How to Prevent a Green Pond This Summer

How to Prevent a Green Pond

Have you ever secretly judged a pond that was overtaken by “green,” closer resembling a bowl of pea soup than a pond? Told yourself my pond will NEVER look like that, but got an unwelcome surprise one particularly scorching summer day?  The invasion started slowly.  You noticed your water clarity was off and you thought saw little bit of green in your pond, but shook it off.  Algae?  Not in my pond!  Over the next few days you tried to tell yourself it wasn’t happening, but one morning you couldn’t deny the facts any longer: your pond was under attack!  Your pond that had been crystal clear all spring was overtaken by thick green algae.  You couldn’t see any of your fish and there was a distinct smell.  Your pond looked like that very pond you had scoffed at.  Algae can be a tough battle especially in the warm summertime, but with a little extra effort you can help prevent your pond from turning green.


What is the “green” exactly?

The more common algae that gives the “pea soup” appearance is a single cell organism called planktonic or free floating algae.  It feeds on nutrients like nitrates and phosphorous, and uses sunlight to photosynthesize.  Direct sunlight is vital to algae, which is one of the reasons blooms tend to occur during the summertime.  Blooms are when the algae quickly multiply and “take over” a pond.


Algae is not the prettiest sight and can actually be harmful to your plants and pond life.  At night and on cloudy days, the algae start using oxygen in the water.  During an algae bloom, the warm water (which already holds less dissolved oxygen than colder water) can become less and less oxygenated and eventually hurt pond life.


Aerobic bacteria (oxygen-needing) quickly break down organic material into harmless compounds.  Anaerobic bacteria (does not need oxygen) work much slower and breakdown the organic matter into nutrients that fuel algae and harmful gases like hydrogen sulfide, which makes the pond smell like rotten eggs.  As the algae deplete the oxygen levels in the pond, the aerobic bacteria die off, leaving the anaerobic bacteria to decompose the debris and sludge in the pond.  Their byproducts create more nutrients for the algae, fueling the bloom.


What causes algae?

Algae need direct sunlight and excess nutrients to flourish.  A still pond, one without fountains, waterfalls, or another source of aeration, allows algae to float on the surface and receive all of the sunlight it needs.  When the water is moving, the algae aren’t able to get as much direct sunlight.  


Excess nutrients, particularly nitrogen and phosphorous, feed algae (commonly found in fertilizers).  When it rains, fertilizers and other runoff can make its way into your pond and create more nutrients than the good bacteria can breakdown.  Nutrients can come from other sources like leaves, lawn clippings, and decaying plant matter.  If you have too many fish, the pond can become overcrowded, creating too much waste.  Overfeeding your fish (giving them more food than they can consume during feeding time) can also contribute to the problem.


How can you prevent algae blooms?

You can maintain a natural pond and prevent algae blooms.  If you can work on prevention before the pond is full-on green, you can avoid using any harsh chemicals to treat the water.


  • Aeration: Aeration is extremely important to help keep the water oxygenated year round.  It will also keep the good aerobic bacteria active during the summertime to help breakdown the nutrients and debris.  You can use a fountain, waterfall, or an aerator to keep the surface water constantly moving.
  • Filtration:  There are three types of filtration systems: mechanical, biological, and UV clarifiers.  Mechanical filtration removes smaller debris by passing the water through a filter pad.  Biological filters utilize good bacteria to help break down nutrients and debris.  UV clarifiers help to break down the algae at a microscopic level using ultraviolet (UV) light.
  • Aquatic plants: Many pond plants are dual-purpose decorations.  Floating plants like lilies, and water lettuce create shade and prevent algae from getting the sunlight it needs.  Marginals like iris and pickerelweed help soak up the extra nutrients in the water.  Also, submerged plants like cabomba and hornwort absorb carbon dioxide and expel oxygen for improved aeration.
  • Minimize debris: Adding a pond skimmer can help remove surface debris from the water before it sinks to the bottom of the pond, where is will begin to decay.  You can also manually scoop out leaves and other debris daily, before it begins to sink and decay.
  • Water quality tests: Checking water quality will help you detect any issues before they get out of control.  You can check for excess nutrients like phosphates or nitrates, and see how your oxygen levels are.  You need to find the source of the excess nutrients so the algae doesn’t keep coming back.
  • Water treatments: Water treatments like beneficial bacteria, sludge remover, and pond tint can help prevent algae from taking over your pond.  Read the smartpond® guide to water treatments to learn about each treatment and how it can help.


Everyone wants a green yard, but no one wants a green pond.  Algae is a greater issue during the warm summer months where the sun shines brightly all day long.  Sitting under an umbrella next to your crystal clear pond with a good book and a glass of iced tea is a great way to spend a lazy summer day.  It’s hard to enjoy your day next to your smelly green pond, covered with algae so thick you can’t see your beloved fish.  There are many all natural actions you can take to help prevent having a pea-soup pond this summer, so you can enjoy your clear water all season!


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