Winterizing a natural pool: the mistakes that cost you dearly in spring, and what to do instead

Every spring, the same calls come into the garden center: green water as soon as the first warm days arrive, a liner folded at the edge, irises that fail to grow back. Tracing things back, it almost always comes down to an autumn or winter task carried out with the best intentions. A natural pool is prepared for winter by doing little, but doing it right: removing organic matter, protecting the equipment, and letting nature slow down. Here are the eight mistakes we encounter most often, and what to do instead.

Mistake No. 1: emptying the pool for winter

It's a habit inherited from ornamental ponds or certain above-ground pools, and it's the most damaging one. A natural pool is designed to stay full: the weight of the water stabilizes the walls, edges, and liner. When empty, the bottom can rise under groundwater pressure, the liner contracts in the cold and pulls away from the ledges, and all the biological life in the substrate dies. Instead: keep the water at its normal level all winter, checking that it does not fall below the top of the liner.

Mistake No. 2: letting leaves sink

A leaf that falls into the water in November decomposes slowly on the bottom and releases its nutrients in spring, exactly when the water is warming up and the plants have not yet started growing again. The algae then have a few weeks' head start. In most of the natural swimming pools we monitor, green water in April is caused by leaves from the previous autumn. Instead: stretch out a net as soon as the leaves start falling, remove it when the trees are bare, and skim off anything that has sunk.

Mistake No. 3: cutting all the plants back in autumn

Pruning is necessary; cutting everything level with the water is not. Reeds, rushes, and cattails have hollow stems: when cut below the surface, they fill with water and the crown may rot. Their dry stems also shelter beneficial insects during winter. Instead: in autumn, remove anything that rots in the water (water lily leaves, excess oxygenating plants, and soft foliage), and leave the hollow stems until the end of winter, then cut them above the water level.

Mistake No. 4: breaking the ice

Hitting the ice sends a shock wave through the water, stressing the wildlife present (newts, larvae, and sometimes a few fish), and a poorly placed blow can damage the liner near the edges. A natural swimming pool without fish can, moreover, withstand being frozen for a few days very well. Instead: if you want to keep an opening for gas exchange, install an anti-freeze float before freezing temperatures arrive, or gently melt a hole with a saucepan of hot water placed on the ice.

Mistake No. 5: putting on a watertight cover

An opaque, airtight cover blocks light and air exchange. Underneath, submerged plants become depleted, decomposition consumes oxygen, and in March you discover dark, foul-smelling water. The weight of accumulated rain and snow also pulls on the edges. Instead: a fine-mesh net is enough to keep out leaves, while still letting in light and air.

Mistake no. 6: forgetting the pipes and equipment

A pump stopped without being purged leaves water in the above-ground pipes, valves, and filter housing. At the first hard freeze, a fitting bursts, and you only discover it when restarting the system. Instead: decide clearly whether the filtration will keep running or stop. If it stops, drain everything exposed to freezing temperatures, bring the UV unit's quartz sleeve and lamp indoors, and remove the floating skimmer. If it keeps running, check that nothing exposed is left with stagnant water in it.

Mistake no. 7: adding bacteria to icy water

The bacteria sold for swimming ponds are living organisms. Below around ten degrees, their activity drops sharply, as Boyd and Tucker point out regarding nitrification in ponds. Adding them in December often amounts to throwing your money into the water. Instead: save them for restarting the system, when the water remains consistently above ten or twelve degrees.

Mistake no. 8: letting de-icing salt run off

A salted terrace or walkway along the edge of the swimming area sends saltwater toward the planted zone every time the thaw comes. Marginal plants do not tolerate this well. Instead: use sand or gravel near the water, and direct runoff from the terrace toward the garden.

The right order, in one autumn day

Once the trees have shed their leaves, generally in November here: remove the net and shake it out, run the fine-mesh net through the swimming area, vacuum up the sediment if it is more than a few centimeters deep, trim anything that is rotting, then put the equipment into winter mode. The rest is patience.

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Frequently asked questions

— Should the water level of a natural swimming pool be lowered in winter?

No. Keep the normal water level; the liner and banks are designed to be supported by the water. Simply make sure it does not drop too much during a dry winter.

— Can a natural swimming pool freeze completely?

The surface freezes during severe cold, but a swimming area of normal depth does not freeze to the bottom in our climates. This is not a problem in itself.

— Can a natural swimming pool be covered with a tarp?

A fine-mesh net is preferable: it stops leaves while allowing light and air to pass through, unlike a waterproof cover.

— When should bacteria be added again after winter?

When the water remains above ten to twelve degrees for a sustained period, roughly speaking. Before that, their activity remains very low.

In summary

• The pool stays full all winter.
• Fallen leaves are the number one enemy: net, skimmer, vacuum.
• Cut back anything that is rotting, and keep hollow stems until the end of winter.
• No breaking the ice, no waterproof cover, no salt near the water.
• The system is either running under controlled conditions or drained: never anything in between.

Last updated: September 29, 2026.