Floating plants like duckweed, blanket weed and the invasive plant Water fern can cause a problem in ponds when the dense growth can form a thick mat over the entire water area, this is called a 'bloom'.
This blocks out light and prevents oxygen getting into the water from the air so is not good for other plants and for wildlife in the pond. Where the mats of growth look thick, this can be dangerous for children and animals because it looks solid.
A small amount of algae or duckweed in your pond is OK. Both occur naturally in ponds with clean water and some animals like to live in them - there are small beetles that live out their lives within the duckweed fronds.
The problem is caused by too many nutrients in the water (especially nitrates and phosphates) cause plants like algae and duckweed to enjoy unrestricted growth.
Blooms in new and recently- dredged ponds
New ponds or ponds that have been dredged often have temporary algal or duckweed blooms. This occurs because disturbing the soil causes nutrients to be released into the water. These blooms are usually short-lived and normally no action needs to be taken because the ponds will recover naturally in time.
Blooms in older ponds
Older ponds with a persistent bloom are more of a concern. This is most likely to be caused by too high a level of nutrients in the water. The following things may cause this:
- nutrients in the pond water supply e.g if the pond is fed by a stream
- fertiliser applied to fields or garden which is washed into the pond
- run-off from bare soil
- duck or livestock faeces
- bread or corn being thrown in the pond to feed ducks
- too many fish
- sewage, silage, or farmyard run-off
- run-off from roads or pavements
Before you act, check what is growing there!
Many people think these blooms look unsightly, but before doing anything, make sure you are not about to destroy a rare species!
There is a chance you might have Fat Duckweed (Lemna gibba) or Great Duckweed (Spirodela polyrhiza) both of which are uncommon and worth protecting.
If you live near the Somerset levels or the South Kent coast, you might have rootless duckweed (Wolffia arrhiza). This is tiny - only one millimetre in diameter and looks like a green grain of sand. Its Britain's smallest flowering plant and one of the rarest.
Most likely though, is that you will have either Common Duckweed (Lemna minor) or the introduced species Least Duckweed (Lemna minuta).
You can contact your local Council Wildlife Officer for advice.
How to control blooms.
