How to Build a Keyhole Garden

Keyhole Gardens are water efficient, easy to maintain and super productive in a small space. Here’s how I built one without breaking the piggy bank.

My Keyhole garden in late May

What is a keyhole garden?

So I was sitting around late last winter reading the week’s Costco circular when I ran across an article about the success of something called a “keyhole garden” in remote villages of Africa. (What Costco had to do with this, I don’t know.) Intrigued, I dropped the brochure and hopped over to the interwebs to find out what was up with these keyhole gardens.

The backstory on the keyhole garden is that it was developed in the 1990s by researchers in Lesotho, an arid mountain kingdom surrounded by South Africa where they were trying to find a way for remote populations with poor soil, little water and desert-like conditions to build efficient, and sustainable, but low tech, food sources.

The garden’s design is basically a circular raised bed made about 6 feet across with a wedge cut out to allow you to reach a basket at the center of the bed. The bottom of the bed is lined with organic material, sticks, leaves, cardboard, etc. and soil is filled in on top of it. The basket gets buried as well, but the top remains accessible so food scraps, plant trimmings and other organic material can be thrown in and compost will leech back into the bed. The width and height of the keyhole garden makes it easy to water and maintain by hand without bending or reaching too much. Best of all, it’s designed for intensive planting so it yields a whole bunch of produce in a very small space.

I read a couple more websites, looked at some photos of the keyhole gardens others have built, and said “I’m going to build one.” A couple weeks later, I did. Here’s how I did it and the results.

Step 1 – Garden Layout

Design the Keyhole Garden

My keyhole garden design on paper

Most of the keyhole garden builds I read used a circular bed about 6 feet in diameter and 2 feet deep with walls made from stones, cinder blocks, bricks, and similar sturdy stuff. The area I wanted to build mine was the old kitchen garden, a 10 ft x 10 ft south-facing patch right outside the door. Not wanting to waste space, I decided to make my garden 8 feet wide with a two foot keyhole opening. I also made the compost basket in the center slightly larger to compensate for the additional planting area.

Because I wasn’t certain this whole keyhole garden thing was going to work out, I decided to skip the cost and effort of dragging (and possibly removing) a ton of stones or blocks to the area and go with a wood wall built like an open top barrel. I chose cedar fence boards as the wood because it was inexpensive, bugs didn’t like it, and held up well in the weather. I used standard 6 in x 6 ft dog-eared fence boards rather than 8 in x 6 ft because the narrower boards were less likely to split. For the “barrel bands” I chose ½-in stainless steel pipe strapping because it was (again) inexpensive and held up well in the weather.

My supplies list:

• 5½-in x 6ft Dog-eared cedar fence boards
• Roll of stainless pipe strap
• ⅝-in screws
• ¼-in x 2 ft wire cloth (for the compost basket)

All of these things are available at any local hardware or home improvement store.

Step 2 – Supplies

Calculate How Much You Need

Thank a geometry teacher if you can read this

With a supplies list in-hand, the next thing I had to do was to calculate how many cedar boards and screws I was going to need. I planned on cutting the cedar to 18 in, giving me 4 boards for each piece of fencing. Each cut board would need 4 screws to attach it high and low to two bands of pipe strapping. So figuring out the circumference of the bed would tell me the number of cut boards and screws I need.

Except… the bed also has a “wedge” cut out of it that’s 2 ft wide at the edge and 18 in wide in the center where it meets the compost basket. Those extra three pieces (left, right and center) actually make the number of boards required more than just the circumference.

Calculating the circumference of the bed was easy enough:

C=2Πr

Or, for my purposes the Circumference = 2 x 3.14 x 4, which just a tad over 25 ft (7.62m). Subtract 2 ft from that for the opening to the keyhole, and you’ve got 23 ft (7m) and change for the outer wall.

(Side note: Mr. Janko, if you’re reading this, I owe you a huge apology. Despite my youthful statements to the contrary a couple decades ago, I did actually end up using geometry for something.)

Figuring out the amount of wood I’d need for the walls of the keyhole was more complicated. I had to determine the base angles of the isosceles triangle that was the “wedge” formed by the keyhole, then determine the intersection of the compost bin and the wedge, and calculate the length of the walls from there.

When that didn’t work, I used a tape measure and eyeballed it.

You should too. It’ll save you a whole bunch of time and some negative consequences that come from calling and threatening your high school geometry teacher in the middle of the night.

Anyway, all told, I needed a minimum of 16 uncut cedar boards, and 18 if I wanted to leave room for splits, cracks, bad measurements and all the other stuff that can happen. I bought 20.

Between the wood, screws and 100 ft of pipe strap, I spent about $100.

