Pond algae and blanket weed

Algae can be a major problem in ponds, causing discoloured water, green scum at the pond edges, or dense mats of green growth under the surface. If conditions are favourable, algae will spread quickly and can harm aquatic life. 

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blanket weed spreads quickly in summer.

Quick facts

Common name Algae
Botanical name Chlamydomonas, Chlorella & Euglena sps. (single celled algae); Cladophora sps. (filamentous algae)
Main causes Excess organic debris, fertiliser run-off in the pond; newly filled ponds; very sunny sites
Timing Tends to be worse in spring and summer, during sunny weather

What is pond algae?

There are hundreds of different types of algae that can cause a problem in ponds, from those that are suspended in water to others that spread across the surface in long fibres. Apart from looking unsightly, algae can lead to the deoxygenation of water with a detrimental effect on pond life. This page looks at options for gardeners when pond algae is becoming a problem. 


There are many different symptoms, depending on the algae present. These are the most common symptoms:

  • A ‘pea soup’ effect caused by fine algae suspended in water
  • Floating green scums
  • Blanket weed or silkweed, are very common pond algae and have dense growths of hair-like green strands that float under or on the surface, or cling to plants at the side of the pond

The problem

Most ponds will have an algal problem at some time or another, but it thrives where pools have been created in a sunny position, with very little planting to provide shade across the surface.

It also forms when there are too many nutrients in the water. This could be due to:

  • A build up of organic matter (sludge) on the bottom
  • Fertilisers leaching into the pond
  • Common pond debris (such as fish faeces, fallen leaves and invasive pond weeds) can also add to the nutrient level in the water
  • Filling or topping up the pond with tap water; ideally use rainwater


The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner. 

New ponds

If you are creating a pond from scratch, site it with at least part of the pond in slight shade and aim to plant up at least one third of the surface with aquatic plants, while ensuring there are also sufficient submerged oxygenators.

Ideally there should be a minimum pool depth of 60-75cm (2-2.5ft) to prevent water warming up rapidly in sunshine, which speeds up algal growth. Pond liner colour is likely to be of limited significance when it comes to reducing algae; a pale liner might reduce temperatures but increase photosynthesis and thus algal growth, whereas a dark liner may decrease light reflectance but increase water temperature which is known to promote algae.

Established ponds

Where blanket weed and algae are already a problem in an established pond, consider the following;

  • Blanket weed and other floating algae are easily removed by twirling them out of the pond with a cane but this will only be a temporary solution
  • All pond algae can sometimes be discouraged by floating a mesh bag of barley straw on the pond – about 50g of straw per sq m (1½ oz per sq yd) of water surface area is ideal. Add the straw in spring and remove it when it has turned black (usually about six months later). Sometimes lavender is added to the barley straw for additional effectiveness
  • Various treatments have been promoted by manufacturers in recent years. Some appear to be effective only in settling suspended debris, others may give short-term control of free-floating algae. Some have been claimed to be effective in controlling the most troublesome types, such as the filamentous algae or blanket weeds. There are, however, several different groups of algae collectively called blanket weed. These individual treatments only affect certain algae, so you might have to try several products to find one that works in your pond. Treatment needs to be repeated at intervals
  • Do not be tempted to change the water in the pond, as it will bring only short-term relief, the problem becoming worse subsequently
  • These methods should all be treated as quick fixes, and the conditions that caused the algae to grow in the first place should be improved

For algae and aquatic weed control options in large ponds and lakes, professional help may be required. Suitably qualified operatives can be obtained using the Find a Contractor section of the National Association of Agricultural Contractors website or Complete Weed Control. There are currently no chemical control treatments available to amateurs.

Pond dyes

Dyofix Pond Black pond dye claims to help prevent algae. It works by colouring the water black and reflecting ultra violet rays away from the surface of the pond so that the algae cannot photosynthesise. Surface plants such as lilies and reeds will still benefit from the sunlight. Once applied, Pond Black will last for several months, and can be used all year round. Pond Blue is paler than Pond Black, enabling fish and other aquatic life to be seen beneath the surface of the water.

Large ponds and ultrasound

Trials at the Centre for Ecology and Hydrology found promising results in the use of ultrasound to control pond algae. A device in the water emits ultrasound waves and works by shaking apart the algae cell wall, leading to algae death within 7-21 days.

There is some evidence that it can even be tuned to different frequencies to target different types of algae, though currently it does not appear to be effective on free-floating algae (e.g. Chlamydomonas and Euglena blooms). There appears to be no adverse effect on pond wildlife but more research is being conducted in this area.

The equipment is not cheap to buy, but may be worth considering for larger ponds or hired for shorter periods where other methods of control have failed. Professional installation is needed for it to be effective. The device is left permanently in the water and runs on electricity or solar power.

Algae control options for larger ponds including ultrasound: WaterLand Management.


Aim to reduce the amount of nutrients entering the pond by the following:

  • Only introduce plants to the water that are growing in low-nutrient, aquatic compost
  • Avoid putting soil in the pond and remove any soil that has collected in the base of a new pond prior to filling with water
  • Apply any fertilisers to lawns carefully to avoid waste running into the pond
  • Place a net over ponds to prevent leaves falling in
  • Use rainwater, if possible, to top up levels

Other options

  • Prevent stagnant water by having a fountain or water feature that will help aerate the water and prevent the build-up of blue-green algae. However, it is unlikely to have any effect on other types of algae
  • Pond filters and ultraviolet clarifiers are useful for removing algae and other debris from the water

Fish and pond snails

  • Algae will be less of a problem where fish are not introduced to the pond. If fish are to be added, the best pool fish are goldfish, orfe and shubunkin. Carp and tench feed in mud and debris at the bottom of pools and can cause clouding of the water. Add fish about three weeks after filling and planting a new pond
  • Keep fish populations in check by occasionally giving some away, or moving them to another garden pond, but never into the wild
  • Ramshorn snails can be introduced but avoid other water snails as they tend to be less efficient scavengers and, in particular, avoid those with long, pointed shells which have a tendency to eat the leaves of the water plants rather than scavenging

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