-step instructions to complete your project.

Preparing the Foundation

A solid foundation is crucial for a stable and long-lasting concrete raised bed. 

Step 1: Excavating the Area

First, you’ll need to excavate the area:

1. Mark out the perimeter of your raised bed using stakes and string.
2. Remove any grass or vegetation within the marked area.
3. Dig a trench around the entire 4-by-8-foot perimeter. The trench should be 8 inches wide and 6 inches deep.
4. Use a level to make sure the trench is even.

Step 2: Pouring the Concrete Footing

Now it’s time to pour the concrete footing:

1. Mix the concrete according to the package directions.
2. Fill the trench halfway with concrete.
3. Lay down two pieces of half-inch rebar in the center of the trench along the footing’s four walls.
4. Fill the trench with concrete, making sure it’s level with the ground surface.
5. Allow the concrete to cure for at least 24 hours.

Building the Concrete-Block Walls

Landscape contractor Byron Moffett showed us his process for laying the walls for a raised garden bed. This involves laying the blocks and reinforcing the structure.

Step 3: Laying the First Course

Carefully lay the first course of concrete:

1. Start by setting one L-shaped corner block in a half-inch-thick mortar bed on the cured footing.
2. Working around the perimeter, set one course of 4-by-8-by-16-inch blocks in mortar, spacing them a half-inch apart.
3. Use a level to make sure each block is even as you go.

Step 4: Adding the Second Course

You may prefer to let the first course set for added stability, or you may decide to continue while the mortar is still flexible enough for adjustments. When you’re ready, begin the second course:

1. Begin the second course by alternating the orientation of the L-block in the corners. To add strength to the structure, make sure that vertical joints don’t line up.
2. Continue laying blocks in mortar, maintaining the half-inch spacing between blocks.
3. Check for level and plumb (vertical alignment) as you build each course.
4. Depending on your desired height, you may add a third or fourth course following the same process.

Step 5: Reinforcing the Structure

After completing two courses, strengthen the walls:

1. Place a 15-inch length of rebar in each corner.
2. Fill the voids in the blocks with concrete. Each block should have two hollows separated by a divider.
3. Repeat this process in every other cavity in the block walls.

After the reinforcement has set, continue adding more courses if desired.

Finishing Touches

These finishing touches will enhance the look and functionality of your garden bed.

Step 6: Installing Capstones

Use capstones to top off the bed:

1. Select two-inch-thick smooth capstones.
2. Apply a layer of mortar to the top of the final course of blocks.
3. Place the capstones on the mortar, allowing for a two-inch overhang on the outside edge.
4. Allow the mortar to set completely before using the bed.

The overhanging capstones create a comfortable perch for tending the beds or simply relaxing by your garden.

Step 7: Adding Drainage Solutions

Proper drainage is essential for plant health. Consider these options:

• Drill small drainage holes in the bottom course of blocks if your soil is prone to waterlogging.
• Install a French drain around the bed’s exterior if your area gets heavy rainfall.
• Line the bottom of the bed with a layer of gravel before adding soil to improve drainage.

Soil and Planting Considerations for Planting Beds

Now, it’s time to focus on creating the best growing environment for your plants.

Selecting the Right Soil Mix

The beauty of raised beds is the ability to control your soil quality. Create a nutrient-rich environment with these tips:

• Use a mix of topsoil, compost, and perlite or vermiculite. There are numerous recipes available online, recommending anywhere from a 50/50 mix of topsoil to 60% topsoil, 30% compost, and 10% soil lighteners like perlite.
• Avoid using soil from your yard, which may contain weed seeds or pests.
• Consider your plants’ pH requirements and adjust the soil accordingly.
• Add organic matter annually to maintain soil health.

Choosing Plants for Your Raised Bed

Select plants that will thrive in your raised bed environment:

• Determine how much sun the bed gets and choose plants that will thrive.
• Experiment with companion planting to maximize space and deter pests naturally.
• Group plants with similar water and nutrient needs together.
• Take advantage of the excellent drainage by planting root vegetables or plants that prefer well-drained soil.

Remember to rotate your crops annually to prevent soil depletion and reduce pest problems.

Maintenance and Care of Your Planting Beds

Proper maintenance keeps your concrete-block raised planting bed productive and attractive for years to come.

Seasonal Upkeep

Follow these tips in the different seasons:

• Spring: Remove any winter debris, add compost, and prepare the bed for planting.
• Summer: Monitor water needs closely, as raised beds can dry out faster than ground-level gardens.
• Fall: Clean out spent plants, add a layer of compost, and consider planting cover crops.
• Winter: Protect the soil with mulch or row covers if you’re not winter gardening.

Long-Term Durability Tips

These guidelines will help keep your garden bed intact and healthy in the long term:

• Apply a concrete sealer to the exterior every few years to extend the blocks’ lifespan.
• Check for wall shifting or bulging and address issues promptly.
• Clean the capstones periodically to prevent algae growth.
• Inspect the mortar joints annually and repair any cracks to prevent water infiltration.

Alternative Designs and Variations

While the basic rectangular raised bed is versatile, consider these variations to customize your garden space:

• L-shaped beds that fit nicely into corners of your yard or along fences
• U-shaped beds that maximize growing space in a compact area
• Tiered beds for sloped yards