An invasive plant that covers the surface of waterways has cost the Environment Agency £40,000 to clear from a site in East Sussex over a year.
The fast-growing Floating Pennywort raises the risk of flooding and starves water of light, nutrients and oxygen.
More than 24 miles (40km) of ditches on the Pevensey Levels, between Eastbourne and Hastings, contain the weed.
It has been attributed to garden pond owners disposing of their unwanted aquatic plants in the waterway.
Environment minister Richard Benyon said: "It's important we raise awareness and encourage pond owners to be more plant wise at this time of year when they are starting to think about tidying their ponds."
Pond owners have been advised to compost their waste away from streams, rivers, ponds or lakes.