An "explosion of duck weed" is swamping Lancashire's canals, according to the Canal and River Trust (CRT).
The trust says it is removing about 70 tonnes of the floating weed each week to ensure the "thick carpet" it creates does not harm aquatic wildlife.
If left to thrive, thousands of tiny plants can accumulate and starve the waterway of oxygen and sunlight.
The trust is asking people to report any sightings of the weed, which is not harmful to humans.
Recent sunshine has seen duck weed spread rapidly through the region's waterways and while each individual piece is tiny, it has left "a thick carpet across the canal", the trust said.
Dogs and other animals have been known to mistake duck weed for grass "and ended up in the water", trust spokesman Diane Rollin said.
Litter can also gather in the weed and cause problems for boats.
Described as being "like a garden lawn, pea soup, or even green porridge", if left to flourish "it can cause problems for other aquatic wildlife by starving it of oxygen and sunlight", she said.
Teams have cleared a mile-long section of the Lancaster Canal, between Hollinshead Fold Bridge and Ashton Basin, the trust said.
The weed can be removed by skimming the surface of the water but will quickly multiply again if any is left behind.