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What is Blanket Weed? And How to Remove It (Video Included)

15th March

What is Blanket Weed?

Blanket weed is a type of algae, scientifically called filamentous algae. It’s also commonly known as string algae or silk weed. Pond owners often struggle to identify blanket weed because it occurs in different forms.

Typically, it appears either floating on top of or submerged within pond water as clumps of thick green, hair-like strands. These dense clumps have no leaves or stems but will attach themselves to pond walls.

Ornamental ponds with high sunlight exposure and organic nutrients create the perfect environment for it to thrive.

Not sure if you have blanket weed?

Watch this video:

5 Pond Problems Caused by Blanket Weed


  • 1. Aesthetics. Blanket weed ruins the appearance of a pond.


  • 2. Not only does it make ponds unappealing, it’s often a sign of chemical imbalance in the pond. (Yes, even if the water looks clear!)


  • 3. It consumes large amounts of oxygen from pond water. This creates a harmful environment because aquatic plants and fish are forced to compete with it for oxygen. Especially in summer, when pond oxygen levels are low.


  • 4. Floating blanket weed deprives submerged oxygenating pond plants of sunlight. Preventing their process of photosynthesis and further reducing pond oxygen levels.


  • 5. Fish can get tangled in submerged string algae. When unable to escape, fish can potentially suffocate and die.

What causes blanket weed?

It has three main causes, excessive sunlight, a high pH and excess nutrients. Nutrients in pond water, caused by the build-up of organic matter, act as a food source for the blanket weed. While sunlight stimulates growth through the process of photosynthesis. Blanket weed, like other forms of algae, thrives in ponds that have a higher pH, typically anything above 8.5. These three factors are also the likely reason for any recurring problem.

How to Remove It

Pond owners often attempt to remove string algae with a pond net or by hand. However, this method alone is not advisable. Moving/pulling it while it’s alive releases spores into the pond water, that will cause more to grow in the future. To remove it from your pond, you will first need to kill it.

For removing floating algae, we recommend you use Blanketweed Klear. This is a granular treatment that you can apply directly on top of the blanket weed and works within 24 hours to kill it, meaning you can safely remove it after a day.

To remove submerged algae, we suggest you use Algae Klear Xtra. A liquid treatment, which effectively travels deep into the pond, to where the submerged string algae has anchored itself to the pond walls, pumps and other plants. Algae Klear Xtra breaks down the proteins found in the algae to completely kill it, down to the root.

Related Products

  • Front view of Envii Blanketweed Klear blanket weed killer

    Envii Blanketweed Klear

    Kills surface floating blanket weed within 24 hours.
    Add to Basket
  • front view of Envii Algae Klear Xtra eliminates submerged algae

    Envii Algae Klear Xtra

    Kills submerged algae and blanket weed.
    Add to Basket

How to Prevent It

Simple tips to help you prevent blanket weed from returning in the future:

  • Fill your pond with aquatic plants that will absorb excess nutrients in the pond.
  • Keep an appropriate stock of fish to suit the size of your pond.
  • If and where possible, provide your pond with shade.
  • Check your water quality regularly with a test kit to make sure your pH, KH and GH levels are balanced. If you find that these parameters are out of the ideal ranges, use Pond Equaliser to instantly stabilise them and buffer them for up to 6 months.

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