Getting water lilies winter-ready: pruning and lowering them deeper

Getting water lilies winter-ready is a job of a few hours, spread over two weekends. In a tub or small pond where medaka also swims, more depends on it than on the lily alone. Leaves left lying around eat oxygen out of the water, right in the months when the fish is at its slowest.

The plant itself hardly suffers from that, as long as the rhizome stays frost-free. That rhizome is the whole story: it holds the reserves for spring and is the only part that can really be damaged. Everything you do above water is meant to get it through the winter.

Why getting water lilies winter-ready is the foundation

As soon as the days get shorter, a hardy water lily stops growing. The plant lets go of its floating leaves, withdraws into the rhizome and goes dormant. Botanists call that dormancy, and it is not standstill but a defence: the tissue above water would not survive the frost, the tissue under water does.

The deeper the rhizome lies, the more stable the temperature. Water is heaviest at 4 °C and sinks to the bottom, while colder water stays on top. In a tub 80 cm deep the bottom therefore often holds on to those 4 °C, even when there is ice on top. In a shallow container the whole water column follows the outside air, and that is where it goes wrong.

You read the moment to start from the weather, not from the calendar. The KNMI gives the night frost forecast per region, and the first frost in the Netherlands usually falls in the second half of October or in November. The national measurement series goes back to 1901, and the severe winter of 1963 still counts as the benchmark for frost damage in Dutch gardens. Wait too long and leaves freeze onto the ice and the stem tears off. Start too early and you remove green leaves that are still feeding the rhizome.

• floating leaves that rot, sink to the bottom and cost oxygen;
• dying stems become an entry point for bacteria and fungi;
• a tub full of dead leaves runs out of oxygen sooner than it cools down;
• without intervention the rhizome grows crooked out of the basket.

Prune the leaves as soon as they turn yellow

Do not cut on the first cold night, but at the moment the leaves turn yellow and the flowers are finished. Usually that is late October. Use a sharp pair of scissors that you clean first with hot water, because a dirty cut brings rot inside. Of all the steps in getting water lilies winter-ready, this is the one most often skipped.

1. Fish out the floating leaves that feel brown or soft.
2. Cut every stem 2 to 3 cm above the rhizome, not into it.
3. Twist off the seed pods, so the plant does not put energy into seed.
4. Take the cuttings out of the tub and do not throw them on the compost next to the water.
5. Check the stems for soft spots and keep cutting until you reach healthy, white tissue.

Leave a few young leaves if they are still green. They make sugar as long as there is light, and that sugar goes to the rhizome. Only when they turn yellow do they come off too. That way you prune in two or three rounds, not in one afternoon.

Protecting the rhizome against frost and rot

The rhizome is vulnerable at one point: damaged tissue. Every time water freezes and thaws again, the plant expands and shrinks back. Hairline cracks form at the bends, and rot bacteria get in through them. Protection is therefore mainly about preventing that freeze-thaw cycle. Anyone who wants to go deeper into frost damage to plant tissue will find background at Wageningen University & Research.

In a mild week, check the rhizome you can reach. Healthy tissue is firm and smells of wet earth. Soft, slimy spots you cut away with a clean knife, until you see healthy white tissue. Getting water lilies winter-ready therefore comes down to one thing: keeping the tissue intact until spring.

Lowering water lilies deeper before the frost

A hardy lily should stand with the top of the basket 40 to 60 cm below the water surface. In a tub 80 cm deep that is the deepest point; in a pond you choose the spot where the water is not warmed by a shallow edge. Put the basket there before the first frost comes, not after. Getting water lilies winter-ready without that step is half a job: the plant then only survives the frost in a mild winter.

If your tub is only 40 cm deep, or the lily stands in a 60 litre mortar tub, moving it is not an option. There are four ways out:

• move the basket to the deepest point of the pond;
• wrap the tub in bubble wrap and put it against a wall;
• take the rhizome out of the basket and keep it moist in peat or sand;
• place a floating ice preventer that keeps aerating and keeps an opening free.

Lift a heavy basket by the handles and not by the leaves, because the stems break off easily. Tie a piece of rope to the basket before you let it down; in March you will find it back without having to grope around in the cold water.

What medaka needs in the cold tub in winter

Medaka, the Japanese rice fish or Oryzias latipes, is a temperate fish that tolerates cold fine. In a tub that does not freeze solid to the bottom, the fish get through the winter without heating. Getting water lilies winter-ready and helping the fish through the winter are two sides of the same tub. What they tolerate badly is a lack of oxygen: under a closed ice layer the exchange with the air stops, and rotting leaves use up the oxygen that is left.

Do not break the ice with a pole, because the shock wave is worse for the fish than the ice itself. Rather keep an opening with an ice preventer or a bundle of reeds sticking through the ice. Feed hardly at all below 10 °C: digestion then runs slowly and feed that is left lying spoils the water.

Your winter job in half an hour per tub, from pruning to ice

The order of getting water lilies winter-ready is simple, and that keeps the job small. One round in October for the pruning, one round in November for the depth, after that only looking. Whoever does it in that order does not have to repair in January what could have been done in December.

1. Prune away leaves and stems as soon as they turn yellow.
2. Put the basket at 40 to 60 cm depth, or protect the tub against freezing through.
3. Stop feeding the lily and stop fertilisers in the water.
4. Keep an opening free as soon as ice forms and check the rhizome every two weeks.

In March, when the water comes above 10 °C, you lift the basket up again, refresh the top layer of soil and start feeding slowly. The lily then sprouts by itself, and the first floating leaves come up as soon as the sun can reach them again.

Verder lezen

• Vijver herfstklaar maken: bladafval, slib en ammoniak
• Slib verwijderen voor de winter: dit is je laatste kans
• Winter-hardy aquatic plants for your medaka pond
• Overwintering medaka: tips for healthy fish

FAQ: getting water lilies winter-ready in a cold tub

When do you start pruning?

As soon as the leaves turn yellow and the flowers are over, usually late October. Watch the night frost forecast and not the date; a mild autumn pushes the moment back.

How deep should a tub be for a hardy lily?

Count on 40 to 60 cm of water above the basket. If the tub is shallower than 80 cm, it easily freezes to the bottom in a severe winter and you are better off storing the plant frost-free.

Can a water lily overwinter indoors?

Yes, that works well for tropical species and for tubs that are too shallow. Take the rhizome out of the basket, pack it moist in peat or sand and put it away at 2 to 8 °C, dark and frost-free.

What happens if you do nothing?

The leaves rot away and the oxygen level drops. In a tub with medaka that is the biggest risk of the winter, because the fish then have few reserves.

How do you see in spring whether the rhizome made it?

A healthy rhizome is firm, smells of wet earth and is white to cream yellow inside. Soft or brown parts you cut away; if little is left, a new plant is the fastest way.