Blanket Weed Algae In Ponds

A blanket weed is a type of algae with a long, stringy filament body. The body floats at or near the surface, covering water with a green blanket. It will choke out all the nutrients, sunlight and oxygen in a pond. Once blanket weed becomes established, it is very difficult to remove, and the best course of action is blanket weed prevention.
These algae do not cause the green colour that is at times seen in garden pond water; the discoloration of ponds is caused by planktonic algae. Individual cells of the planktonic algae float in ponds and cause the green water. Blanket Weed Algae, on the other hand, is made up of strands of cells that form rather long strings. People who look after ponds refer to it by a number of names, including filamentous algae, long string algae, and others.

Regarding how blanket weed algae enters ponds, its spores can get into ponds via rain, tap water, or wind. Parts of this algae can also enter ponds attached to plants, including iris and water lilies. It spreads around a lot when the conditions are optimal for it to do so. Namely, for this algae to grow in a pond, high levels of nutrients, direct sunlight, and more need to be prevalent on a consistent basis. This algae also tends to form on waterfalls.

While blanket weed is unpleasant to look at, it does in fact have some good qualities that need to be noted. In particular, this type of algae keeps out the planktonic algae. This is since it consumes the same nutrients that would be taken in by the planktonic algae. Given this dynamic, it supports a vegetative type of filtration for the pond’s water. This kind of algae also serves as an excellent fish food.

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Despite some of the good qualities of algae that is filamentous, it can most certainly cause problems if it gets out of control. It can suffocate small fish, and it can also strangle good plants in the pond.

Some of the things that can be done to rid a pond of filamentous-type algae is to harvest it with a rake or a stick. Shading ponds or adding more plants to them that will compete with the filamentous-type algae for nutrients is also rather helpful. Water lilies can accomplish both of these goals, though fifty percent of the pond would need to be covered in order for the plant to have a noticeable effect. Water hyacinth are also able to effectively remove nutrients that would otherwise be consumed by the filamentous pests.


In conclusion, algae that are filamentous can be a nuisance to ponds. While they have some good qualities, filamentous strands of algae can strangle small fish and good plants if they get out of control. With this said, to control the growth of filamentous algae in a pond, a person can shade the pond or fill it with plants that will compete with the nutrients that would otherwise be consumed by the filamentous pieces of algae.

1 thought on “Blanket Weed Algae In Ponds”

  1. Patrick Bailey

    My new one-year-old pond was nicely balanced, early algae had come under control, and oxygenating plants were flourishing. But almost all my new water plants had become badly potbound, so I planted them up, adding small amounts of clay soil from the garden (as I couldn’t find a source of pond clay for planting up). Since then, blanket algae has been a real problem, and is interwoven with the oxygenators. I remove large amounts every couple of weeks, but it keeps building up again. I’m pretty sure it is the nutrients from my re-potting (following instructions from an online video) that have caused the problem. Do I just stick it out, and the blanket algae will eventually die out, or do I need to remove all the plants and start again (hope not)?

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