How to control string algae

Written by kent page mcgroarty
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How to control string algae
Algae can really take over a pond. (abstract algae image by maxthewildcat from

String algae, also known as pond scum, blanket algae and pond moss, is a filamentous algae floating on the surface of ponds. It bonds with other filamentous algae to create strings and lime green mats that can take over pond surfaces. It can also grow in pond waterfalls, shallow streams, on the sides of ponds with the most sunlight, and on rocks. Barley straw is the best way to keep this kind of algae from invading your pond as it releases a chemical that controls the growth of algae. It works as a preventive method for such algae; it will not be effective on existing algae. Existing algae can be dealt with manually and by keeping the pH level of your pond under control.

Skill level:

Things you need

  • 1 bundle of barley for every 1000 gallons of water
  • Toilet bowl brush
  • Duct tape
  • Plastic or metal pole
  • Onion sacks or nylon stockings
  • String
  • Large rocks

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    String Algae Prevention

  1. 1

    Use one bundle of barley straw for every 1,000 gallons of water in your pond. Barley straw is non-toxic and will not harm fish or other plant life. Spread the bales apart and place them in onion sacks or nylon stockings.

    How to control string algae
    Barley straw will work wonders at preventing algae. (gerstentopf image by Ewe Degiampietro from
  2. 2

    Anchor the sacks in your pond with large rocks. Do this by tying a string that connects the sack and anchor. Since barley consumes the oxygen in your pond place the sacks underneath or near any fountains or waterfalls where water is moving frequently. This will keep oxygen around the straw and helps spread the chemicals the barley straw creates as it decomposes.

    How to control string algae
    Use large rocks to keep the sacks in place. (large boulders image by Yali Shi from
  3. 3

    Make sure the sacks float in the upper 3 feet of the pond water so they get the most sunlight. The barley straw will control the algae for about 4 to 6 months, after which they need to be replaced. Effects occur in 2 weeks if pond water is 21.1 degrees C or over and can take up to 2 months if water is 10 degrees C or below.

    How to control string algae
    Keep sacks near the surface of your pond. (purple water lilly in a water pond image by Jorge Moro from

    Removing Existing String Algae

  1. 1

    Check the pH balance of your pond. String algae can thrive in water with a high pH balance, so balance must be lowered to 7.0 to 8.0 to control the growth of algae.

    How to control string algae
    Check the pH balance of your pond water. (algae on the riverside image by McDanny from
  2. 2

    Attach a toilet brush with duct tape to a thin plastic or metal pole.

    How to control string algae
    Use duct or other heavy tape to secure the brush to the pole. (plastic tape image by timur1970 from
  3. 3

    Use the brush to wind the string algae up and around the brush. This is most effective when dealing with smaller ponds or small amounts of algae. Manually removing string algae will also remove the nutrients that caused the algae to begin with.

    How to control string algae
    Toilet brushes are very effective for reducing the amount of algae in your pond. (scrub brush image by weim from

Tips and warnings

  • The best time to put barley straw in your pond is early spring. This will give it a chance to break down and prevent algae before the stuff starts to appear. Do not wait until all straw has been broken down to replace it. Overlapping new straw with old straw means new straw will already be breaking down when it is time to remove old straw.
  • If you decide to use anti-algae chemicals in conjunction with the straw make sure such chemicals will not harm your fish, other water plants, or you.

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