Q My pond has stringy algae and water like pea soup. I keep cleaning it out, but this doesn’t help. What’s going on?

A It sounds like you have a problem with blanketweed, an algae that grows in long strands. Pea-soup water points to another alga that remains suspended in the water. Blanketweed can be removed by winding it out on a stick or rake. Leave it on the edge for two days, so that small creatures can return to the pond, then compost it.

To deter regrowth, and help to clear the pea-soup algae, you need to reduce the nutrients and light available. Add oxygenating plants and marginals to mop up nutrients, and floating plants to increase shade; aim to cover about half of the pond surface. Avoid fertiliser run-off from lawns, don’t overfeed fish and use only aquatic loam as a planting medium. Do not wash out the pond or change the water, as it takes about two years for the pond to settle into its own natural balance. 

Caption: Physically removing blanketweed helps reduce the problem

Q Are there any effective treatments?

A In a Which? Gardening trial, we had success with Nishikoi Clear Waters, Hydra-Aqua Hydra Lakeshade and King British Barley Straw Extract.

Q  What about barley straw?

A Adding a bale of barley straw to your pond is a traditional way of getting rid of blanket weed. As the straw rots, it releases chemicals and encourages fungus and bacteria that kill off the blanket weed. However, it can take a long time to work if there is no flow of water or aeration in the pond. And if you have a small pond where the straw is visible, it can be unsightly. We would suggest you use one of our recommended treatments instead.