Why is it in my pool?
The spores of blanketweed can blow in on the wind, or traces may come in on plants or on birds’ feet. It tends to become a problem where the nutrient levels in the water or sediments are high. In the Cotswold area the soft limestones also leach nutrient minerals. Nutrient rich water is common in new and recently refilled ponds where mineral rich tapwater has been used (see our article on phosphates), or where fresh planting soil has been used. In these cases the problem may subside after a couple of years. It often becomes a worse problem where large amounts of fish food are fed; where soil washes into the pool from the surrounding garden; in shallower ponds; or where a pool leak requires large amounts of tap water to be regularly added to the pond. It is less of a problem in well-balanced, established ponds.©
A seasonal issue
A little blanketweed is to be expected, especially in the late spring when warm sunshine encourages its growth. At this time of year it is still too early for much competition or shade from water plants. Net or rake out unsightly blanketweed (use a soft plastic rake to avoid liner damage, see below). It is easier to rake out during the afternoon, when more of the weed is floating. Very occasionally certain types of pond blanketweed (the ‘blue-green’ species) can have some toxicity. To be on the safe side, handle using rubber gloves, and make sure that pet dogs do not come into contact with the scums. www.watergardensolutions.co.uk
Avoiding Blanketweed and Scums
Some blanketweed is inevitable in most new ponds, but tends to subside as a natural balance is achieved. Ensure that nutrients/soil/leaves and debris are not washing into the pool. Where fish are present and being fed, consider using less weight of food and changing to a higher-quality floating-pellet fish food. Having ample plants of various types (submerged, lily-like, marginal, floating) can also help, as these compete for the nutrients and help to shade out the algae. Topping up with clean rainwater rather than tapwater can help.
For persistent blanketweed, we have altered the management routine of the pond or used various chemical and biological treatments to reduce the problem. The most appropriate treatments will depend on the type of pond and the sorts of plants and fish stocked in it. Your local specialist pond store might be able to advise on available brands, and we can visit local customers to test their pond water and advise on treatments.
Some of the stronger chemical treatments contain herbicides, oxidizing agents, or heavy-metals and can have unwanted side-effects so must be used with particular care (though the strongest aquatic herbicides have been banned for some years). Barley-straw treatments work on some types of algae, but not all. Certain treatments aim to reduce nutrient availability. In deeper ponds, shading dyes can restrict the full spectrum of light from penetrating the water. Most treatments need repeating. Specialist electromagnetic devices can sometimes weaken the strands, resulting in shorter growths of blanketweed. In pools without plants or fish it is possible to use dosing devices that release a constant trickle of algicide.
Ultra-Violet units are effective for green water control but have no effect on established blanketweed. Indeed, in crystal-clear filtered ponds there is no green or cloudy water to compete with the blanketweed algae, allowing the strands to grow at deeper levels in the water.
If you are having persistent blanketweed problems Contact Us to arrange a visit. We can test the water to see which nutrients are part of the problem, and offer advice on management and treatment options.