Winter Care for ponds & Water Plants

Putting the water garden to bed for winter - by Peter J May.

This is the season of our discontent, us poor water gardeners. The registered temperature slowly crawls down the thermometer and the world of the water garden slows to slumbering. With only a gradual change it is hard to know precisely what to do and when to do it, but certain pond events are a definite indication that the time has come to put the pond to bed for the winter months. 

Putting the pond water plants in order

With the diminishing hours of daylight and the reducing temperature, the plants in and around the garden pond have decided to call it the end of the season.

Amongst the Marginals, the fleshy stemmed and thick leaved pond water plants like the Marsh Marigolds (Caltha palustris and varieties) may have succumbed weeks before the real winter cold comes, preferring an early rise at the dawn of spring. These will need cutting back to the crown of the plants.

Any exotics like Canna hybrids lilies should have been removed at the end of September, and planted in ordinary compost and kept in a frost-free green house. Arum Lilies can either be settled deep in the pond or alternatively over wintered in pots in a greenhouse. Gone are the days when we would plunge the pots into clinker beds in cold frames along with the dark leaved Lobelia cardinalis in trays. With the current spate of mild winters in the south, gardeners find that as long as the pond water plants are in large enough groups, they survive quite happily outside.

 Some of the thin sword shaped leaved rushes and reeds will still seem very much in their element, perhaps acquiring attractive russets that subtly flash changes as the autumn winds rock the swathes of pond water plants en masse. It is always a dilemma in cutting these back, as the movement from these grassy pond water plants lends life to a scene that is otherwise dormant.

For small water gardens it perhaps wiser to at least cut off the seed heads before they shed, since all these marsh reeds and rushes carry an armoury of seed to perpetuate their species, scattering them on the wind to all four corners of your water world.

In the water garden that is a wildlife haven, the remaining standing leaves will keep a cover for the ingress and exit of insomniac amphibians and thirsty mammals, but unfortunately will also serve as hide for herons. If you have to net your pond against the autumn fall, cut them back to one third. The net will then double as a heron deterrent.

Floaters. A sure sign that winter is on its way is when the floating Water Soldier (Stratiotes aliodes) and the Frogbit (Hydrocharis Morsus Ranae) sink out of sight. Frogbit will lie as winter buds on the bottom of the pond. Rescue it if you have a mind for a thorough clearout of the pond. Mid October was best for this, as the water was still warm and the drowsy wildlife would recover before the cooler temperatures really affected the water. Other floaters may need to be taken indoors.

Some floating pond water plants that will definitely need some tender loving care:

The lilies, if they are still struggling on through October, their leaves will be looking diseased and mottled. If you can reach any of the worst leaves, give them a gentle tug to rescue them from rotting in the bottom the pond.  Every little helps in preventing the build of detritus in the bottom of the pond. The lilies themselves want as little disturbance as possible until late spring.

Pond pumps

Remove any submersible pumps for waterfalls or filters to give them a thorough clean and overhaul, even gently scouring away any build up of lime or silt on fins and bearing surfaces.

Put them back at a level less than 30cm(1ft) below the surface so that only the surface water is in the cyclical flow of water in the pond. This is so that the water at the bottom of the pond, if left undisturbed, is more likely to remain at a more even temperature. 

The pumps can be left in operation until it gets really cold, but there will be very little biological activity from a biological filter cooler than 10C (45F). If there is any chance of freezing, shut it all down and drain the filter box. You ought to start from scratch in the spring.

Pond filters

If you have not looked already the filter is in need of a major service. The Ultra Violet clarifier needs cleaning too if you have one.

The filter needs back flushing if you have that facility, otherwise scoop out the medium and rinse it in pond water or rainwater to flush out the worst of the muck. It does not need to be spotlessly clean, but check the chamber in the base because if the system has been overloaded at some time in the summer the evidence will be there to see. Scoop out this mess and rinse with pond or rainwater water.

The filter will continue to operate mechanically at all temperatures above freezing but there is little point in using the U/V unless the temperature of the water rises well past 10C (45F). If this is likely then many U/V clarifier manufacturers advise you fit new a bulb. If it freezes, drain the filter and start again in the spring as though from scratch.

GET NETTED

If you have not got a biological filter, then all the microbial activity that happens in biological filters has to happen in the bottom of the pond. All the muck that falls has to be digested by bacteria down there. But with the advent of winter this activity slows right down at a time of year when the potential load is increased by falling autumn leaves. This has to be cut to a minimum.

  • First, any leaves that have found their way in to the pond need to be dredged out with a net. Leave the resulting muck to drain away on the side of the pond for 24 hours. This will give time for any wildlife you have accidentally scooped up in it to make its way back into the pond.
  • Serious fish keepers suggest a partial water change of about a third of the water if it has been a hot summer. This will dilute any concentrated soluble salts that may have built up through evaporation. Treat any fresh tap water with proprietary pond conditioner.
  • If the pond is small enough, cover the water with a net. Stretchy nets that can pegged or held in place by bricks or stones can be obtained from your nearest aquatic store. If the net sags into the water, support it with long battens of wood or even a ladder.

Fish

If there are any particularly fancy fish, like fantails or bubble eyes, that may have difficulty competing in an environment with bigger more robust fish, contemplate keeping them in a tank indoors over winter.

Koi keepers know that fish will still come to be fed, if feeding is a routine, way past the temperature at which they are capable of digesting food. Being cold-blooded animals their systems can only summon enough reserves to digest very low protein foods between 10C (50F) and 7C (45F). So only feed a little winter feed or wheat germ two or three times a week. Below that temperature, feed nothing at all. As the temperature of water in the pond drops to around 5 C (41F) the fish begin to hibernate. Arm yourself with a thermometer that will register these temperatures accurately so you wont have to guess what is going on.

As water turns to ice in winter

Something very strange happens to water at 4C (39F). Instead of warmer less dense water being at the surface, as in the summer, the density changes and the water that is closest to freezing becomes the least dense and floats to the top. The result is that the bottom stays at a fairly steady temperature under a blanket of cold or frozen water.

If the pond freezes, there is nothing to worry about for a few days.  There is plenty of oxygen in cold water even if the fish needed it. But if there is any rotting matter in the bottom of the pond, toxic gases produced in the muck could build up.  In order to maintain gaseous exchange between the water and the air, float a ball or a piece of wood in the pond. A pond heater is the most effective method of maintaining an air hole. But even in recent years, I have seen these frozen into ponds.

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