Blanket weed is a type of algae. There are over 20,000 different named species of algae, so working out which one you have is understandably tricky! Monocellular algae is the type that turns the water green and Lemna Minor, commonly known as duckweed, is the tiny little leaf that floats all over the surface, see separate post links.
Of all the people we speak to on a daily basis, blanket weed, I would say, is the most common algae we come up against. It is one of the fastest growing algae and can grow at the rate of 2 metres or more per day. This can rapidly clog up your pond and pond filter and, left untreated, will become a hazard to aquatic life.
Filamentus Algae (Blanket Weed)
There are over 500 species under the umbrella of “blanket weed” and it has many other nick names, like ‘string algae’, ‘horse hair’ and ‘woolly algae’. The colour may vary from yellowish green to green to brown. Blanket weed has no leaves, stem or roots that are embedded into the ground. It is a thread like algae that free floats, and can attach itself to the side of ponds and rocks. It also clings to beneficial oxygenating plants, smothering them, thus reducing oxygen levels which is always bad news.
What is the cause?
Whatever type of algae you have they all have the same cause which is over nutrition. The causes of over nutrition are many and varied; the usual culprits are fish or duck excrement, rotting leaves, dead weed, lawn fertilizer washed in from surrounding areas by rain or too much sun. Natural ponds can be polluted via the water table from a source far beyond your own boundary, as can stream fed ponds. Likewise, one of the biggest culprits these days can be spray from farmers’ fields which can be carried for miles in the wind and deposited on unsuspecting ponds.
Aquaplancton stimulates the beneficial, aerobic bacteria in your pond which multiply and spread. They then consume any over nutrition, causing algae and blanket weed to die out naturally. For more, here’s some product information.
A lot of our customers use barley straw and this can help but often isn’t the complete answer as the blanket weed can develop immunity and will come back even stronger. If you do use barley straw make sure you remove it before it starts to rot otherwise this will only add to the over nutrition if left in your pond. You can use Aquaplancton in conjunction with barley straw if you wish, one won’t affect the other.
Blanket weed loves the sun.
Water lilies are good, because they not only give shade from the sun and shelter for fish, 
but also look pretty and use up nutrients leaving less for the blanket weed to feed on. Trees are also good for shade but have the disadvantage of leaves falling into the water which is not good as they’ll rot away and be neat nutrition for blanket weed, however, they can be netted off before this happens.
As a last resort, if we have a particularly hot day, you could always position a garden umbrella or parasol during the hottest part of the day.
In conclusion, be vigilant at keeping nutrients to the minimum and you will go a long way to controlling the growth of blanket weed,
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