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Filamentous Algae: Control and Prevention

Filamentous Algae: Control and Prevention

Rootless and primitive, filamentous algae refer to microscopic plants that are known less formally to many as ‘pond scum.’ Filamentous algae typically grows underwater where it may attach to rocks or other objects. However, colonies of filamentous algae can break loose from these objects and float to the surface, forming mats that continue to grow and, if allowed to grow unchecked, can cover the surface of the water in large tracts. Although filamentous algae produce oxygen, they can choke off pond water from oxygen and nutrients, leaving the water stagnant. Knowing how to prevent and control filamentous algae can help you protect pond life as well as the health of your pond’s water. Here, we'll explore how to keep pond scum in check.

What Is Filamentous Algae?

Filamentous algae are microscopic plants that join together with long hairlike strands. The algae grows quickly so that in a matter of weeks, it can form mats that float to the surface of ponds and along lakefronts. Typically, pond owners will begin to see mats start to grow by July, but by late summer, large sections of the pond may be covered by late summer. It’s not uncommon for the algae to cover the surface of smaller ponds completely.

While there are different varieties of these algae, most prefer the warm, stagnant water of ponds, especially if they are rich in nutrients. Many pond animals feed on filamentous algae, so it is necessary for the pond’s ecosystem, but it can quickly bloom out of control, blocking oxygen from the atmosphere from reaching the water. When the algae covers the pond, it also feeds off its nutrients, depriving fish from both these nutrients as well as oxygen.

Preventing Filamentous Algae Overgrowth

Filamentous algae has an important role to play in the ecosystem of ponds and lakes. It only becomes a problem when its growth is unchecked and it’s allowed to form large mats that cover the water’s surface. Scientists advise pond owners to keep surface algae growth below 20% in order to protect fish and other aquatic life. Once filamentous algae surpasses the 20% mark, it can cause the water to stagnate, develop unpleasant odors, and affect the overall health of the pond water and life within it.

The best way to prevent algae blooms from covering too much of your water’s surface is to reduce nutrients from entering ponds and small lakes. Fertilizer from soil that washes into the pond or lake can trigger a filamentous algae bloom. It’s important to avoid fertilizing the ground surrounding your pond, especially along slopes where the fertilizer can easily wash into the water during rain storms. Also, avoid feeding your pond’s aquatic life as you’ll inadvertently feed the algae too.

Controlling Filamentous Algae

Filamentous algae blooms can be controlled by physical, biological, and chemical controls. There are even some technological resources that can help keep algae growth in check like diffusion circulators. To control filamentous algae growth in your pond or along your lakefront, consider these options:

Physical Filamentous Algae Controls

Although labor-intensive, physical controls of pond scum like raking up the algae mats from the surface of the pond is effective, at least for smaller ponds. You can rake the mats off the water’s surface and dispose of them elsewhere in your landscape (just keep them away from the pond or they’ll decay and become an abundance of nutrients that triggers subsequent algae blooms). This method can be time-consuming, however, and may not be enough to prevent blooms from recurring a short time later.

A parachute skimmer is an effective tool to help you physically remove algae mats from your pond. Simply drag the skimmer along the water’s surface to collect the filamentous algae mats. This skimmer is also helpful for removing leaves and dead vegetation that can trigger large algae blooms.

Biological Controls of Filamentous Algae

Some pond owners successfully control filamentous algae by introducing blue tilapia to their ponds. However, these fish are tropical and are unlikely to weather the cold season in many areas, so they’ll have to be restocked each year after the winter. Avoid adding grass carp to your pond. Many people mistakenly introduce them to the pond’s ecosystem in the belief that they’ll eat the algae, but these fish will only eat filamentous algae as a last resort; they prefer other types of pond vegetation.

Many pond owners have found success for tamping down algae growth by adding barley straw to their pond in early spring. You can submerge the straw in small bundles secured by a small wire. People have had mixed results using this method, and its exact mechanism for preventing algae growth is not entirely known. It’s thought that a natural chemical in the barley grass interferes with the algae’s development in some way. Even so, if you attempt to use the barley grass method, you’ll likely have to employ other methods to control algae blooms too.

Chemical Methods for Controlling Filamentous Algae

Many pond owners rely on chemical controls such as aquatic herbicides to control filamentous algae. These herbicides can be effective, providing they’re safe and designed for pond use. Usually, these herbicides only need to be applied to the pond water once per year. Cutrine Plus Algaecide is regarded as one of the safest herbicides you can rely on to control filamentous algae growth. If you plan to use a chemical control, be sure that the herbicide you choose is specifically designed for filamentous algae, or you could wind up targeting the wrong plants.


Many pond owners rely on a combination of the techniques outlined here in order to keep their filamentous algae colonies under control. Weeders Digest features a wide range of products you can rely on for controlling pond scum and maintaining the health of its water and aquatic life. 

30th Aug 2021 Weeders Digest

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