When the subject of edges on water gardens comes up, I cannot help but think of the phrase “sedges have edges.” In other words, if the stems of a marginal water plant have distinct edges, you can be reasonably sure that it is a sedge.
All ponds also have edges, and each type of water garden has a set of circumstances that helps determine the best type of edge for your pond.
Think about the purpose of the pond edge. Protecting and anchoring the waterproof liner are initial considerations. The aesthetics of creating an edge that enhances a beautiful water garden and blends with the overall garden design is the fun part. It is also critical that the edge be stable and last for many years.
There are other important aspects to be aware of when designing an edge. Besides being stable and long lasting (i.e., will not deteriorate over time), it also needs to deter critters, provide entry and exit points for pond maintenance, be itself easily maintained, include wildlife considerations, and, most of all, be beautiful. So, when I mull over all the choices of edge designs, these aspects are considered.
Something I like to promote with any edge plan is varying the design between two or three different types of edges. This reduces the likelihood of a “necklace” look around the perimeter of a water garden. This look, while often used successfully in formal edge designs, is encountered way too often among misguided attempts to edge informal, natural-looking ponds.
Lay the Groundwork
The first step with the installation of any edge is to dig and shape the soil so that it supports the chosen designs. Water levels must be known and appropriate for any edge. Some edges allow for a wide fluctuation in water levels, whereas others look odd if the levels are not spot on.
When planning your pond, determine the edge designs before any soil is removed. Once the soil is disturbed, it is very difficult to reestablish the soil structure so that it will be stable enough to support the edge. In your pond’s design, consider where a concrete underlayment would be useful, like in areas where the soil is unstable, the pond is steep-sided and deep, or where heavy, large rocks may be placed. In areas where concrete is used, be aware of utility lines going in and out of the pond. You may need to create a gap to allow for those lines. Do a good job of planning before doing any digging.
Part of every edge design is figuring out the depth of the ledges you want to create. I am a huge fan of going to a depth of 12 inches to the first ledge. At this depth, you will deter some critters from being effective predators — in particular, raccoons. As I like to say, when a raccoon is doing a back stroke, it cannot catch fish. It is also a good depth for when a person steps into the pond to perform maintenance. If it is deeper than 12 inches, then it is a huge step!
In addition, marginal plants in pots can be set at this depth without the pot showing. This depth is perfect for most of the designs that I will be discussing. When looking at the design drawings, this depth is apparent in most of them.
EPDM-lined ponds that have a sloped ledge or even an entire sloped bottom can be a huge design mistake. If the entire bottom is sloped and lined with rock, the rock on the slope of the pond leading up to the edge is dependent on the rock in the deeper part of the bottom to hold it in place. If the liner is not covered with a stable layer of rock, it is almost impossible to stand up without slipping or sliding while maintaining the pond. I aim for a slight slope on the bottom to allow sediment to accumulate in a sediment basin or bottom drain, but this slope must be minimal.
I incorporate a collar of raised soil around almost the entire outside perimeter of the pond edge in all my edge designs to prevent any surface water in the surrounding terrain from entering the pond during heavy rain events. In areas with a flat landscape, there is still a grade where water will pool or run off. This water will contain both surface debris and anything that was applied to the surrounding landscape, like fertilizers and chemicals.
I cannot overstress the importance of at least a 2-inch raised area that is gradually sloped away so that it is barely noticeable. In areas where water normally flows, like a depression or gully, this area of raised soil will need to be higher. It concerns me when I recall the number of times I have heard people say they need to put their water garden in a low spot. This is a natural impulse, since natural bodies of water occur in the low areas. However, unless you install this collar, these can be the worst places in the landscape for a water feature.
Rock it Out
Let’s start with the traditional, widely used informal design of placing rocks down to the first ledge in the water. Rocks are placed down to the shelf and are interlocked with each other for stability.
