How to Control String Algae

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 preventative 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.

String Algae Prevention

Algae can really take over a pond.
Barley straw will work wonders at preventing algae.

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.

Use large rocks to keep the sacks in place.

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.

Keep sacks near the surface of your pond.

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 70 degrees or over and can take up to 2 months if water is 50 degrees or below.

Removing Existing String Algae

Check the pH balance of your pond water.

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.

Use duct or other heavy tape to secure the brush to the pole.

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

Toilet brushes are very effective for reducing the amount of algae in your pond.

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.

Things You Will 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


  • 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.

About the Author

Kent Page McGroarty has worked as a writer since 2006, contributing numerous articles to various websites. She is a frequent contributor to the health and fitness sections of the online magazine EDGE Publications and holds a Bachelor of Arts in English from Saint Joseph's University.

Photo Credits

  • abstract algae image by maxthewildcat from Fotolia.com
  • gerstentopf image by Ewe Degiampietro from Fotolia.com
  • large boulders image by Yali Shi from Fotolia.com
  • purple water lilly in a water pond image by Jorge Moro from Fotolia.com
  • algae on the riverside image by McDanny from Fotolia.com
  • plastic tape image by timur1970 from Fotolia.com
  • scrub brush image by weim from Fotolia.com