Officers have identified the invasive plant, Hydroctyle ranunculoides, which can grow up to 20cm a day, at multiple locations in the River Thames.
River management specialist professor Alastair Driver told the BBC yesterday that an "invasive alien plant with Triffid-like growth" was about to take over parts of the river and was particularly prevalent downstream of Reading.
He warned that the Environment Agency needed to eradicate the floating pennywort this autumn or it would be too late.
The EA said that the invasion in the Sonning area of Berkshire is believed to have originated in a lake in Green Park.
Hydrocotyle ranunculoides - image: Flickr/Dick Culbert (CC BY 2.0)
"Despite best efforts at control, has worked its way downstream via the Foudry Brook and the Kennet and Avon Canal. This summer’s low river flows and warm temperatures have contributed to its rapid spread," a spokeswoman said.
"Landowners have a legal responsibility to control the growth and spread of certain invasive species on their property including rivers. However, in this case the Environment Agency is using its permissive powers to quickly control this new infestation to prevent further spread, which will be more difficult and more expensive to deal with if not tackled early. Our teams completed site visits around Sonning in early October to assess the situation and have an action plan already in place."
Floating pennywort was banned from sale in the UK in 2014 and was added to the EU List of Invasive Alien Species of Union concern in 2016.
Large infestations of floating pennywort choke up rivers and can increase flood risk and impede the amenity use of a river. It also displaces native plants and animals and may reduce oxygen levels in water.
The EA plans to mechanically remove floating pennywort in the River Thames at Sonning in the coming weeks. Measures will be put in place to capture any floating fragments that have broken off during mechanical removal, essential to prevent downstream spread, the agency said.
Officers will also walk the reach between Reading and Sonning to ensure that floating pennywort is not present in other locations.
In the spring, experts will re-assess the extent of floating pennywort and record sightings on the EA’s Plant Tracker app, which ask members of the public to report sightings of invasive plant species. The app enables anyone to take a picture of the invasive species and to log its location and other details to help build a picture of its distribution.