Guide to Pond Blanketweed

This is my guide to pond blanketweed which is very common and it's likely that most experienced pond keepers will have come across this unsightly algae before.

Created by Andy Hobson (Alumni) on Tuesday, 22nd of April, 2014.
Updated on Wednesday, 1st of February, 2017.

Blanketweed removal using a Bermuda Blanketweed Brush.

Blanketweed is a hair like algae with long fibres, it rapidly grows and spreads in ponds with crystal clear water becoming choked with what can only be described as green gunk.

Found growing under or on top of the water surface, blanketweed will also cling to the sides of the pond. The problem is not only aesthetic, as blanketweed removes oxygen from the water and can be detrimental to any pond life


A combination of warm water, sunlight, and nutrients from decomposing organic matter, provide the ideal conditions for the growth of algae while poor filtration and shallow water make matters worse.


Ponds which are constantly in sunlight with no shading are likely to suffer massively. Sunlight will feed the algae just like regular plants and this can be a major problem as blanketweed can grow anywhere in your pond and has lots of surface area.

Fish Waste

Fish stocking levels can affect algae in your pond and high amounts of fish with an inadequate filter means lots of fish waste in the pond, this feeds the blanketweed and allows it to thrive.

Sludge and Silt

A build up of sludge and silt in your pond should also be avoided. It is caused by uneaten fish foods, fish waste, leaves and other debris sitting at the bottom of the pond and rotting. This rotting process releases nutrients that feed algae.

Tap Water

One of the most common causes of blanketweed and other algae is too much tap water being put into the pond. The chlorine contained in tap water can upset the balance of your pond, killing vital bacteria and feeding algae blooms at the same time.


Preventative measures can be taken. Your pond may be positioned where leaves collect in the autumn, so either relocate the pond or use netting to prevent them entering the water.

Shade is important, make sure that plants such as water lilies cover 50% of the water surface, but don't forget to collect the dying foliage of aquatic plants before it falls to the bottom.

Ideally the water should be 3- 4ft deep and not overstocked with fish. To prevent the build up of nutrients, make sure that filtration is adequate and well maintained.

Don't add soil into the pond unless using aquatic planting baskets as the fertilisers in soil can feed algae and cause blooms. If you vacuum your pond regularly this will also prevent a build up of sludge at the bottom. Any soil which has come out of baskets will be removed during this process too.


GardenSite sell a wide range of Blanket Weed Treatments and Remedies, but first you have to clear as much blanket weed as you can using a net or a stick. 

Barley straw is a proven remedy and is best placed at the base of a waterfall or fountain around Easter time. You'll need approximately 2oz of straw to 10sq ft of surface area (50gms for every sq metre) and, as it breaks down, a chemical is released that inhibits the growth of the algae.

Pre-packed straw is available, alternatively fill some net sacking or even an old pair of tights and weigh them down with a stone. The effect can take over a month to begin, and the straw will need to be replaced every six months. If your pond is quite small you may be able to use barley straw extract.

A product that's received great customer reviews is Cloverleaf Blanket Answer. It's a natural enzyme and won't take oxygen out of the water, making it safe to use even during very hot summer weather. It's also completely plant and lily safe, and the treatment will stay in the pond for a period of time consuming any new strains of blanketweed that form.

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