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 David Coton on Tuesday, 22nd of April, 2014.
Updated on Monday, 30th of April, 2018.


Blanketweed removal using a Bermuda Blanketweed Brush.

What is Blanketweed?

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 'slime'.

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

Causes of Blanketweed

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.

1. Sunlight

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.

2. 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.

3. 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.

4. 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.

Prevention of Blanketweed

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 the leaves from entering the water. If neither are practical or desirable may sure you remove the leaves before they have time to rot and break down.

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 too before they fall to the bottom of your pond.

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 to the pond unless using aquatic planting baskets as the fertilisers in the 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.

Control of Blanketweed

GardenSite sells 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 safe to use around water plants and lilies, 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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