From the Digest

Free-Root-Run Raised Beds for Cacti and Succulents

Part 2: Building a Raised Bed

Carefully leveling the entire area chosen for a raised bed is a crucial step that literally forms the base of the finished project.

By J. Eric Driskill (May 2023)

Part 2 of a three-part series about using raised beds to grow cacti and succulents.

I do not recall the dimensions or configuration of the first raised bed I built. The second bed was relocated to the back of my yard, primarily due to the increase in direct sunlight that area of my yard receives compared to the first location. I also do not recall the initial length of that relocated bed, but what I have today is a bed with the same width but longer. What I like about how I built this bed is you can expand it a little or a lot by extending it and making it longer.

In this article, I will explain how to build your own raised bed, beginning with a smaller bed. My guess is after you see the results the first year, you will be expanding yours before you start putting plants in there the second year.

Your first task is to find the ideal location for your bed. You can do a lot of research to determine this alone. Although time-consuming, this can be done as simply as visiting your location at various times of the day between April and September, and noting where direct sunlight is shining. You can find information online that will help if you want a more scientific approach, or if you do not want to wait for an entire summer to observe your location.

Online resources include sunrise/sunset graphs, solar path diagrams and cartesian sun charts, just to name a few. These tools can help you determine sun angles, sun path and how to calculate when a tree, house or other object will shade an area.

In general, I suggest finding the area that receives the maximum number of hours of direct sunlight. I also suggest orienting the bed to run east and west. To increase the size of the bed, I suggest making it longer rather than wider, so take that into consideration with your location, as well.

Apart from finding the location with the maximum amount of light, there are a few other considerations. You should try to avoid building your bed under or near a tree, which could not only shade your bed, but also drop branches on your plants in storms or high wind; or leaves, seeds or acorns that could damage your plants or require you to weed your bed when those acorns or seeds sprout.

There are a couple of eastern redbud trees in my yard, and I am constantly amazed at not only how many seeds those trees drop, but also how far the wind carries them. Those are the seeds that I am constantly picking out of my bed.

Once you determine your location, you will want to prepare that area. My current bed has an outside measurement of 23’6” long and 5’2” wide. In this article, I am going to detail a smaller bed to start with, but will include information for expanding it later. The smallest bed I recommend will have an outside footprint of 7’10” long and 5’2” wide with a depth of two cinder blocks.

Here is a quick materials list for a bed of this size:

  • 36 cinder blocks
  • 10′ x 4′ roll of quarter-inch or half-inch hardware cloth, cut down to 7’5″ x 4′
  • 34 cubic feet of growing medium.
The position of the cinder blocks in the second layer should be offset from those in the first layer. If you completely level your area to begin with, the process of laying blocks will go much more smoothly.

Note: All measurements in this article are approximate. Although cinder blocks are typically sold as 8” x 8” x 16”, most are a bit smaller when measured. Their actual dimensions are closer to 7 5/8” x 7 5/8” x 15 5/8”. Be sure to buy cinder blocks that are not damaged, broken or chipped.

The four basic steps to prepare your raised bed are leveling the area, laying down the hardware cloth, setting cinder blocks and filling the bed with medium.

Step 1: Completely level a 7’10” long and 5’2” wide area.

Level the entire area where you will place your raised bed. This is a really crucial step you should do right. The cinder blocks are placed in two layers and held together by gravity alone. This allows you to dismantle the bed to remove or relocate if wanted. It also allows you to extend the bed, which I bet you will do when you see how well it works.

If your area is not level, your cinder blocks will not sit level or evenly against one another and result in gaps between the blocks. When this happens, soil will spill out between the cinder blocks, which can only worsen the problem over time. If your blocks, on the other hand, are not level inside to outside, your wall may lean in, or worse yet, lean out, thus compromising the integrity of the wall of your raised bed.

These instructions do not include securing the cinder blocks together with mortar. Using mortar would increase overall integrity but would also make moving or expanding your bed a real headache. Using mortar would also require you to make new measurements to account for the mortar between each block.

Step 2: Lay down the 7’5” X 4’ roll of quarter- or half-inch hardware cloth.

Prior to laying out your hardware cloth, assure that your cinder blocks sit on a solid footing. This may require you to tamp down the soil until it is compact so the cinder blocks will not shift under their weight or the weight of the soil within the bed. This may also require you to add some small or large gravel prior to laying out the hardware cloth.

Once your raised bed footprint is level, roll out the hardware cloth. It should cover the entire bottom of the raised bed with the cinder block border sitting on the edge all the way around. This is a step you could skip, but only if you want to risk voles, gophers, moles or other burrowing critters visiting your plants from underneath and discovering their newest favorite root fare. I am not certain whether any of these animals or the groundhogs I have in and around my back yard have ever eaten or damaged anything from below, but the cost of the hardware cloth is completely justifiable to not ever have to find out.

The first layer of cinder blocks will sit on the edge of the hardware cloth. On each end, the 7’5” length hardware cloth should extend 2.5 inches under the cinder blocks on the 4’-wide side. The cloth will extend .75 inches under the blocks along the long sides. Be sure to take these measurements into account when laying the hardware cloth into the bed footprint.

The completed bed will be hard to resist. Planting out your bed will be covered in Part 3 of this series.

Step 3: Set 18 cinder blocks down to build the first layer, then the other 18 blocks to form the second layer.

Prior to laying the cinder blocks you might want to measure and either add a chalk line or stakes and twine pulled tight for your blocks to follow. To begin, start by laying your cinder blocks in a corner. The first block is important, as it defines the vertical and horizontal levels for subsequent blocks. Be sure to get your first block level across the long end of the block as well as side to side.

Add blocks until you have six blocks end to end, creating the first long wall. Then go back to the first block and add a block on a short wall. Add two more blocks to complete that first short wall. At the end of that third vertical block, set the other corner as a horizontal, which will continue down to make your second long wall. Looking at the first finished end, you should see three cinder blocks sandwiched in between a cinder block on each end.

Set six cinder blocks to complete your second long, horizontal wall. Add the three additional blocks to complete the second short wall. This will result in two parallel rows of six cinder blocks horizontally with two parallel rows of three blocks at each end.

As you add each block, assure it is level both ways, following the chalk line or
twine, and tamped tightly and completely against the previous block. To add the second layer of cinder blocks, offset the blocks and do not stack them directly on top of the first layer. The second layer will have four vertical blocks and only five horizontal blocks.

It takes lots of planting material to fill a raised bed. The author includes chunks of charcoal, chert, gravel and other components in the mix.

Step 4: Fill with 34 cubic feet of growing medium.

Fill the bed with growing medium. The material I use is remarkably like the soil mix I use when potting plants in small or large pots. The biggest difference is the size of some of the materials in the mix. Because the volume of the raised bed is so large, I have some larger chert or large gravel that would be too big to use in a pot.

My soil mix for pots often contains horticultural charcoal. I also add bigger pieces of charcoal to my raised bed. I have a fire ring near my raised bed,
and I often put sticks that fall out of my trees in there for a small but hot fire. Then I douse it with water to put it out. The wood breaks up easily because it is burned through and crumbles into pieces of about half an inch all to 4 or 5 inches in size.

Extending your raised bed.

To extend your raised bed, simply take all the cinder blocks off one of the short ends and add blocks to extend the long wall by one or more blocks on each side, then replace the initial blocks to rebuild the short wall.

Of course, you will need to level the ground for the new footprint and add hardware cloth underneath before extending the walls. Be sure the added hardware cloth overlaps the previous cloth.

Once you get your bed built and filled with soil, now comes the fun part – planting it out, which will be covered in Part 3.