Algae pond problems … How to rid your pond of blanketweed.

Blanketweed or string algae is a big concern for many pond keepers. Green water is another unsightly algae problem, making a pond resemble pea soup. Blanketweed is another type of algae that forms an unsightly and smothering ‘blanket’ across the pond surface and throughout the pond. Blanketweed floats because of the oxygen that is trapped within it buoys it up. Blanketweed can smother a pond in days in perfect summer conditions, causing pumps to block up and submerged plants to be choked. Blanketweed thrives in clear water ponds, that are typically fitted with a UV clarifier. Blanketweed is best controlled by addressing the cause – which are the combination of nutrients (nitrates and phosphates), warm pond water and bright sunlight.

Blanket weed is an alga, it is a relatively primitive form of plant life that is very successful at colonising and dominating an environment under favourable conditions.

Blanket weed spreads and recolonises vegetatively. Goodbye Blanket Weed & Blanc-Kit Excel are very effective in controlling blanket weed.

How to rid your pond of blanketweed. What is blanket weed?

Blanket weed spreads and recolonises vegetatively, by branching off and breaking off parts of its own structure which will then colonise new areas.

Biologically speaking, this can be a risky strategy, as if the environmental conditions change, it does not have extensive means of adapting to the changing environment. However, where the environment provides consistent conditions, (as in a koi pond), and the blanket weed is adapted to those conditions, it is a very successful strategy, allowing it to thrive and spread rapidly.

Blanc-Kit Excel is specially formulated to create a pond environment resistant to blanket weed without the use of harsh chemicals

Are there different types of blanket weed.

Blanket weed (also referred to as string-algae) is a collective term given to a number of very similar algae that both look and behave identically. The most common genera are Cladophora, Oedogonium and Spirogyra. Cladophora means ‘branched plant’ and when viewed under the microscope, it is possible to see the regular-branding filaments, each of which is divided by cross walls. Absorption of light and nutrients is very efficient in such small structures and so growth can be incredibly rapid. They reproduce both sexually (releasing gametes that unite and develop into new plants), and asexually (releasing small motile spores or simply smaller fragments that break off from the main body).

Does it pose any health risks to koi?

We don’t resent blanket weed’s grip on our ponds because it poses a direct threat to the health of our koi, as in fact, it can actually lead to improved water conditions. When there is a thin, beardlike covering on areas of the pond, koi will browse and graze on the soft, lush growth. However, koi find it less appealing when the beard has grown into lengths of weed several feet in length (hence its other names such as hair or thread algae). Blanketweed will also provide an excellent nursery, both feeding and protecting developing koi fry. If your pond also contains sterlets, blanketweed can prove to be a real hazard for these weak swimmers.

Quite surprisingly, blanketweed is beneficial to a pond in that it will very actively take up minerals and nutrients from the pond water (just like a vegetable filter), the only difference being that this one is in the pond! So vigorous is the growth and uptake of nutrients by blanketweed that should we find a way of confining it to a vegetable filter, it would be our number one plant choice. Unfortunately, like all other weeds, blanketweed does not know its right place and freely enters any koi pond, doing so at its own risk, as its presence is likely to be challenged.

Another redeeming feature is that blanketweed is a very effective oxygenating plant. Its fine filament structure and submerged position lend it to producing a ready supply of microscopic oxygen bubbles. So intense may be its aerating effect that in strong sunlight, rafts of blanketweed will rise up to the surface, buoyed up by the mass of oxygen bubbles caught within its filaments.

What does it need to grow?

Blanket weed is not some sort of aquatic curse that we all fall under for keeping koi. We only have ourselves to blame, because blanket weed will only grow where it finds suitable conditions (these conditions just happen to be similar to the conditions found in most koi ponds). In fact if we wanted to farm blanket weed, we would probably provide it with the same conditions in which it thrives – a koi pond!

The 3 factors that enable blanketweed to thrive.

Clearwater that sunlight can penetrate.

It is no coincidence that blanket weed problems have increased in line with the sale and use of UVCs. Mud ponds in which koi are farmed are characterised by their murky water, and even though they represent a nutrient-rich environment, will rarely suffer from blanket weed. Although blanketweed is present in mud ponds, it is out competed and shaded by a combination of the turbid conditions created by the suspended clay and the blooms of single-celled algae. In a filtered and clear koi pond, we have removed the clay and the algae from the equation, leaving ponds exposed and ripe for blanket weed attack and colonisation. The sunlight is required to fuel the process of photosynthesis which allows blanket weed to manufacture food for new tissue growth. By providing clear water conditions for koi we are leaving ourselves exposed to an unhindered blanket weed attack.


Algae will readily absorb nitrates and phosphates to satisfy their need for nitrogen and phosphorous as they grow. These are readily available in tap water or indirectly through fish metabolism. Wherever nutrients abound, so will this opportunistic algae, being the first to capitalise on ideal growth conditions.

By killing green water with UVCs, we are perpetuating the imbalance that Mother Nature is trying to fill. The nutrients will continue to accumulate, until an opportunistic algae (such as blanket weed) can take advantage of these conditions. If blanket weed was also susceptible to UVCs then it too, like green water would not be a problem in koi ponds. – Unfortunately it is not.


A warmer temperature will accelerate algae growth considerably and blanketweed growth will be greatest in the shallower areas such as cascades and waterfalls and along the pond perimeter. In winter, the cooler water (and shorter daylengths) prevent blanketweed from growing. Unfortunately, it will only die back, ready to thrive when suitable conditions return in spring.

Why can a pond that has been free of blanket weed suddenly succumb to it?

