By Sarah Mukherjee
Environment correspondent, BBC News
Care is needed when dealing with water-fern (Azolla filiculoides)
The waders we are all standing in begin to sink into the mud as the clouds of gnats home in on their rubberised meal.
The TV gardener Charlie Dimmock and Trevor Reynolds, an invasive species expert from the Environment Agency, are on lookout for species of aquatic plant that, while attractive in garden ponds, can cause havoc in the wild, literally choking the life out of rivers and streams and killing off the wildlife that reside there.
The UK government is launching a campaign on Wednesday to highlight five of the species, all common in garden ponds and available at garden centres and aquatic supplies, that gardeners have to use special caution with.
They are pennywort, New Zealand pigmyweed, water-primrose, parrot's feather and water fern
We're on the River Wandle, in South London - and Trevor and Charlie have found one growing already, despite the recent icy weather.
"This water primrose can grow eight inches a day at the height of the growing season," Trevor says.
"It can form a thick mat of vegetation that could cover this river very quickly, suffocating anything underneath. It can be so thick, it can look like dry land, which makes it a hazard as people can fall in."
Many people have heard of plants like Japanese knotweed, but ministers say not as many are aware of these "aquatic triffids", which can costs millions of pounds to clear up every year.
Floating pennywort (Hydrocotyle ranunculoides): Careful disposal needed
"We recognise that the army of gardeners are our biggest defence, and we're going to try to increase knowledge amongst gardeners," says the Environment Minister Huw Irranca Davies.
"For example, when you're going to tidy up your pond, make sure you destroy these species, either in a green bin or properly composted. Don't lob it over the fence!"
The minister speaks with some passion about this - he is building his own pond at home. "It's not a case of digging a hole in the ground, filling it with water and hoping for the best. You need to think little bit longer about what you're buying."
The government is launching this campaign in association with several societies like the Horticultural Trades Association, or HTA.
"Many garden centres now have specialists who work between centres on aquatic plants, so there may not be someone at your centre to give advice," says David Gilchrist from the HTA.
"But centres that are members of our association should be able to get you in touch with someone to help."
The government is consulting on whether to ban some species from sale.
"We're not going to make a decision until the consultation is over," says the minister. "But even if we do decide on a ban, it will, in many cases, be closing the stable door after the horse has bolted. But I would never rule it out."