Emma Townshend: 'Who needs chlorine? Open-air ponds offer a different swimming experience'

Emma heads down to King's Cross Pond Club, a new open-air pool in London

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I feel I may have been unwise. I'm standing in just my swimsuit, trying to be brave in what I think could accurately be described as "hostile conditions". There's an edgy, biting wind coming in sideways from the east, laden down with that sneaky category of rain, which can soak right down under your coat collar. If you were in fact wearing a coat, a woolly hat, a scarf and some tights, rather than just a swimsuit.

I'm also in the middle of a building site. Did I mention the building site? Plenty of builders in high-vis. Also, plenty of finished offices, with people working in them. In fact, I can see the Post Office Tower from here, so, logically, the people in there can see me and all. To add the final blow, a Eurostar train just went past, heading into St Pancras. Probably full of sophisticated French women deciding I'm a bit odd, stripping off in the rain.

I'm here, sans wetsuit, at the opening of King's Cross Pond Club, an open-air wild-swimming pool of a kind that's popular on the Continent, but rarely seen over here. We've wandered up from the Tube, over the canal and between the fountains in Granary Square; and here, behind the impressive building housing Central St Martins School of Art, we've found this sweet little hillocky mound, holding a 400sqm pool. The bottom is covered in a soft sand, and the wooden sides are visible. The water is clear, not muddy: wild swimming ponds are designed to be self-cleaning, with about half the surface area taken up with aquatic plants that filter the water till it sparkles.

Wild swimming ponds are a passion for Biotop, the Austrian company doing the science bit at King's Cross. They are experts on the problems with regular swimming pools – do not get CEO Peter Petrich started on the horrors of chlorinated water – and advise on the careful planting required to provide correct filtration, in balance with the human beings who'll use the site (limited at King's Cross to 168 per day). Biotop has been making these ponds for 30 years, some bigger than others: they've done one in Istanbul that's 12,000 sqm – 30 times larger than the one in London. It's a strangely appealing vision, even to me, with my phobia of fish, mud and pondweed.

Such havens are very much of the moment: Jo Thompson, designing the flagship M&G Garden, "The Retreat", for this year's Chelsea Flower Show, has also included a wild swimming pond in her design. "Natural pools provide a place to swim, but look good all year round," she explains of her decision. "And they can be any shape you like." Jo's pool is also surrounded by soft planting, with striking hostas, irises and astilbes, as well as more conventional pond planting such as good old-fashioned water lilies. "Some natural swimming pools are glorified carp ponds, but everything here is as nature intended. The water is kept clean by plants, not chemicals."

The system works by pumping water from the swimming bit into the green reeded areas, so that both the plants, and tinier micro-organisms, can help to sift out the rubbish. The pool is lined with a very thin polymer membrane that makes it feel soft to the feet. Physical filters such as gutters are also included, and natural pools do still need skimming over to remove leaves.

Nonetheless, wild swimming pools feel like they might be becoming a trend. To have one built in a garden costs between £40,000 and £70,000 (and waterfalls and rockeries can take it over £100k), though it's easier (and cheaper) to convert an existing concrete pool.

Come in from a garden swim and not smell of chlorine at all? Unanticipated bliss.

Four more: How to capture Jo's look

Water Iris 'Black Gamecock'

Dramatic dark velvety petals to these striking iris flowers, striped centrally with yellow. £9.99, crocus.co.uk

Nymphaea alba

The simplest and loveliest of water lilies, with wide green leaves providing shade to prevent the spread of algae. £24.99, crocus.co.uk

Trollius x cultorum 'Alabaster'

The Globeflower has the poised prettiness of a creamy water buttercup, but mustn't dry out at the roots. £7.99, crocus.co.uk

Hosta 'Patriot'

Jo has the rarer "Thomas Hogg" in her garden, but "Patriot" is very similar, with smart white edging to shapely green leaves. £7.49 (down from £9.99), crocus.co.uk