Monday, October 06, 2008

Garden Diary: Making a natural clay pond

Placing the Pond
At first I wanted the pond to be far out from the house. I thought it should become a destination. But I waited and observed the land. I let it tell me where the pond should be.

Stormwater runoff from the slope above us flows like a flood during wet weather and is directed by the contour of the land, as you can see in the photo above, around the end of the little hill on which the house sits. It was clear a pond in this area would be repeatedly filled whenever it rained. Availability of a natural source of water was ultimately the deal breaker. The pond would go here.

Considering the constraints of the space, I decided on a canal-like shape, long and narrow, with a couple of barely suggested S curves. I started by outlining the pond with a garden hose in the fall of 2007 and left it there over the winter. I couldn't dig while the ground was frozen, so I was forced to look at the shape for a long while. That was good. It gave me time to be sure.

Over that winter I reconsidered using a liner, but a natural pond a couple of hundred feet out in the woods was proof enough to me that an unlined pond could hold water in our heavy clay. This evidence of natural impermeability, coupled with a high water table, and a ready source of rain water, gave me the confidence to proceed without a liner. Even with some leakage, I thought the runoff would keep the pond full, and that has proven to be true. Lesson learned: put the pond where conditions are appropriate.

The dry laid stone wall at the base of the house was finished in February. I understood the wall would make a curve, but I hadn't anticipated its visual power. Figuratively speaking, it creates the feel of a centrifugal force that "throws" the eye around the curve, down the length of the pond, and into the distance. That shape helped me finally decide on the exact shape and position for the pond, and I juggled, poked and prodded the hose quite a bit as the digging proceeded.

Having the pond close under the house was a good decision. I can enjoy it from the terrace above without leaving the area of the house, and the pond has given me a new entry point, entirely changing my original concept of how to enter the garden.

Building the Pond
We started digging the pond during a late February thaw. This part involved a lot of improvisation. After we dug the far end of the pond, it completely filled with water overnight.

I decided to leave a foot-wide dam of earth in place, dig the second part ...

... leave another earthen dam to hold in the water, then dig the third section.

This work could have been completed in three days, but my diggers could come only occasionally. Allowing for bad weather and my helpers' schedules, it took about six weeks to finish the project.

Joining the Compartments
Once the pond was finished, all three compartments were full of water, so we needed a day just to join them. We used a small pump made for emptying flooded cellars to do the job, then dug out the dams between the compartments. We used this earth to make the dam we discovered we needed at the far end to raise the water to an appropriate level.

The Dam
One thing I couldn't measure in advance was the change in land elevation from the upper to the lower end of the pond, a distance of about 40 feet. As the pond was completed, it quickly became clear the change in land elevation from the bottom end to the top was a serious problem. The simplest solution was to build a dam at the lower end. This raised the water level about 8 inches and allowed the entire pond to fill to a satisfying level. The next photo shows the beginning of the dam on the left.

The dam was easy to make using excess earth from the pond excavation. We simply piled up the earth around the lower end of the pond, tamped it down, then added more until we achieved an acceptable elevation. To prevent breaks, I made the dam wide, four to five feet in places, wet it thoroughly, and repeatedly walked over it to compress the earth as much as possible. I placed a few rocks in strategic positions to help hold the dam in place initially, until it settled and stabilized, and started planting the dam slope to further stabilize it.

The Drain
Another danger was water overtopping the dam and washing it away during a heavy storm. To prevent that, I bought a cheap five-inch flexible plastic drainage pipe from Home Depot and installed it in a small channel I dug at the side of the pond, just above the dam. I placed the pipe at an elevation that would keep the pond full, but would allow any excess water to drain out well before it reached the top of the dam. I used rocks to visually screen the pipe opening at the pond end and at the outlet end, and recovered the length of pipe with earth. This was a very simple, but absolutely critical, operation that took only about 30 minutes. In the following photo, the outside end of the drain is shown covered by a rock in the middle right, with drainage water flow visible below the rock. You may need to click on the photo to see this detail.

I used a rather wide pipe to be sure it had capacity to carry off sufficient water, even in very heavy rainstorms, to prevent overtopping the dam. This was a hit or miss operation since I don't have the mathematical skill to calculate drainage volumes. I decided I would keep close watch and, if I ever saw the water approach the top of the dam, I would simply add another drainage pipe, but this has not been necessary.

Pond Depth
A large boulder in the lower end of the pond dictated a maximum depth of about 30 inches. We simply couldn't get that rock out of the ground, and I wanted to avoid the expense, inconvenience, and damage of bringing in equipment heavy enough to do that job. The pond bottom rises considerably moving from the lower to the upper end, where the water depth is only about ten inches.

I wanted to keep the upper end shallow to make it possible to create a sort of beach where birds and other animals could easily get to the water.

"Naturalizing" the Pond
Planting the pond edge was easy. I quickly added two large clumps of Darmera peltata, Ligularia 'Othello', several water loving irises, ferns, Pontadera cordata, Spartina pectinata 'Marginata', Chrysogonum virginianum 'Eco-Lacquered Spider', Carex muskingumensis 'Oehme', and assorted other plants that came my way.

In the next photo, the native, and highly invasive Equisetum arvense had crowded the pond edge in only four to six weeks. The scum on the water surface is tree pollen, distasteful but soon gone.

The next photo, taken at about the same time, shows how quickly the pond area revegetated.

By the end of May, frogs were present in abundance.

The final photo shows the pond at four months, near the end of July. We used wood chips to create a sitting area on the right, between the pond and the stone wall, and to build a path from the upper terrace, around the curved stone wall, to the pond, and continuing off into the larger garden. This has created a new point of entry into the garden, and a new focal point from which the rest of the garden, in a metaphorical sense, "flows".


  1. Great story, thank you. I find your garden rather fascinating as all the elements (heavy soil, too much water, invasive weeds etc) combined often make the unadventurous gardener run screaming for them thar hills. As you so ably demonstrate it is better to bend in the winds rather than fight them.

  2. When we moved to this house, I realized I couldn't have a conventional garden, at least not without bringing in many tons of soil and spending a small (large) fortune to transform the land into something it isn't. I've taken a lot of inspiration from Noel Kingsbury's books and from the work of his associates at the University of Sheffield (Nigel Dunnett and others whose names I apologize for not being able to recall at the moment).

  3. What a great post. Its interesting to read about a garden thats different. I'll be back to find out what else you are getting up to.

  4. Thanks for the visit. I really enjoyed your post on your visit to Lytes Cary Manor.



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