Step 3 – How to Assemble the Bed

Cut your boards to size

Cut the cedar boards to 18 inches long

I decided to make my keyhole garden 18in instead of 24 in high because there’s a nice view out the dining room window and I didn’t want the garden wall to block half of it. Plus, dropping the size 8 in let me get 4 pieces per fence board rather than 3.

Somewhere between 20 and 350 cuts later, I had my stack of wall pieces.

Step 4

Building the main wall

Line up the boards and cut a length of pipe strap

The first thing I did was build the main portion of the keyhole garden’s wall — the whole circle except for where the 2 ft keyhole opening would be. For my project that was 51 pieces of 18 in cedar boards (total length divided by the width of each board).

To put the wall together, I laid the boards down side-by-side, evened them up with a 6 ft level, and screwed two sets of pipe strap, one at 6 in from the top edge and another 6in from the bottom. Two screws per strap, four total on each board. When I got to the last board, I extended the the pipe stripe 12 in more and cut it. That piece would be used to join it to the interior wall of the keyhole.

Attach the pipe strap to the boards with 5/8 in screws

Finally, with the assistance of a helper, we brought the long “snake” of joined boards and stood it up in the garden. We knew where to place the wall because we had used a couple of stakes and a 4 ft length of twine to mark the circle’s boundary.

Stand up the assembled wall in the garden

Step 5

Assemble the keyhole walls

Once the main wall was in place, we used the same double strap technique to assemble the right and left walls of the keyhole along with the center section where the compost basket would sit.

Then we stood those up and screwed them to the main wall using the extra 12 in of strap to join the pieces.

Step 6

Make the compost basket

Assemble the compost basket with a length of weld wire cloth

To make my compost basket, I unrolled a few feet of weld wire, shaped it into a tube 3 ft across and used some wire to tie the ends. Then I dropped it in place in the center of the garden and secured it with two garden stakes to keep it from collapsing when we filled the soil in around it.

Place the compost basket in the center of the garden bed to complete the keyhole

With that complete, it was time to fill the bed.

Step 7 – How to Fill the Bed

Add debris to the bottom of the bed

A 6 in layer of twigs, leaves and sticks

The bottom portion of the keyhole garden is a layer of course plant matter and compostable materials — branches, twigs, leaves, even bark and cardboard — to provide drainage and organic matter. Fortunately, I had plenty of that, so the bed was lined with feijoa branches and leaves, spent grapevines and tomato vines left over from fall. After adding and smashing it down we had about 6 in of coarse organic matter.

Step 8

Add Soil to fill the bed

Adding soil and manure to the bed

Next step was to figure out how much soil it would to take to fill the bed. (If you already have plenty of soil, skip this and simply fill the bed.) To do that, I had to know the radius of my bed (4 ft), and how high the soil would be when the bed was full (1-½ft). Then using my high school math again, I calculated the volume of the bed like this:

Volume=Πr2h

(You can also just and plug it in to the Google.)

Once I knew how much soil I needed, I checked prices at both the local home improvement stores and landscape centers, but didn’t find much difference. I chose the home improvement store because their 1 cubic foot bags were easier to carry to the garden than navigating the paths with wheelbarrows of loose dirt. I spent about $100 on the soil.

Add your soil to the bed. (That’s me. King of dirt mountain)

After I got the soil, I filled the beds and mounded it to slope away from the compost basket and help nutrients and water to leech from the compost to the outer edges of the bed. Finally, I gave it a good watering and let it sit for a couple days so air pockets between the soil and debris would settle so I didn’t wind up with “sink holes” later in the season.
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Step 9

Plant Something!

The keyhole garden complete and ready for planting

At this point the garden was good to go, so I started moving seedlings out of the greenhouse and into the bed. Here’s what I planted over the course of the growing season:

Some of the articles I read about keyhole gardens mentioned that wide spreading, shallow root vegetables tomato and zucchini wouldn’t perform as well as other crops, but my results didn’t match that. The tomatoes eventually took over half the bed and the zucchini produced so much fruit that the ducks and chickens ended up getting plenty as well (yes, they love squash).

I also had to wrestle with sweet potatoes I apparently missed when we cleared the area to build the keyhole garden. They didn’t grow in the bed, but sprouted from the bottom and took off from there, eventually creeping over the top and swallowing the compost bin (which I still used).

Summary

Yes, it’s a great garden design

My Keyhole garden in late May

All told the yield from the keyhole garden was easily twice that of the standard raised beds down in the vegetable garden, and watering and upkeep was far easier. I did have some problems early with the soil drying too quickly (we only get rain between November and March), so I laid down a soaker hose and placed about 3 in of leaf mulch over it to control evaporation.

After that it was basically zero care and continuous production from May through September. Definitely worth the $200 and the few hours I put into building it. If you build one, I have no doubt you’ll be equally pleased at how much food production you’ll get out of such a small area.

A look at the compost bin

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