You can always tell when someone has been to a swimming pool, by that characteristic smell of chlorine clinging to skin and hair. Wouldn’t it be nice if we could swim in pure water, polluted by neither chlorine nor the harmful bacteria and viruses that thrive without it? Increasingly, gardeners and nongardeners alike in Germany and Austria are doing just that, in swimming pools kept clean and safe through entirely natural processes. “While many people in the past may have swum in their garden ponds,” says German water-plant expert and professor Wolfram Kircher, “it was only in 1984, in Austria, that the first true swimming pool with natural filtration and cleaning was developed.” Since then a great number of natural pools have been built across Europe.
Not only are these natural pools an exciting development from a healthy living standpoint, but from a gardening and garden design one as well. Through these pools, gardeners can introduce a whole new palette of plants, many of them natives of low-nutrient habitats, such as bogs. And for the design-conscious gardener, these natural swimming pools mean that they no longer need to sacrifice the looks of their garden to accommodate a visually dominating blue cement pool and its associated paving. Natural swimming pools use waterside plants as part of the cleaning process, and these fit more easily into a garden setting, especially if the pool area is blended into the landscape by means of neighboring lush perennial or naturalistic plantings. Pools are often shaped with organic curves and have wooden boardwalks alongside them, instead of paving.
Natural swimming pools are composed of two parts: the swimming area and the planted area, or cleaning zone. To work effectively, the pond’s planted area must be about the same size as the swimming area—water is gently, constantly circulated between the two. These bodies of water can be adjacent to one another but separated by a low shelf, or they can be completely separate. Plants and people are generally kept apart, partly because human activities would damage the plants. Helpful bacteria and algae attached to the plants’ roots feed on nutrients in the water, in turn feeding the plants. These nutrients are derived from organic materials that are introduced to the water by plant debris, insects, and of course, gardeners swimming after a hot and grubby day in the garden. In all pools, it is these nutrients that need to be removed, or else they allow for the increase of harmful bacteria.
Keeping the pool as low-nutrient an environment as possible helps to keep it clean, because the plants and associated beneficial microorganisms stay hungry and constantly absorb all the nutrients that come their way. Subsequently, the cleaning area is not generally filled with luxuriant vegetation, but by sparse and low—but still attractive—plants. Plants are grown in gravel, rather than compost or soil, providing a home for the microorganisms that do the cleaning. It also functions as a trap for debris.
Most swimming pools are planted with species that are typical of marginal habitats, such as cattails (Typha spp.), irises (Iris spp.), rushes (Juncus spp.), and reeds (Phragmites spp.), although certainly a wider variety of plants can be used. Wolfram Kircher’s own swimming pond includes a particularly interesting plant selection. He notes, “I built a pond for several reasons: we wanted a place for the children to swim, and I wanted somewhere to grow a collection of bog plants.” Kircher designed a system where the water is steadily pumped out of the pond through a fountain and past marginal plants on each side of a narrow stream, which removes most of the nutrients. At one side of the pond, he has his peat bog plants, denizens of very infertile environments. They are particularly good at extracting nutrients from water. Among those growing in Kircher’s garden are the dramatic-looking hardy pitcher plants Sarracenia spp. and Darlingtonia spp. Kircher notes that in many swimming ponds “there is a conflict between needing to maintain low nutrient levels in the water… and the high nutrient requirements of the vigorous marginal plants commonly used in natural ponds. I decided to look to nature for alternatives. My inspiration comes from naturally low-nutrient wetland habitats.” In doing so he has created a biologically efficient and visually attractive planted area.
Natural swimming pools are constructed much as conventional garden ponds are, with a liner to retain the water. In theory, puddled clay could be used, but this can result in cloudy water because the turbulance caused by swimmers constantly disturbs the bottom— not to mention that most swimmers tend not to like the squelchy feel of clay between their toes. Pool liners, generally made of PVC or rubber, are usually black but also come in green. These are worth seeking because they make it easier to see the bottom.
A pond can be any size, but for swimming purposes 500 square feet is regarded as a minimum. Sun is a prerequisite, obviously, as swimming and sunbathing are of a pair, but it is also important to keep a pool away from trees because leaves falling in the water add to the nutrient content. Any leaves that do fall in will need to be collected and removed, as with a conventional pool. Cleaning the bottom of accumulated debris is occasionally necessary, and the planted zone needs some attention every year, during autumn or winter, with dead plant material being removed. Conventional pools can be converted, most easily by the addition of a separate plant-filled cleaning pool and more creatively by surrounding the existing pool with a shallow extension filled with gravel and plants.
For many people, the feel of swimming in open and pure water is infinitely preferable to that of exercising in what can seem like a chlorine-filled box. Others opt against having a conventional pool because of the difficulty in making them work visually in the garden. Natural swimming pools are much more garden friendly. With some hope, we can look forward to seeing many more being built in Europe and North America.