Algae pond problems ... How to rid your pond of blanketweed.
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. Blanc-Kit effective in killing
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
Blanc-Kit Excel is specially formulated to create a pond
environment resistant to blanket weed without the use of harsh
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
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
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 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
- 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
- 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
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.
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
- Is it the high levels of nutrients? - Test for nitrates and
- 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
- 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
- Environmental Changers a. Blanc-Kit Excel
blanket pond water growth nutrients koi algae control sunlight
environment plant "green water" "barley straw" thrives gallons