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. Blanketweed-type algae can also grow on the nutrient rich sediments on the pool base, frothing up and billowing to the surface as 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 rarely 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. 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 has 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; 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 other water plants. Blanketweed is also common in the Cotswold area with its naturally mineral-rich spring-water. If it becomes unsightly, net or rake it out (with a soft plastic rake to avoid liner damage), and take steps to reduce the amount of nutrients getting into the water.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 shade out the algae.


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. Most treatments might need repeating. In pools without plants or fish it is possible to use dosing devices that release a constant trickle of algicide, or dyes that prevent light from penetrating the water. Various electromagnetic devices can sometimes weaken the strands, resulting in shorter growths of blanketweed. Certain treatments aim to reduce nutrient availability.

UV units are for green water control and 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 it to grow at deeper levels in the water.

If you are having persistent blanketweed problems why not 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.


See more Frequently Asked Questions

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