What is blanket weed and why is it a problem for my pond?

Blanket weed, also known as string algae, silk weed or filamentous algae, is a type of fast-growing green algae that forms long, hair-like strands. It often floats just below the surface or attaches to rocks, plants, pumps, and pond liners — creating unsightly, tangled masses that can quickly take over your pond if left unmanaged.

Why is blanket weed a problem?

 1. Reduces oxygen levels

Blanket weed consumes oxygen during the night, which can reduce the oxygen available to your fish, especially in warm weather when oxygen is already low. This can lead to fish stress or even death in extreme cases.

2. Disrupts water flow

It easily clogs filters, pumps, waterfalls, and UV clarifiers. This can slow down or stop water circulation, reduce filtration efficiency, and create stagnant areas in your pond.

3. Competes with plants

Blanket weed competes with aquatic plants for nutrients and light, often outgrowing them and causing your beneficial plants to decline. This upsets the natural balance of your pond’s ecosystem.

4. Aesthetic nuisance

It looks messy, forms thick floating mats, and detracts from the visual appeal of your pond. It can also make routine pond maintenance more difficult and time-consuming.

What causes blanket weed to grow?

Blanket weed thrives in warm, sunny ponds, especially those with:

• High levels of nutrients (nitrates and phosphates)
• Still or slow-moving water
• Excess sunlight
• Limited plant competition
• Overfeeding of fish (which increases waste and nutrients)

 How to control and prevent blanket weed

1.     Use pond treatments specifically designed for blanket weed (like clumping or biological controls).

2.     Limit nutrients: Reduce fish feeding, avoid run-off from lawn fertilisers, and use beneficial bacteria to outcompete algae.

3.     Add plants: Floating plants, water lilies and oxygenators can shade water and use up nutrients. Water cress can be a very useful temporary solution and will grow from salad cuttings whilst more ornamental plants catch up.

4.     Add fake floating plants: It might sound odd but fake floating lilies or similar will provide instant shade for the least green-fingered water gardener. The fish have no strong opinion and won’t judge you!

5.     Install a UV clarifier: Helps reduce algae spores in the water.

6.     Manually remove it: Regularly twirl it out with a stick or algae brush before it builds up.

7.     Ramshorn snails: Don’t damage plants and will graze hard surfaces to control blanket weed before it develops into threads.

8.     Grass carp: Can be added to ponds where plants aren’t a focus, such as Koi ponds. They prefer to eat pond plants but will happily eat blanket weed if these aren’t available.

9.     Barley straw: Releases chemicals that inhibit algae growth as it decomposes. Bear in mind that rotting vegetation is a nutrient source for algae and you’ll see why this isn’t always effective. Barley straw extract can be a more efficient way of trying this method as it’s faster than waiting for the straw to break down. Given the price of barley straw, it’s worth trying to see if it works in your system – make sure oxygen levels remain high though to avoid making things worse.

Seasonal Blanket Weed Control Plan

Spring (March–May): Prevention Is Key

Spring is when blanket weed starts to appear as temperatures rise and light levels increase.

Action Steps:

• Test your water for high nitrates and phosphates
• Remove any early growth manually with a stick or net
• Add beneficial bacteria like:
• Microbe-Lift Clean & Clear – boosts your pond’s natural balance
• Microbe-Lift Sludge Away – removes organic waste where algae feeds
• Introduce or repot oxygenating plants (e.g. hornwort, elodea)
• Start using a UV clarifier if you have one — clean it and check bulbs [LINK TO CLARIFIERS]
• Add extra pond plants to keep pace with nutrient availability.

Optional treatment:

Blanketweed treatment: Start dosing with a safe anti-blanketweed product like:

• AquaPond Anti-Blanketweed
• NT Labs Blanketweed Control

Summer (June–August): Keep it in check

Warm water, bright sunlight, and active feeding can trigger sudden algae growth.

Action Steps:

• Limit fish feeding (only what they eat in 2–3 minutes)
• Use floating plants (like lilies or water lettuce) to shade the surface
• Continue weekly doses of beneficial bacteria (Clean & Clear or TheraP)
• Skim and remove any visible blanket weed before it spreads
• Check pumps and filters weekly — blanket weed can clog them quickly
• Top up water and use a dechlorinator to avoid disrupting bacteria

Optional treatment:

• Apply an enzyme-based or natural clumping product if growth spikes
• AquaPond Blanketweed Resolve or Microbe-Lift AlgAway (follow instructions carefully)

Autumn (September–November): Clean and protect

As temperatures cool, pond activity slows, but decaying leaves and leftover nutrients feed winter algae.

Action Steps:

• Use a pond net to catch falling leaves
• Dose with Sludge Away to reduce organic buildup
• Reduce feeding or stop altogether when water drops below 10°C
• Trim plants and remove any remaining blanket weed manually
• Prepare your pond for winter with a bacteria booster (helps in spring)

Winter (December–February): Minimal intervention

Blanket weed often lies dormant but can bounce back early in spring if you don’t manage nutrients.

Action Steps:

• Keep the pond surface clear of ice using a floating de-icer or air pump
• Don’t feed fish if water is below 8–10°C
• Avoid disturbing the pond too much; bacteria slow down in cold water
• Plan your treatment schedule and supplies for spring

Recommended Products from Maidenhead Aquatics

Shop Pond Water Treatments https://www.fishkeeper.co.uk/pond-products?product_types=Water%20Treatments

Final Tips for Success

• Consistency beats crisis control – dose beneficial bacteria regularly
• Shade and aerate to reduce algae-friendly conditions
• Don’t over-clean – leave some biofilm and good bacteria on surfaces
• Start early – it's much easier to prevent than to cure

For specific advice about your pond, pop into your local Maidenhead Aquatics store and speak to one of our friendly staff.