Written by: Aaron Fox
The idea of natural swimming pools is intriguing, but these landscape features are often not well understood. Finding useful information about them can be quite challenging, especially in North America where they are far less common than they are in Europe. Understanding their management and maintenance requirements can be even more difficult. As a horticulturist and landscape architect from Ontario, Canada, I had a natural swimming pool installed in our backyard 10 years ago, and I have been taking care of it ever since. While I am by no means an expert on natural swimming pools, I would like to share some practical considerations related to their management and maintenance I’ve learned from maintaining my own pool If you are interested in the design and construction of natural swimming pools, I highly recommend the book How to Build a Natural Swimming Pool by Wolfram Kircher and Andreas Thon, as it is one of the best resources I’ve come across to date.
Our natural swimming pool was designed and installed using similar methods as a conventional artificial pond, with the addition of a large planted biofilter surrounding the swim area. The biofilter was made from clear gravel embedded with perforated pipes that are connected to a pump. The pump draws water down through the gravel where it is biologically filtered by “good” bacteria in the gravel, as well as the plant roots. The main measure for water quality in a natural swimming pool is water clarity, and by this measure, our biofilter has consistently succeeded in creating a balanced system where algae growth is kept in check by the removal of excess nutrients in the water, particularly phosphorus. I regularly add beneficial bacteria, which I suspect isn’t necessary, but I haven’t mustered the courage to try going a season without using it yet.
For those considering installing a natural swimming pool or landscape professionals who may have clients asking about them, there are a few things to be aware of based on our experience. First, a natural swimming pond is a living, dynamic system. It contains bacteria, microorganisms, biofilms on most surfaces, some algae, debris from plants and all kinds of insects. Even local fauna will more than likely make use of it. If any of that makes them even a little uncomfortable, I’d be inclined to suggest a natural swimming pool might not be the right option for them. Another point to consider is that there’s always the potential risk of “bad” bacteria, such as E. coli, which can come from fecal contamination. I’ve encountered references that suggest certain plants may have the capacity to remove E. coli from the water, but I’ve never allowed babies or dogs in our pool due to this risk, and I always make sure people are aware of it before using the pool. Unlike conventional pools, there is no way to “shock” a natural swimming pool..
I would consider our natural swimming pool to be fairly low maintenance for the most part, but I am a practically trained horticulturist and I really just think of it as an extension of the garden. Regular maintenance consists of manually keeping the swim area as clean as possible using a net and brush on an extension pole. Startup and shutdown in the spring and fall can be quite labor-intensive, especially in the fall when all of the plants in the pond need to be cut back and removed, and any leaves from surrounding trees must be kept out of the pond to prevent them from breaking down and adding excess nutrients to the water. Using a leaf net to cover the pond can help, but as that’s not something I can manage on my own, I use a net and pole, sometimes needing to clean it daily due to several large trees on our neighbors’ property.
What has probably been the most significant challenge has been finding plants that survive and thrive in the biofilter. Plants play an important role in the overall filtration of the pool, and they also shade the water in the biofilter area which prevents it from getting too warm in the summer and helps prevent algae growth. They also contribute to the “natural” aesthetic of the pool. I plant them in a small burlap sack with just enough soil for them to start off, but the premise of the biofilter is that the roots need to grow out into the gravel. There are very few plants that seem to be well-adapted to growing in gravel constantly submersed in approximately 6 inches of water and the ones that do are often too aggressive. Most plants really struggle, and I suspect it’s also in part due to the nutrient-poor growing conditions. I try several new plant species almost every year and there’s only a handful that have really succeeded, with Iris versicolor (northern blue flag) and Saururus cernuus (lizard’s tail) being two of the best species to date. I’ve also begun supplementing with urea fertilizer (for nitrogen) on a regular basis which also seems to have helped a fair bit.
Last, but not least, there are two large pumps running continuously: one for the biofilter and one for the waterfall. I’ve recently replaced the waterfall pump with a more efficient magnetic drive pump which I turn off at night, but the significant amount of electricity used to operate the pumps is worth considering. Pumps also fail, and you need to ensure you have a replacement readily available in the event they do, particularly for the biofilter. I’m not sure how long an established biofilter can function without water circulation and what the implications would be without it. Since we live in an older neighborhood with above-ground power lines and lots of mature trees, I also installed a backup generator in case of a power outage.
There’s definitely much more I could discuss, but I believe these are the main points that anyone considering a natural swimming pool should know up front. But all that said, I can’t even begin to articulate how much enjoyment we’ve gotten from our natural swimming pool and the pristine experience it offers. Our kids, neighbors, and friends’ children have all grown up swimming in our pool, and I’d like to think the hours they’ve spent chasing dragonfly nymphs underwater, watching them emerge from the water, witnessing their molting on the Acorus calamus (sweet flag) leaves, and continuing to visit the pool as adults has contributed to their awareness and appreciation of the natural world around them. For that reason alone, it has been entirely worth it.
