Blanketweed and String Algae in Ponds

(Causes and How to Control It)

Blanketweed and string algae are among the most common pond problems, especially in spring and early summer. While a small amount of algae is natural and even beneficial, excessive growth can quickly spoil the appearance of a pond, smother plants, and interfere with filtration.

Understanding why blanketweed and algae appears — and how to control it properly — is the key to long-term success.

What Is Blanketweed (String Algae)?

Blanketweed, also known as string algae, is a type of filamentous algae that forms long green strands. It often attaches itself to rocks, plants, waterfalls, and pond liners, creating clumps or floating mats that can spread quickly.

Unlike green water (which is caused by microscopic algae), blanketweed is visible to the naked eye and can usually be removed by hand.

Why Blanketweed Appears in Ponds

Blanketweed thrives when conditions favour algae over plants and beneficial bacteria. Common causes include:

Excess nutrients from fish waste and decomposing leaves
Increased sunlight, particularly in spring
Low plant growth early in the season
Poor circulation or undersized filtration
Ponds that were not fully cleaned after winter

Spring is the peak time for blanketweed because pond plants are still waking up, leaving algae free to take advantage of available nutrients.

Is Blanketweed Harmful?

In small amounts, blanketweed isn’t dangerous. However, if left uncontrolled it can:

Smother pond plants
Reduce water circulation
Trap debris and waste
Compete with plants for oxygen, particularly overnight

Managing it early prevents more serious water quality problems later in the season.

How to Control Blanketweed & String Algae

1. Manual Removal

Physically removing blanketweed using a net, brush, or algae twister is the quickest way to reduce visible growth. While this won’t solve the underlying cause on its own, it removes nutrients locked within the algae and gives you a clean starting point.

2. Improve Filtration & Circulation

Ensure your pond pump and filter are correctly sized and fully operational. Good water movement discourages algae growth and supports beneficial bacteria.

👉 Read our full guide to Pond Filter Problems

3. Reduce Excess Nutrients

Remove fallen leaves and sludge
Avoid overfeeding fish
Clean filters regularly (without killing beneficial bacteria)

The less food available for algae, the harder it is for blanketweed to thrive.

4. Encourage Healthy Plant Growth

Aquatic plants compete with algae for nutrients and light. As plants establish, blanketweed naturally becomes less dominant.

Using Barley Straw for Blanketweed Control

Barley straw is a popular natural, preventative method for controlling blanketweed. As it breaks down slowly in the pond, it releases compounds that help inhibit algae growth.

Barley straw works best when:

Added early in the season
Placed in areas with good water flow
Used as part of a wider maintenance routine

It’s important to note that barley straw is not an instant cure, but it can be very effective at preventing blanketweed from becoming established.

When to Use Blanketweed Treatments

If blanketweed is persistent or widespread, targeted treatments may be required. These should always be used according to instructions and alongside good pond maintenance.

Blanketweed treatments work best when:

Excess algae has already been removed manually
Filtration is running efficiently
Nutrient levels are under control

If you’re unsure which treatment is suitable for your pond, getting advice before dosing is always recommended.

Preventing Blanketweed Long Term

The best way to manage blanketweed is prevention:

Maintain good filtration year-round
Remove organic waste regularly
Support beneficial bacteria
Avoid sudden changes to feeding or stocking levels

A balanced pond will naturally resist excessive algae growth.

Need Help With Blanketweed in Your Pond?

If blanketweed keeps returning or you’re unsure how to tackle it safely, the team at Birchwood Aquatic Centre can help.

We stock a wide range of blanketweed treatments, barley straw products, filters, and pond maintenance equipment — and we’re always happy to offer honest, practical advice. Bring in a water sample or visit us in store for tailored help getting your pond back under control.

Frequently Asked Questions

What is the difference between blanketweed and green water?

Although both are forms of algae, they are very different. Blanketweed (also known as string algae) grows in long green strands that attach to rocks, plants, and pond liners. Green water is caused by microscopic algae suspended in the water, making the pond appear cloudy or pea green.

If your water is green but you can’t see long strands of algae, you’re most likely dealing with green water rather than blanketweed.

What causes blanketweed in ponds?

Blanketweed is usually caused by a combination of sunlight and excess nutrients in the water. Fish waste, decaying leaves, uneaten food, and organic debris all release nutrients that algae can use to grow. It is particularly common during spring when pond plants and beneficial bacteria are still becoming established.

Is blanketweed harmful to fish?

Blanketweed is not usually harmful in small amounts. However, heavy growth can reduce water circulation, trap debris, compete with pond plants, and contribute to lower oxygen levels, particularly overnight during warm weather. Removing excessive blanketweed helps maintain a healthier pond environment.

What's the best way to remove blanketweed?

The quickest method is to remove as much as possible by hand using a pond net, algae brush, or blanketweed twister. This should then be followed by improving filtration, reducing nutrient levels, and, if necessary, using a suitable blanketweed treatment.

Treating the underlying cause is the best way to stop it returning.

Does barley straw get rid of blanketweed?

Barley straw works best as a preventative rather than a cure. As it slowly decomposes, it releases natural compounds that help discourage algae growth. It should be added early in the season and used alongside good pond maintenance and filtration for the best results.

Will blanketweed disappear on its own?

Sometimes. As aquatic plants become established and compete for nutrients, blanketweed may naturally reduce. However, heavy growth is unlikely to disappear without some form of intervention and can continue spreading if the underlying causes aren’t addressed.

Can I use a UV clarifier to remove blanketweed?

No. UV clarifiers are excellent for clearing green water because they target microscopic algae suspended in the water. Blanketweed is attached to surfaces and won’t pass through the UV unit, so it needs to be removed manually or treated using other methods.

How can I stop blanketweed coming back every year?

Regular pond maintenance is the best long-term solution. This includes removing leaves and sludge, avoiding overfeeding fish, maintaining good filtration, encouraging healthy aquatic plant growth, and checking water quality regularly. Preventing excess nutrients from building up makes it much harder for blanketweed to establish.

Should I remove all of the blanketweed?

It’s usually best to remove as much as practical, but don’t worry if tiny amounts remain. Completely sterilising a pond isn’t necessary, and a healthy pond will always contain some algae. The goal is to keep blanketweed under control so it doesn’t affect water quality or spoil the appearance of the pond.

When should I seek advice about blanketweed?

If blanketweed keeps returning despite regular maintenance, or you’ve tried several treatments without success, it’s worth seeking expert advice. The underlying issue may be poor filtration, excess nutrients, or another water quality problem that needs addressing.

Bring a water sample into Birchwood Aquatic Centre, and we’ll be happy to help identify the cause and recommend the most suitable solution for your pond.