How to Keep Outdoor Ponds From Getting Green With Algae


Thick mats of algae ruin the look of a garden pond and can cause serious problems for fish and plant life. Two main types of algae grow in outdoor ponds. Planktonic algae are suspended in water and provides food for tiny organisms important in the pond food chain. However, this type of algae can bloom in population, causing water to turn green. Filamentous algae is another algae type that creates slimy green strands on the water's surface. The latter tends to be more straightforward to control.

Step 1

Scoop up some water and matter from the pond surface, using a pool-skimming net or rake. If green, slimy strands come away with the net, the pond is likely suffering from filamentous algae. Regularly scoop up as much as possible to keep the algae under control. Use it as garden mulch or compost, according to Ohio State University.

Step 2

Grow a strip of grass and plants around your pond. This helps absorb any nutrients before they wash into the pond. Sudden floods of nutrients can result in green algae blooms.

Step 3

Remove any dead fish floating in the water. Keep livestock and pets away from the water to avoid any excrement or manure runoff entering the water, according to Cornell University.

Step 4

Nail a white plastic lid or object to a yardstick. Push the white object into the pond. Note the point at which the lid disappears. At 24 inches you should see the white lid in healthy ponds, according to West Virginia University. If you can't see it at that depth, chances are that a planktonic algae bloom is taking place. A serious bloom may require chemical algaecide treatment and further advice from a specialist. Target early signs with bales of barley straw.

Step 5

Fill an onion bag with barley straw. You'll need 225 lbs. of barley for every acre of pond, according to the Environmental Protection Agency.

Step 6

Add the barley bags to the pond in March or April before large blooms develop, if possible. Leave the bags in shallower pond areas, away from the deepest parts. Tie or anchor the bags to stop them from drifting.

Step 7

Run your pond pumps and filters to maintain oxygen levels in the water. Barley requires oxygen to help decompose the algae.

Step 8

Replace the bags with fresh barley after six months, according to Purdue University.

Tips and Warnings

  • Check with your local Environmental Protection Agency division for rules on use of herbicides such as algaecide and barley straw. If your pond has any public access, you'll need additional permits to treat the algae with chemicals.

Things You'll Need

  • Pool skimmer or rake
  • Barley straw bale
  • Onion bag
  • String
  • Algaecide (optional)


  • Purdue University: Barley Straw for Algae Control
  • Ohio State University Extension Fact Sheet: Planktonic Algae in Ponds
  • Ohio State University Extension: Controlling Filamentous Algae in Ponds
  • West Virginia University: Pond Scum Wipe-Out--Control of Nuisance Algae in Ponds
  • Cornell University: How to Prevent and Control Algae in Farm Ponds
Keywords: prevent pond algae, pond algae blooms, prevent algae blooms, stop pond algae, stop green algae

About this Author

Based near London, U.K., Peter Mitchell has been a journalist and copywriter for over eight years. Credits include stories for "The Guardian" and the BBC. Mitchell is an experienced player and coach for basketball and soccer teams, and has written articles on nutrition, health and fitness. He has a First Class Bachelor of Arts (Hons.) from Bristol University.