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.

Things You'll 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

String Algae Prevention

  • 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.

  • 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.

  • 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

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