The Key to Keyhole Gardening

By Amy Grisak
Photography by Amy Grisak
Great Falls

A few years ago, my friend Kari posted pictures of her eye-catching, raised bed, stone garden in her backyard. But there was something different to this design; instead of a simple round structure, there was a pie shaped wedge, allowing easy access to the center.  Kari called it a “keyhole garden,” which I decided I had to have after learning more.  

Besides being a beautiful, functional garden which raises plants to an easy-to-reach level, keyhole gardens incorporate biomass to create a moisture retaining system, combined with a compost basket which turns kitchen scraps into nutrients for the plants. These were enough reasons, as well as an excuse to work with rocks, to inspire to build my own. 

After gathering rocks, I measured and leveled an area approximately six feet in diameter. While I chose to use rocks, there are countless options that would work – landscape bricks are a popular choice as they are readily available and easy to stack. Some stack wood or use galvanized metal for the perimeter. There’s even been some creative enough to use wine bottles for an artistic design.

After completing the rock work, it was time to create the compost basket and fill the bed. For this garden, I formed hardware cloth into a one-foot diameter column and placed it in the center near the narrow part of the pie-shaped wedge. To prevent the soil from spilling into it, I wrapped it in newspaper, which would eventually break down.

Traditional keyhole gardens use logs and chunks of wood in the bottom, reminiscent of the hügelkultur concept where the garden is created as a mound with these materials buried at the base. As they slowly break down, they create a bio-sponge which retains water, making it a self-sustaining system when used in large scale settings. A keyhole garden lacks the surface area to achieve the same results as a massive hügelkultur mound, but the concept does reduce the need for additional watering.

Because the native soil at my Great Falls home is claylike, I used a combination of potting soil, compost, and manure to create a soil mixture. The garden grew well the first season, and I was particularly impressed by how quickly the kitchen scraps broke down in the compost basket. It was nice to have a place to discard vegetable trimmings, while feeding the herbs and vegetables growing in the garden. 

But, after speaking with Deb Tolman, PhD., who authored Soiled Rotten: Keyhole Gardens All Year Round, I’m going to build my next keyhole garden a bit differently. Seeing the results of her work, she is certainly on to something. 

"What I'm promoting is the process, not the product,” she explained. “What’s important is to get the soil right.”

As written in her book introduction, "I live in North Central, Texas, where there are tons of rocks and almost no soil, and, at that time, temperatures had topped 100° for 70 consecutive days. Any trace of ‘soil’ I could find, was pottery-ready clay that can hold water for weeks with no air.”

That sounded familiar to me. Granted, living in Great Falls, summers are brief, but during those years of extreme heat and no rain, it’s practically impossible to keep enough water on the garden to make it thrive. Experiencing survival mode in those circumstances, I’m always seeking a better option for my garden. 

Building a classic keyhole garden, Tolman focuses on creating a space which holds a cubic yard. 

"You have to have a certain mass,” she said. “The keyhole garden is the vessel that holds the microbiology. It's always about the living creatures underneath."

But the difference is how she fills it. Instead of mixing compost and potting soil as I did, Tolman’s concept is to create layers of brown (carbon) to green (nitrogen rich) materials. 

"You're actually growing on an active compost pile,” she explained. “There’s no native soil.” 

Many of the materials for the fill are very cheap, if not free.  The brown materials consist of items such as cardboard, sticks, newspapers, magazines, and dried leaves. The “greens” include kitchen scraps, coffee grounds and fresh grass clippings (only those which are free of herbicides).  Be sure to incorporate some fresh manure into this mix to really kick start the process. 

“The kind of manure doesn't matter except there's a huge difference between rabbit, manure and chicken manure,” she said. 

Chicken manure is particularly potent, making it a better choice for a garden which won’t be planted for a month or more, while rabbit manure won’t burn growing plants. Overall, it’s important to be certain of the manure source. For instance, if you’re using horse manure, and the horse was fed weed seed-free hay, the powerful herbicides will prevent the manure from breaking down and will kill plants, particularly tomatoes, peppers, potatoes and other members of the Solanaceae family. 

"It's about the microbiotics. The brown to green ration is three to one. You need tons of cardboard and you’ll need about two wheel barrows of green manure,” Tolman explained.

To fill the garden, Tolman wets cardboard and lines the bottom with two rows, along with the sides. On top of this she adds sticks, grass clippings, kitchen scraps, and manure. Then, it’s time to add another two or three layers of cardboard before soaking it with water and adding a layer of green materials. Repeat the process until the bed is filled to six inches from the top. For the final planting layer, she adds a planting medium so seeds can be sown or transplants added.

Once planted, her garden requires minimal watering, even in temperatures cresting 100 degrees. 

“And you never have to rotate the soil. Never,” she added. 

Instead, simply add more layers of brown and green materials each year as the pile “cooks” down. She recommended checking the health and appearance of plants to know how well the system is functioning.

“If you have a kitchen basket that is stinky, it’s a message something isn’t right,” she explained.

If it’s too wet, shredded newspaper can be incorporated to aid the decomposition process. If it’s too dry, simply add water. 

Besides creating a self-feeding garden with phenomenal soil, keyhole gardens filled with Tolman’s soil layering technique also provide heat to the plants, so a garden built in March in Montana or Wyoming will have warmer soil temperatures and could potentially be planted earlier when the garden is covered by a dome. 

The possibilities are encouraging. Keyhole gardens are a practical way to handle poor soil conditions in many areas throughout the region while building something beautiful. What started as a simple project to create an eye-catching garden — and a reason to play with rocks — has shifted into a new gardening design altogether.  Stay tuned for the results.