Blanket Weed & Green Water
Author: Bernice Brewster
Blanket weed and green water in garden ponds, The causes of Blanket weed and green water in garden ponds
Nuisance algae such as blanket weed or green water are a very common problem in
almost every garden pond. Blanket weed may also be called 'silk weed' and
describes the filamentous threads which are comprised of long chains of algae cells.
The blanket weed grows as the cells at the tip divide on a regular basis, in the
summer this algae can grow at a phenomenal rate and can be removed from the pond
on a daily basis. Occasionally, the blanket weed indulges in a form of sexual
reproduction, which results in the formation of spores, that are shed into the water.
It is most likely these spores are how the blanket weed gets into the pond in the first place.
Green water is caused by microscopic single celled algae suspended in the water column and
perfectly describes the effect it has on the water colour! The algae which cause green
water are very simple plants and reproduce by dividing, although there are usually several
different species involved in colouring the water green. Given the right conditions, these
single celled algae reproduce rapidly, so that a single millilitre of water can contain
anything from 7, 000 to 15,000 individual cells and in hot sunny weather, it is possible
to see the algae swirling around in the water as the cells rise to the surface.
What causes green water or blanket weed to grow in the pond?
Green water is often associated with new ponds and filtration systems, where the addition
of fish alters the balance of nutrients in the pond, which encourages these nuisance algae
to thrive. Often once the green water problem has been solved and the water is clear, the
blanket weed appears to plague the pond owner, as the nutrient source remains in the water
Plants including algae have few basic requirements in order to grow, the first and most
important is sunlight, the second phosphate and lastly a source of nitrogen and in the
presence of all three, they will thrive. Phosphate is often described as the
'limiting factor' for plant growth as it is an essential nutrient and if there is very
little, the plants, including aquatic species, show poor growth. Fish also require
a certain amount of phosphate, so their formulated feeds contain plenty but any excess is
shed into the water as waste.
The nitrogen also comes from the fish, as they produce waste in the form of ammonia, which
can either be utilised directly by plants, including aquatic plants and nuisance algae.
If the pond is filtered, the ammonia is broken down to nitrite and then nitrate through
the activity of bacteria but both nitrite and nitrate form a good source of nitrogen
for plants.
What can I do to get rid of green water?
There are a variety of chemicals on the market, which can be used to control green water,
some colour the water, preventing enough sunlight from reaching the algae, so they cannot
produce any nutrients and die, other cause the cells to stick together, so they can be
scooped out of the water. It may be realised the chemicals can work in different
ways to get rid of the algae but these are only temporary solutions and as soon as the
chemical becomes diluted through water changes or time the water becomes discoloured once
again. It can help to add plants to the pond to compete with the algae for available
nutrients, although submergent species such as Canadian pond weed and Parrots Feather cannot
get enough light where the water is green and they will die. Water cress is very popular
as it will readily compete with the green water for available nutrients, however, this is also
quite an invasive plant and needs to be controlled by regular cutting to prevent it from also
becoming a nuisance.
When treating a pond with any chemical, always add extra aeration, as oxygen levels may
be affected directly by the chemical. Once the algae have been killed by these
chemical methods it is important to test the water as the dead organic material rots,
it will pollute the pond with ammonia and nitrite, as well as reducing available oxygen
levels.
Green water can be permanently controlled by fitting an ultra violet light unit to the
pond system. Ultra-violet is a wavelength of light which has unique properties,
we are very conscious of its strength as it tans our skin but over exposure can lead
to skin cancer. Ultra-violet light damages the algae cells which prevents them
from converting energy from sunlight into sugars and in the absence of a food resource,
they die. The light unit is left to run continually and once the algae that cause green
water have died it will keep the water clear, by preventing further blooms. The
ultra-violet light needs to have an electrical source and to be connected to a pump in
order to circulate the water through the unit. There is a range of filter units which
have ultra-violet units fitted as an integral part of the system but there is also a
wide range of manufactured lights which can be installed separately. The
ultra-violet light bulb needs to be replaced annually, as it gradually loses its strength.
Occasionally the quartz sleeve, which protects the bulb from coming into contact with the
water needs cleaning. The quartz sleeve can become coated in slime or lime scale,
which reduces the ability of the light to penetrate the water. Lime scale can be
removed from the quartz sleeve by soaking in warm, white vinegar. Care should be taken
when removing or replacing the quartz sleeve as it is extremely delicate.
Will barley straw keep my pond free of green water or blanket weed?
In recent years, barley straw pads have been marketed by a number of Companies for
controlling green water and blanket weed, aimed principally at the smaller pond.
As the barley straw rots, through the action of bacteria and fungi, it gives off a
special chemical, which prevents the algae that cause green water and blanket weed
from growing. The bacteria and fungi which produce this special chemical will
only thrive in very well aerated water, so the pads need to be placed near a water
fall, or the surface of the pond. If the water is not well aerated, the straw
simply decomposes, adding further pollution to the pond but without the release of
this special chemical. Where barley straw is used correctly, it is a very effective
treatment for green water or blanket weed.
I've got rid of the green water and now my pond is choked with blanket weed.
The green water excludes light from the water column and therefore prevents the blanket
weed from growing. As soon as the water becomes clear, in addition to the planted
submergent plants the nuisance algae blanket weed very quickly appears. It is
possible to physically remove blanket weed from the pond and there is an assortment of
tools available to help twist the blanket weed and lift it out of the water. Where
there is only a little blanket weed growth, these tools are quite useful in controlling
this algae but some ponds are overwhelmed by this algae and it would be impossible to
remove in this way.
There are chemical treatments available for controlling blanket weed growth. The ideal
time to begin using these treatments is in the late winter or early spring, most of us
wait until the summer time, by which time it becomes really difficult to use these
treatments effectively. Blanket weed begins growing very early in the year, as
soon as the days are noticeably longer, so by treating at this time of the year, it
prevents the algae from getting such an early start and it is easy to use maintenance
levels of the chemical through the summer to maintain control.
Water cress or fool's cress are very effective plants at reducing blanket weed growth
as they also begin to grow very early in the year. The ideal place for these
plants is in a water course, such as a water fall but they do need regular attention
in the form of pruning, or they too can become an invasive nuisance.
In recent years electronic devices have been developed, which are fitted to the pond
system and which appear to have some effect on the salt and mineral content of the
water, making changes to its make up. The effects of these changes are to also
prevent blanket weed growth although the underlying mechanism is not yet understood,
although being investigated by a number of Universities. Many people have found
these devices to have worked in controlling blanket weed growth and if it works,
that's all that matters!
Are there any fish or other creatures which will control blanket weed?
Grass carp have been sold for many years to the pond keeper as an effective means of
controlling blanket weed. The reality is that it is much easier for the grass carp to
consume fish food than chew their way through the fibrous mats of algae. So, after
introduction to the algae choked pond, they quickly change habit and eat the fish
food and the blanket weed remains. Carp which includes ghost carp and koi will
occasionally eat the blanket weed but often not in sufficient quantities as to be
regarded as controlling this algae. Lastly, crayfish will eat blanket weed
but also the plants and marginals. In the UK, there are very stringent regulations
governing the introduction of any crayfish to any open water, including garden ponds,
failure to comply will result in severe fines. These regulations have been imposed
following the introduction of exotic species of crayfish, which have taken over the
habitat of the native species, which are also being decimated by 'crayfish plague'
introduced by the invading species.
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Copyright Pets Parade Ltd. 2000