Can Pond Algae Still Grow In The Winter?

I’ll share a recent and common question that always comes in during the colder months of the year.  This one is from Robert who has a small pond that he keeps open year round.  He was somewhat surprised to find algae growing in the water, even with the pond being surrounding by snow and the temperatures are still quite cold.  He asks if this is at all normal?

Answer: As odd as winter algae may sound, it’s really not unusual for small pond owners who may choose to keep their pond open all winter long.  I remember as well that large ponds can be affected too.  It was very early on in my pond management work and I visited a pond in early March that was completely covered in ice.  However there was a very healthy algae bloom growing under the inch or two of ice.  It was quite apparent that there was ample sunlight getting through to stimulate it, and the pond had historically  had algae issues in the summer too so nutrient loading was quite high as well.

For small ponds it can still come down to an issue of nutrients and these, for the most part will run unchecked in cold weather.  The normal bacterial systems such as a biofilter and active bacteria will either be shut down or go dormant in really cold temperatures.  There are winter blends of bacteria but even the best of these will only work down to about 34 degrees and most “normal” bacteria will stop working below about 45 degrees.

The very healthy beneficial and desirable plants that you might use in summer time are also long gone from the pond, yet some sunlight and some degradation of organic elements in the pond can still continue on, albeit more slowly than in warm weather.

With all of these things combined, algae has a fairly free run and growing.  Algae can be a very hardy, adaptable plant and there is often some kind of species of it that will survive in almost any environment.  It’s one of the oldest plants known to be in existence today and something doesn’t live that long without having some unique qualities.  Simply put, algae can grow, when other things may not function well.

To ease some of the concern, it should be kept in mind that algae isn’t necessarily a bad thing in every single instance.  Fish may eat it from time to time and in small quantities it’s probably not anything to worry about.  Once everything is operational again, meaning the pond’s filtration system, and the addition of aquatic plants, the algae most likely will go away on it’s own.

If you find algae troublesome in the winter time, applying a bit of hydrogen peroxide (3% solution) or a hydrogen peroxide based treatment like Algae-Off algaecide will usually kill it fairly quickly.  This is one of the best ways to treat any algae growing on rocks and will help to avoid having to srub it off.  If you find thicker string algae growing, it’s best to remove this by hand, or by mechanical extraction if possible.  A pond-safe algae rake would likely do the trick.

Winter time pond algae may seem out of place in a world of white and ice but in fact, it can feel right at home in nearly any condition or environment.  Fortunately there are answers to remedy a winter algae bloom if it’s necessary, and with a bit of patience, you may find the problem to be short-lived as the seasons begin to change.

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