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Filamentous algae occur in single cells that quickly form long chains or threads in and on the surface of a pond. While these algae normally start to grow on the bottom of a pond, large numbers of filamentous algae will often form large, floating mats that most people refer to as pond scum. Large quantities of pond scum can ruin the aesthetics of your pond and may also present a risk to any fish, which may die as a result of low oxygen levels. If you've got a filamentous algae problem in your garden pond, and you don't want to use chemicals or herbicides, find out how you can deal with the issue.

Physical or mechanical controls

Many gardeners choose to physically or mechanically remove filamentous algae from a pond. By using nets, rakes or other tools, you can effectively drag the algae off the surface of the water, and if you only have a small pond or water feature in your garden, this is often the simplest and quickest way to deal with the problem.

There are other benefits to this method. By physically removing the algae in this way, you will also often remove various nutrients from the water. In turn, this can help cut the risk of future filamentous algae growth.

Of course, this method is relatively time-consuming and labour-intensive and is sometimes too physically demanding for some people. It's also important to make sure you dispose of the unwanted algae carefully. Some gardeners simply scrape the algae out of the water and dump the waste by the side of the pond. If you do this, nutrients from the algae may leak back into the pond as the algae decays.

Biological controls

There are also various ways you can tackle filamentous algae growth using biological methods.

Some gardeners add barley straw to pond water. Barley straw doesn't kill any existing algae in the pond, but the presence of the barley will sometimes deter algae growth.  You should take a bale of barley straw and submerge loose bundles within wire or cloth under the surface of the pond, allowing water to flow freely through the straw. This acts rather like a natural filter, pulling water through the barley and disrupting the algae's growth. However, this method is normally only effective in larger ponds, where there is sufficient water flow.

You can also add natural water additives that cut filamentous algae growth. These additives contain bacteria or natural enzymes that consume nutrients in the water and stop algae growth. An additive is more suitable for a smaller pond, as the cost is more prohibitive when dealing with larger water features.

Some pond owners believe that the addition of grass carp will also control the problem. These fish eat aquatic plant life and can effectively control the growth of certain algae species, but grass carp do not normally eat filamentous algae. Tilapia will eat this type of algae, but this warm-water species cannot survive below 55 degrees, so think carefully before choosing this option.

Aeration

Aeration is a method that some people use to control filamentous algae levels in ponds. By adding oxygen to the bottom layer of the pond, you can encourage phosphorus to bind with the pond's natural sediment. In turn, this means that phosphorus levels in the water drop. Filamentous algae rely on phosphorus to grow, so if the chemical is absent or in short supply, you shouldn't see so many algae.            

You can buy machines that aerate the pond for you. Choose a model that is effective in deeper water, and make sure the machine can aerate water throughout the pond, including the bottom layer. Some devices will only aerate the surface of the water, which won't help you control filamentous algae.

Filamentous algae can spoil the look of your pond and may even harm your fish.  Talk to a pond expert in your area for more information and advice about how you can manage the issue. For advice on how to add a pond liner to your pond, talk to a company like Billboard Tarps.