Duckweed in Ponds
(Causes, Control and Prevention)
Duckweed in ponds is a common issue, particularly from spring through summer. While duckweed isn’t harmful in small amounts, it can spread extremely quickly and cover the pond surface, blocking light, reducing oxygen levels, and spoiling the appearance of the pond.
Understanding why duckweed appears — and how to control it properly — makes it far easier to manage long term.
What Is Duckweed?
Duckweed is a tiny floating plant made up of small green leaves that sit on the surface of the water. It reproduces rapidly, often doubling in coverage within days when conditions are right.
Duckweed is frequently mistaken for algae, but unlike algae it has visible leaf structures and small rootlets hanging below the surface.
Why Duckweed Appears in Ponds
Duckweed thrives in nutrient-rich, still water. Common causes include:
Excess nutrients from fish waste and decomposing organic matter
Low surface movement or poor circulation
Sheltered ponds with little wind disturbance
Warmer temperatures and longer daylight hours
Spring is a peak time because nutrients have built up over winter and filtration systems are often just restarting.
Is Duckweed Harmful?
In small amounts, duckweed isn’t dangerous and can even provide shade. However, if left unchecked it can:
Block sunlight from reaching submerged plants
Reduce oxygen exchange at the surface
Trap debris and waste
Create poor water conditions during warm weather
Early control prevents duckweed from becoming a persistent problem.
How to Control Duckweed in a Pond
1. Manual Removal
Regularly removing duckweed with a fine net is one of the most effective control methods. Frequent removal reduces nutrients and slows regrowth.
2. Improve Water Movement
Duckweed prefers still water. Improving surface circulation using pumps, waterfalls, or returns makes it harder for duckweed to settle and spread.
3. Reduce Nutrient Levels
Remove decaying leaves and debris
Avoid overfeeding fish
Keep filters clean and functioning efficiently
Reducing nutrients tackles the root cause of duckweed growth.
4. Encourage Healthy Plant Balance
Well-established aquatic plants compete with duckweed for nutrients, helping to keep it under control naturally.
Can Barley Straw Help With Duckweed?
Barley straw is best known for helping control algae rather than floating plants like duckweed. While it may help reduce overall nutrient availability over time, it shouldn’t be relied on as a primary solution for duckweed.
Barley straw works best as part of a wider pond maintenance routine rather than as a direct duckweed treatment.
Preventing Duckweed Long Term
The best defence against duckweed is prevention:
Maintain good filtration and circulation
Remove organic waste regularly
Avoid sudden increases in feeding or fish stock
Keep nutrient levels balanced
A healthy, well-maintained pond is far less likely to suffer from excessive duckweed.
Other Common Spring Pond Problems
Duckweed is just one of several issues that can appear as ponds wake up after winter. Other common spring problems include blanketweed, green water, filter performance issues, and changes in fish behaviour.
👉 Read our full guide to common spring pond problems
Understanding how these issues are connected makes seasonal pond care much easier.
Need Help With Duckweed in Your Pond?
If duckweed is taking over your pond or keeps returning each year, the team at Birchwood Aquatic Centre can help.
We stock pond treatments, filtration equipment, nets, and water care products — and we’re always happy to offer honest, practical advice. Bring in a water sample or visit us in store for tailored help.