Blanketweed and Green Scum?

Frothy green scum is common on the surface of ponds in late spring

What Can I Do About Blanketweed Algae    and Green Scum?

The long green strands of blanketweed (also known as string algae, thread algae, and silkweed) can cling to pool sides and waterfall lips, and entangle submerged plants and wildlife. Blanketweed-type algae can also grow on the nutrient rich sediments on the pool base, frothing up and billowing to the surface as thick green scum - especially in hot sunny weather in the late spring, before lily leaves are providing much cover .

There are many different types. Some are soft and silky, others strong and tough, and a few are gloopy and gelatinous.

Just like green pea-soup algae (see Help! My pond has gone green), blanketweed is natural, and only on rare occasions harmful to fish or wildlife.

Why is it in my pool?

The spores of blanketweed can blow in on the wind, or traces may come in on plants or on birds’ feet. It tends to become a problem where the nutrient levels in the water or sediments are high. In the Cotswold area the soft limestones also leach nutrient minerals. Nutrient rich water is common in new and recently refilled ponds where mineral rich tapwater has been used (see our article on phosphates), or where fresh planting soil or fertilizers have been used. In these cases the problem may subside after a couple of years. It often becomes a worse problem where large amounts of fish food are fed; where soil washes into the pool from the surrounding garden/lawn; in shallower ponds; or where a pool leak requires large amounts of tap water to be regularly added to the pond. It is less of a problem in well-balanced, established ponds.©

A seasonal issue

A little blanketweed is to be expected, especially in the late spring when warm sunshine encourages its growth. At this time of year it is still too early for much competition or shade from water plants. Net or rake out unsightly blanketweed (use a soft plastic rake to avoid liner damage, see below). It is easier to rake out during the afternoon, when more of the weed is floating. Very occasionally certain types of pond blanketweed (the ‘blue-green’ species) can have some toxicity. To be on the safe side, handle using rubber gloves, and make sure that pet dogs do not come into contact with the scums.  www.watergardensolutions.co.uk

Avoiding Blanketweed and Scums

Some blanketweed is inevitable in most new ponds, but tends to subside as a natural balance is achieved. Ensure that nutrients/soil/leaves and debris are not washing into the pool. Where fish are present and being fed, consider using less weight of food and changing to a higher-quality floating-pellet fish food. Having ample plants of various types (submerged, lily-like, marginal, floating) can also help, as these compete for the nutrients and help to shade out the algae. Topping up with clean rainwater rather than tapwater can help.


For persistent blanketweed, we have altered the management routine of the pond or used various chemical and biological treatments to reduce the problem. The most appropriate treatments will depend on the type of pond and the sorts of plants and fish stocked in it. Your local specialist pond store might be able to advise on available brands, and we can visit local customers to test their pond water and advise on treatments.

Some of the stronger chemical treatments contain herbicides, oxidizing agents, or heavy-metals and can have unwanted side-effects so must be used with particular care (though the strongest aquatic herbicides have been banned for some years in Europe). Barley-straw treatments work on some types of algae, but not all. Certain treatments aim to reduce nutrient availability. In deeper ponds, shading dyes can restrict the full spectrum of light from penetrating the water. Most treatments need repeating. Specialist electromagnetic devices can sometimes weaken the strands, resulting in shorter growths of blanketweed. In pools without plants or fish it is possible to use dosing devices that release a constant trickle of algicide.

Ultra-Violet units are effective for green water control but have no effect on established blanketweed. Indeed, in crystal-clear filtered ponds there is no green or cloudy water to compete with the blanketweed algae, allowing the strands to grow at deeper levels in the water.

If you are having persistent blanketweed problems Contact Us to arrange a visit. We can test the water to see which nutrients are part of the problem, and offer advice on management and treatment options.

removing blanketweed with a plastic rake_4636  ( aquapic)

As the blanketweed drains, pondlife (like this newt) has a chance to wriggle back into the pond. Lift off the blanketweed after a few hours and rinse any pondlife back into the water. The waste blanketweed can be composted. Don’t leave it more than a day at the pondside in summer, otherwise it will start to decompose and release waste back into the pond.

Use a plastic lawn rake like a scoop, to help pick out blanketweed from the pond whilst reducing risk to pond liners. Rakes block more slowly than nets, and are easier to empty. Rest the blanketweed at the pool side to drain...

a newt returning to the pond  ( aquapic)

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