Pruning and protecting natural pool plants in winter: what to cut, when, and what to leave

In November, the planted area of a natural pool often looks like a battlefield: yellow water lily leaves floating, reeds fallen over, brown irises. The temptation to cut everything back is strong, to “make it look tidy.” Yet this is the best way to weaken the plant-based filtration system. The golden rule: in autumn, remove what will rot in the water, and leave until the end of winter whatever remains dry and upright. Here is how we proceed, plant by plant.

Why prune, and why not everything

An aquatic plant that dies back in autumn returns to the water what it absorbed during summer. If its leaves decompose on the bottom, the nitrogen and phosphorus they contained become available again in spring, just before algae resume growing. Pruning means exporting these nutrients out of the pool: it is the logical counterpart to summer purification.

But part of the plant serves a purpose over winter. The hollow stems of helophytes, such as reeds and cattails, carry air to the rhizomes buried in oxygen-poor substrate; this gas transport through aerenchymatous tissues is well documented in limnology. Cut underwater, these stems fill with water and the crown may suffocate. The dry stems also provide shelter for many insects, and they look beautiful under frost.

The pruning calendar, group by group

The dates depend on the year: here, autumn pruning generally takes place between late October and early December, once the first white frosts have passed. Late-winter pruning is done as soon as new shoots appear at the base, often in February or March.

Water lilies: the priority in autumn

These are the plants that add the most organic matter to the water because their large leaves sink as they rot. Cut the leaf stalks flush with the crown, pulling on the basket as little as possible. Leave the water lily at its usual depth: beneath the ice, it is sheltered. Tropical water lilies, however, do not survive winter outdoors in Belgium; bring them indoors in a tub of water protected from frost or treat them as annuals.

Oxygenating and floating plants

Submerged oxygenating plants, such as hornwort and Eurasian watermilfoil, slow down in winter without disappearing. An autumn thinning is enough: remove about one third of overly dense clumps with tongs to prevent them from decomposing en masse. Some native floating plants, such as frogbit and water aloe, produce winter buds that sink to the bottom and rise again in spring: do not look for them; they return on their own.

Beware of tropical floating plants: water hyacinth is banned in the European Union under Regulation (EU) 1143/2014 and has no place in a natural swimming pool, in summer or winter.

Protecting cold-sensitive plants

Most plants sold for constructed wetlands are hardy in our climate, provided their crown remains submerged. This is the key point: a crown trapped in ice suffers much more than a crown beneath the ice. A few simple steps are enough.

Lower the baskets. Marsh plants grown in baskets on a very shallow shelf can be lowered one level for winter, then raised again in spring.

Bring in frost-sensitive plants. Umbrella papyrus, colocasias, and some thalias cannot withstand our winters. Bring them indoors in a bucket of water, in a bright, cool room protected from frost.

Mulch the damp bank. For recently planted bank perennials, a light layer of dry leaves or straw around the base, out of the water, limits damage from severe cold. Remove it in spring so it does not end up in the pond.

Work without damaging the liner

Pruning a planted area often means entering it. Waders and measured movements help avoid trampling the crowns and puncturing the liner with a sharp tool. Work from the banks or designated access points whenever possible, and never pull a clump by its stems: remove the basket or cut it.

Also worth reading

To complete or renew your planting: Which aquatic plants should you choose for your natural swimming pool?.

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Frequently asked questions

— When should reeds in a natural swimming pool be cut?

In late winter, generally in February or March, before regrowth. Always cut above the water level so the hollow stems do not fill with water.

— Should water lily leaves be cut before winter?

Yes, yellowed leaves sink and decompose at the bottom. Leave the water lily at its usual depth.

— Can plants in a natural swimming pool withstand frost?

Most lagoon plants are hardy if their crowns remain underwater. Tropical species should be brought indoors.

— What should you do with pruning waste?

Remove them completely from the pool and compost them in the garden, away from the banks, so their nutrients do not return to the water.

In summary

• Autumn: remove anything rotting in the water, especially water lilies.
• Late winter: cut reeds, cattails, rushes, and grasses above the water.
• Oxygenating plants should be thinned, not shaved down.
• Keep the crowns underwater, protect frost-sensitive plants, and mulch the banks if needed.
• Pruning waste must be removed from the pool.

Last updated: September 29, 2026.