Even though blanket weed needs specific environmental factors to be in place for it to grow, and these factors are usually unavoidably provided in a koi pond, there are instances when blanket weed will not proliferate in a specific pond. This can be mystifying as blanket weed is like any other living organism in that it has specific requirements for growth, and will only grow under the correct conditions. So even when two ponds provide these conditions and only one is afflicted by a green plague, it is clear that other factors are coming into play. Blanket weed does tend to form tougher structure in more alkaline and calcium-rich water, while deeper, shaded ponds that take longer to heat up are less accommodating to blanket weed. What can be even more puzzling is when you have (rather smugly) managed to keep your pond blanket weed-free for years, only for it to succumb this year. If this happens, try to retrace your steps and look at any of your pond keeping practices that may have changed your water chemistry. Different pond additives, treatments, water sources and food can be the most likely causes of change to a pond that will lead to a blanket weed to bloom.

Does it affect any particular area of a pond?

For the reasons discussed earlier, blanket weed growth can be considerably greater in shallow areas of the pond, particularly waterfalls. Blanket weed will thrive in fast-flowing shallow water, benefiting from the higher light intensity and warmth of water temperatures. Water flow will also ‘tease out’ the blanket weed encouraging it to grow in greater lengths.

How can you prevent your pond from getting blanket weed?

This is the million-dollar question. For reasons discussed earlier, blanket weed growth is affected and controlled by a number of factors. Blanketweed will find your pond. It is adapted to finding and colonising new environments – so why should your pond be the exception?

The answer to controlling blanketweed lies in reducing one of their 3 key requirements; sunlight, nutrients, and a suitable temperature for blanketweed growth. As we want our ponds to be as warm as possible (to stimulate koi health and growth), we should look at reducing sunlight and dissolved nutrients.

Sunlight. Sunlight penetration can be reduced in a number of ways.

Shading. Erecting shading on a pergola will reduce sunlight straight away and reduce blanketweed photosynthesis. It can also help against heron predation.

Adding dyes. Several blanketweed and algae controls work by adding dark vegetable dyes to the pond, filtering out the sun’s rays. This will give the water a tint, and will need to be topped up when the natural dyes are broken down by the filter, but proves effective as a long-term control of blanketweed.

Compelling natural evidence that shading works is evident when a pond suffers from green water. The microscopic single celled algae that turn a pond into a ‘pea soup’ out compete and shade blanketweed out of its valuable light. Blanketweed and green water have a mutually exclusive relationship, where ponds tend to suffer from either one or the other. Unfortunately, one of the side effects of installing a UVc (which is a guaranteed method of clearing green water), is that blanketweed will proliferate unhindered in the crystal clear, nutrient rich pond water.

Nutrients. Several pond treatments are available that control blanketweed growth by locking up or removing the vital nutrients from the pond water, starving the growth of blanketweed. Upon adding to the pond, they will bind up nitrates and phosphates. Other additives will act indirectly, but achieve the same ends using micro-organisms rather than chemicals to ultimately reduce the nutrient levels in pond water.

Other methods of control.


While all other methods simply control algae growth, the addition of algicides (chemicals that kill algae), work by interfering with vital biological processes. These products are the only ones on the market able to clearly state they kill algae – all others control or reduce it.

Barley straw

Barley straw is a green method of controlling blanketweed and green water. Upon its degradation, which can take several weeks, a cocktail of humic acids are released which react to release hydrogen peroxide, reducing algal growth. To speed this ‘natural’ process up, barley straw extract is now available.

Electronic controllers.

This method is reported to work on most ponds that have a suitable water chemistry by interfering with calcium ions. This apparently upsets algae metabolism, reducing blanketweed growth.

Top tips for blanketweed control

Adopt the strategy of prevention is better than cure. If you continue to treat blanketweed (which is a symptom of an unbalanced pond) then it will always return, once the treatment has worn off.

Try to determine the factors that are the problem in your own pond:

Is it the high levels of nutrients? – Test for nitrates and phosphates.

Is it excessive sunlight?

Once you have assessed the dominant factor that is making your pond hospitable to blanketweed, act accordingly:

If the nutrients are high, identify the source(s) of the nitrates and phosphates. (is it tap water?)

Also, for back-up, use nutrient-removing remedies (and re-test your water to see what effect they are having on your pond’s nutrient levels).

If your pond is completely plant-free, but suffers from blanketweed, feel free to use an algicide (the only type of product that can claim it kills blanketweed) as there is no risk of affecting other aquatic plant life.

If you prefer a completely natural remedy to blanketweed, use those that offer a ‘greener’ remedy.

Blanc-Kit Excel uses natural plant extracts

Viresco uses nutrient digesting bacteria

Barley straw (in its many forms) uses natural by-products as it breaks down

What’s available? Method Cost How it works

Barley Straw (Chopped, palletised, liquid extract) From 1.99 for 500 gallons for 12 months The breakdown of barley straw releases compounds that inhibit algae growth

Dye-Based remedies eg NishiCare Blanketweed Control 5.75 to treat 3000 gallons. Blocks sunlight, thereby preventing blanketweed from photosynthesising and retarding its growth.

Screening Approx 3/m2 + construction of pergola. Blocks sunlight, thereby preventing blanketweed from photosynthesising and retarding its growth.

Algicide eg Blanc-Kit 7.99 for 1500 gallons. The only product on the market that kills blanketweed. It works by interfering with blanketweed’s bio-processes with terminal effects.

Nutrient Removers a. Phos-Kit/Pond Balance

Microbial nutrient removers e.g. Viresco 7.99 for 1500 gallon 11.75 for 3000 gallons Binds up nutrients from the pond water, reducing blanketweed growth. Bacteria breakdown and digest the nutrients that would otherwise lead to blanketweed growth.

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