Blanket Weed (Spirogyra Adnate), the nemesis of all amateur and professional pond and fish keepers alike in the UK and around the world! spirogyra adnate is one of the most prolific aquatic weeds you could possibly hope to come accross. Know as 'long string algae' in the United States, because in fact spirogyra is not actually a weed or plant at all, but a filamentous algae.

There are several other so called 'aquatic weeds'  which are filamentous algae, such as rhizoclonium, vaucheris, cladophera, entermorphia and ulothrix. Ulothrix is a chlorphyte like spirogyra and looks almost identical. If that wasn't enough there are over 400 known species of spirogyra, which is more commonly know as 'water silk', 'silk weed' or 'tangled pond scum' and over 20 species of rhizoclonium but to name a few.

So what is it?

Blanket weed is a long thin filamentous algae which thrives in garden and ornamental fish ponds. It's physical composition contains helical shaped chloroplasts which give that unslightly green appearance associated with blanket weeds and other forms of algae. Growth is stimulated by bright sunlight, a process known as photosynthesis, and it's preferred environment is shallow, fast running water hence it is usually found in waterfalls, pond returns, and un-shaped parts of the pond itself. In ornamental fish ponds the waste from our fish and the by-products of our filter systems, i.e. dissolved ammonia and nitrate provide the spirogyra with an abundance of nutrients to suppliment the photosynthesis, which accelerates the growth rate even more. It's not uncommon for it achieve growth rates 2 metres per day with bright sunlight and the right nutrient levels.

During the night the process of photosynthesis is reversed; these filamentous algae normally suspended in the water by tiny oxygen bubbles begin to slowly sink. They then begin to absorb oxygen from the water for cellular respiration.The process depletes the oxygen levels in the water and increases the the carbon dioxide levels which in turn can lead to sudden changes in pH levels, from which stress and even death to your fish can occur. This is in addition to the stress being endured by larger fish as they struggle to swim through it.


Unlike plankton algae, a singular celled algae, which can be eradicated in garden and ornamental ponds by simply installing ultraviolet clarifiers or sterilizers, (which have proven very effective at agglomerating the single cells into clumps for removal by the filter system) ultraviolet lights have little or no effect on filamentous or macrophytic algae. Other methods of control are required. The department of the environment effectively banned the use of chemical weed killers to remove blanket weed, and in any case none of us would want to put such chemicals in our ponds. The preferred option would be to introduce wildlife and plants into the eco system that would feed on and compete for food with the blanket weed. This is also not really an option for fish keepers. That leaves the bacterial approach and electric devices that will feed on or destroy the spores from which blanket weed grows. Just removing blanket weed from the pond can have an adverse effect as the spores are released into the water and the process starts all over again more vigorously.

Bacterial Treatments:

There are many good products for eradicating blanket weed. Bacterial treatments work by introducing bacteria from the strains belonging to the the azotobacter, bacillus, pseudomonas and chaetomium genra. The component strains include both aerobic and faculative organisms. These safe non-pathogenic bacteria compete with the filamentous algae for any waste products like dissolved ammonias, nitrate, fats, oils, grease, proteins, sugars, starch, cellulose and phosphates. This deprives the filamentous algae of a food source forcing it into recession, thus making the water clearer. Please note that this is not an immediate solution, it can take time for the bacteria level to establish and the equilibrium of in the pond to stablilise. It must also be mentioned that bacteria treatments are not as effective against singular celled algae and if this is the preferred option bacterial treatmnents should be used in conjunction with an ultraviolet UV filter.

Blanket Weed Treatments