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Caring For a Koi Pond In Winter

15th April

Back in 2015 we wrote an article for Koi Carp Magazine explaining the impact weather has on a koi pond in winter. Here it is –

Winters in the UK are rapidly warming up! The average temperature in December daytime in the UK is a balmy 8°C and January isn’t much colder. Plants are blooming months earlier than they should and insects don’t know whether to hibernate or pollinate. It’s almost as though the difference between the seasons are becoming indistinguishable. But what effect will this have on your pond’s biological balance when the ‘real’ spring arrives?

Let’s start from the top. Your pond is naturally full of organic matter. Whether it’s the twigs from overhanging trees, droppings from an unwelcome pigeon or your beloved koi’s own bodily functions. Every pond is full of organic material that will decay. This process of decay introduces substances such as phosphorous, ammonia and hydrogen sulphide into the pond. Small amounts are fine, in fact they’re normal, but too much can easily affect water quality and the health of your fish. We call this the “organic loading” of a pond. The more organics, the more decay, the more adverse effects on water quality.

This is the essence of most pond water problems. If the pond’s biological balance is out of kilter, you open the door for excessive amounts of ammonia, nitrites, nitrates, phosphates, urea, faecal matter and dead organic waste. Therefore most owners use various pond treatments to tackle this issue by introducing task-specific ‘good’ bacteria into the pond environment.

The reduction in these contaminants improves the quality of the water and that combats a wide range of aquatic pathogens and diseases such as: streptococcus, pseudomonas, aeromonas, vibrio and burkholderia. In essence ‘good’ bacteria tackle the ‘bad’ bacteria that are effectively feeding off the organic matter in the pond.

But surely a warm, mild winter is good for us and there will be less adverse effects in Spring?

This is where the answer gets a little muddy (pun very much intended). Organic matter decays as low as 6-7°C, which means in the very cold weeks of the year, the pond becomes almost dormant and you can almost leave it alone. However, winters are now becoming very different, with the average temperature for December being 8°C. This is an ideal temperature for organic matter to decay without any competition creating sludge that builds up into an anaerobic environment to emit toxic gases such as hydrogen sulphide.

To add insult to injury, we are now starting to experience storm like conditions with heavy rain and windfall. Gale force winds have distributed a veritable smorgasbord of organic detritus around our gardens and some of it will have inevitably found a home in your pond.  High levels of rainwater, packed full of nutrients such as sulphur (think acid rain) and minerals, provides a readily available supply of nutrients to help organic matter decay. Consider how much better your plants grow when fed with rain rather than tap water! Or how when you collect rainwater in a water butt, invariably there will be a layer of algae over the top. The ‘bad’ bacteria in the pond thrive off this free buffet of nutrients, which only speeds up the decaying process and helps generate sludge.

So we have in essence the makings of a perfect storm. Lots of organics and nutrients being added to the pond, mild temperatures of around 8°C aiding decay and the fact that almost all traditional pond treatments are only effective from above 10-12°C.

This is why you can expect to see some real problems when spring eventually starts.

There are a few tried and tested methods of reducing the impact of green water, sludge and other algae problems in spring –

  • Physically removing as much sludge and organic matter from your pond – You can do this with a net, bucket or even with your bare hands but it’s not always practical.
  • Keep your filters cleanso you can catch as much organics in the water column as possible
  • Add plantsIn a natural pond, plants will absorb a lot of the bad nutrients, but they can’t cure the issue!
  • Add plenty of task-specific bacteria at the start of spring – Providing the bacteria in question is safe in large amounts, add a double or triple dose for the first few times in spring. Bacteria grow logarithmically – 1 becomes 2, 2 becomes 4, 4 becomes 8 – so the more you add the quicker the results.
  • Avoid adding any materials that can act as a nutrient to the ‘bad’ bacteria – with the high organic loading’s, the last thing you want to do is add a nutrient.

Have pond treatments kept up with our evolving climate?

Bacteria can be genetically modified (although there are ethical and moral question marks with this) but it’s far more common that new strains are discovered in natural occurring environments. In many ways it’s a game of chance, thousands of strains can be examined without finding anything that can improve on the strains you already have. This means that when a new strain is discovered it can be a big deal.

In late 2013, a strain of bacteria was sampled in a stream in Pennsylvania. Like many others it was catalogued and its characteristics examined and it showed signs of very good activity at low temperatures. In fact, it showed the same activity at 4°C as similar bacteria do at 10-12°C. It was so unique that the unusual step was taken to patent it!

Its natural environment was in water which made it incredibly suited for water treatments and was very active in breaking down organic waste, therefore just what you would want in a bacterial based pond water treatment.

This activity means that it’s food source are the same organics that create problems in your ponds and also provides a stable and continued treatment even during temperature spikes. Why you may ask is this important? Well imagine going for a run. It’s much easier to run for longer and with less effort if you keep a rhythm, rather than stopping at traffic lights, crossing the road or opening a gate to a field. Well, it’s the same principle for bacteria.

In spring we see periods of really warm weather which raises the water temperature in our ponds above 10-12°C. This allows traditional bacterial strains to become active, but periods of cold weather stops any activity. In fact the mean averages in the UK are; March – 6.5°C, April – 8°C and May – 11.5°C!. For the best bacterial activity you want the bacteria to concentrate on digesting organic matter rather than wasting energy stopping and starting due to temperature fluctuations, as they become much less effective.

These new low temperature strains look to be the ideal answer as they provide you, the pond owner, with more constant and reliable bacterial activity much later into the year, and arguably more importantly much earlier in the year to tackle the inevitable build-up of organic matter that the warmer and wetter winters are bringing.

Envii’s range of bacterial pond products incorporate the unique patent-pending bacterial strains that are fully active at temperatures as low as 4°C.

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About the author


Chief Executive Officer

If eating was an Olympic sport, Olly would hold the gold medal. He once fit 15 giant marshmallows in his mouth, consumes exactly 4,000 calories a day and even admits that 90% of his dreams involve food. At Envii, he specialises in all things e-commerce, spending time devising ways to improve sustainability and delivering exceptional service. All whilst eating, of course.

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