It takes some practice to become a good “rocker” — i.e., choosing and then placing the stone appropriately. Always vary the size of the stone in this design: anything from huge stones (18 to 24 inches in diameter) to small stones. You could create an upper, shallower shelf of 6 inches so that it does not take as much interlocking stone. This shelf is for just the stone. You would still want to create the 12-inch shelf below this that would not have any rock placed on it.
You have the option of rocking the entire shelf, but I prefer not to do this. I like having a place to step when entering a pond and placing pots of water plants on this shelf. When the shelf beyond the edge is entirely rocked, these activities become a nightmare. Also, most pond plants, if planted among the rocks, will proliferate throughout the rocks. Then even the plants become a problem. An added design element is to create a place in the edge to be able to easily enter and exit the pond. Find the perfect, mostly flat stone and place it in a stable configuration so that the edge can be safely stepped on. Do this in a couple of places, if possible.
Another informal design that is much simpler requires the availability of more mostly flat stone. The rocks themselves do not have to be perfectly flat, but they do need to be stable when stacked. This does not take as many tons of stone. In a lot of areas, this type of stone is normally limestone, which naturally is relatively flat. If necessary, more flat stones can be stacked so that water levels can fluctuate more without seeing the liner.
The roots of the plants will need a constant supply of water over the top to keep the soil in this bog aerated. Ideally, the best bogs have plants with only a couple of inches of water over their crowns.
A variation from this stacked design is to use human-made concrete decorative blocks of many designs. This is more of a formal design and needs to be installed perfectly flat. In almost every situation when I have used this type of stone, I have laid a concrete base so that these blocks of stone will not move over time. The base is a critical element of this design.
Another critical consideration is that concrete is very abrasive to the liner. So, the concrete will need to have a protective underlayment installed over it before the liner is laid down. Any freeze-thaw cracking of the concrete that occurs over the winter is normally not an issue, since it is under the flexible liner. The very stable base really will not move very much.
Bog EdgesAn edge design that is fun and can be incorporated into any informal design is a bog edge. This is simple to create, but water levels need to be determined exactly. Dig a pocket in the edge approximately 12 inches deep, varying this depth depending on the plants you choose for this wet area. (In most cases, I think 12 inches is good.)
Place a raised area that is stable and made of either concrete or stone with liner overlaying it between the bog and the main water garden. This maintains the integrity of the pocket of soil that is created so that it doesn’t wash out. On the turf side of the bog, use stone to anchor the liner and cover it. The soil in the bog will need to be equal parts topsoil, peat moss and sharp sand (also known as play sand). Top the soil with a thin layer of ¾-inch gravel. I would not use any fertilizers or animal manure in this mix.
The roots of the plants will need a constant supply of water over the top to keep the soil in this bog aerated. Ideally, the best bogs have plants with only a couple of inches of water over their crowns. Some plants can tolerate more water over their crowns than others. Two inches allows water levels to fluctuate without exposing the liner. I would not use this design around the entire edge of the water garden — only on the far side, where you are not normally viewing the pond or maintaining it. This is a good example of using multiple edge designs to create interest.
A wildlife-friendly type of edge consisting of a shallow slope can be a good one, but it does have its drawbacks — especially the tendency for string algae to grow prolifically in these areas. Wading birds, raccoons and other fish predators love using a shallow edge to enter and exit a pond. However, so many other great critters are attracted to it, so it’s worth considering in certain situations.
This is easy to create and does not take a lot of effort to rock. A ledge or raised area of about 18 inches will need to be created out of either stone or concrete under the liner to keep the gravel from sliding into the pond. This creates a small, sloped area only in the edge that goes from zero depth inward about 8 to 12 inches. Again, I would use this edge in combination with several other edge designs. I would not use this type of edge around the entire perimeter.
All ponds have edges — some that are formal, and a lot that are informal and more natural looking. Your imagination and creativity are the fun part of deciding which edge design fits best in your landscape. Whatever design you choose, just remember that your edge needs to be stable, long lasting, easily maintained and beautiful.