Controlling and treating blanketweed and string algae in ponds

Blanket: “Heavy or dense covering”.
Weed: “A herbaceous plant, not valued for use or beauty”.

Blanketweed and Koi share two things in common:

1. They both have names that accurately describe their physical features.
2. They are rarely seen apart!

It is fair to say, that blanketweed is the scourge of the koi pond. Designed, constructed and maintained for their beauty, never was a weed so out of place as in an ornamental pond – particularly when stocked with koi.

Where the presentation of living beauty is the prime objective, anything that detracts from such a masterpiece is definitely a misfit and blanketweed is the number 1 offender.

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.

Furthermore, 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.

So are we simply aesthetic snobs when it comes to disliking blanketweed? Blanketweed can pose an indirect threat to koi health through blockages in pipework or pump intakes caused by fragments of blanketweed. This may merely reduce the pond turnover, or even lead to blockages in a bottom drain. In extreme cases, I have known that people have thought twice about going on holiday through fear of what blanketweed may do to their pond in their absence.

Blanketweed – what is it?

Blanketweed is the common name given to several closely related forms of filamentous algae that look and behave in much the same way. The most frequently encountered genera in garden ponds are Cladophora and Spirogyra.

Under magnification, Cladophora — meaning “branched plant” — reveals fine, branching filaments divided by regular cross walls. These simple thread-like structures are extremely efficient at absorbing both light and dissolved nutrients. Because of this efficiency, growth can be astonishingly fast once conditions are favourable.

Blanketweed reproduces in two ways. It can reproduce sexually by releasing gametes that fuse to form new algae. It also spreads asexually by releasing microscopic motile spores or through fragmentation, where small pieces break away and establish elsewhere in the pond. This ability to multiply rapidly is what makes string algae so persistent.

The Three Key Factors That Drive Blanketweed Growth

Understanding what fuels blanketweed is the first step in controlling it. Three environmental factors stimulate rapid growth: sunlight, nutrients and warmth.

1. Sunlight

Blanketweed relies on sunlight to power photosynthesis. Ponds in full, direct sun are far more prone to infestation. Shallow areas, waterfalls and pond margins often show the heaviest growth because light penetration is greatest in these zones. When sunlight is intense and prolonged, algae can quickly dominate exposed surfaces.

2. Nutrients

Like all plants, filamentous algae require nitrogen and phosphorus to grow. In pond systems, these nutrients are typically present as nitrates and phosphates.

Sources include:

• Fish waste from feeding

• Decomposing organic debris

• Nutrient content in tap water

Blanketweed is highly opportunistic. When dissolved nutrients are abundant, it is often the first “plant” to capitalise on them. Clear water combined with elevated nutrient levels provides the perfect recipe for rapid string algae growth.

3. Warmth

Higher temperatures accelerate metabolic processes in algae, just as they do in fish. Growth is most vigorous during warm summer months and particularly noticeable in shallow, sunlit areas such as cascades and waterfalls where water warms quickly.

Preventing Blanketweed from Taking Over

The reality is that blanketweed spores are almost impossible to exclude. At some stage, it will find its way into most ponds. Some pond keepers appear lucky, experiencing minimal growth without understanding why. In many cases, their ponds are simply less favourable environments for algae.

For the majority, however, blanketweed control requires active management. Since warmth is beneficial for koi health and growth, reducing pond temperature is not a desirable strategy. That leaves two realistic control points: sunlight and nutrients.

Reducing sunlight can involve:

• Introducing surface shade with water lilies

• Using marginal plants to soften and partially cover exposed areas

• Installing pergolas or shade sails where practical

Reducing dissolved nutrients involves:

• Sensible feeding practices

• Effective biological filtration

• Regular partial water changes

• Removing accumulated organic debris

The key to long-term blanketweed control lies in limiting the resources it depends on rather than simply reacting once it has become excessive. By managing light exposure and nutrient availability, you reduce the conditions that allow string algae to flourish in the first place.

1) Sunlight.

Sunlight penetration can be reduced in a number of ways.
a) Shading. Erecting shading on a pergola will reduce sunlight straight away and reduce blanketweed photosynthesis. It can also help against heron predation.
b) 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.

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.

2) 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.

3) Other methods of control.

1. Algicides. 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.

2. 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.

3. 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.

Interesting facts about blanketweed:
a) Under the most favourable conditions, particularly in strong currents, blanket weed can grow over 2 metres a day.
b) If treated correctly, blanketweed can be removed from a pond and used as a substitute for moss in hanging baskets. It swells and retains moisture, but must not be allowed to completely dry out.
c) In the Hokkaido district of Japan, each summer there is a festival celebrating mythology about Cladophora balls, a form of blanketweed growth in a local lake. The folk lore maintains that a young man and a girl who both drowned in the lake had their hearts turned into Cladophora balls – what a way to go!