Walk the bank of almost any Fenland drain in high summer and you will meet something that should not be there. A raft of round green leaves jammed against a sluice. A stand of pink flowers taller than a person. A crayfish in the landing net that is far too big, with red patches under the claws. Fenland's waterways, from the Great Ouse and the Nene to the Old and New Bedford rivers and the grid of straight drains between them, are flood defences, irrigation channels and wildlife corridors all at once. They are also remarkably easy for a non-native plant or animal to travel through.

You do not need a biology degree to help. You need a phone, a rough idea of where you are standing, and enough patience to take a photograph that somebody else can actually use.

Why a single odd plant matters here

Most Fenland water is contained, embanked and slow-moving, which suits invasive species down to the ground. A floating mat of pennywort or water fern can seriously reduce the amount of water a drain will carry, and a drain that carries less water is a drain that floods sooner. The same mats shade out native plants, lower oxygen levels for fish, and make life harder for the water voles, dragonflies and wading birds the Fens are known for.

Clearing an established infestation is slow, costly and often has to be repeated the following season. Catching something early, when it is one patch beside one bridge, is a completely different job. That is the argument for reporting: five minutes of your time can save somebody else five years of work. The Ouse Washes, the Nene Washes, Wicken Fen and the reserves around them are exactly the places where a new arrival does the most damage.

Plants to look for

• Floating pennywort. Small round leaves, like a miniature lily pad on a fleshy stem, forming dense floating mats against banks, sluices and moorings.
• Himalayan balsam. Head-height stems, pink helmet-shaped flowers in summer, hollow reddish stalks that snap easily. Seed pods fire their seeds when touched.
• Japanese knotweed. Bamboo-like canes with shield-shaped leaves and a zigzag growth pattern, often on bank tops, disturbed ground and near bridges.
• Giant hogweed. Umbrella-like white flower heads on stems that can reach several metres, marked with purple blotches and stiff hairs. Its sap makes skin sensitive to sunlight and can cause serious blistering. Do not touch it, and keep children and dogs clear.
• New Zealand pygmyweed. A bright green carpet of tiny star-shaped leaves over mud and shallow water, like a lawn where no lawn should be.
• Parrot's feather. Feathery submerged foliage with grey-green spikes above the surface, a common escapee from garden ponds.
• Water fern. A tiny floating fern that turns red-brown in autumn and can blanket a sheltered dyke in weeks.

Animals to look for

• Signal crayfish. Chunkier than our native white-clawed crayfish, with red patches under the claws. It carries crayfish plague, which is fatal to native populations. Never move one, alive or dead.
• Chinese mitten crab. Hairy, mitten-like claws, most often seen in tidal reaches and the lower Ouse.
• Zebra mussel. Small, striped, triangular shells stuck in clusters to pontoons, sluices, boats and stones.
• Killer shrimp. Small and striped, usually turned up by anglers and surveyors rather than spotted from a footpath. A clear photo is worth sending in.
• American mink. Dark, low-swimming animals with a bushy tail, often seen on banks at dawn or dusk. A serious predator of water voles.
• Terrapins and non-native geese. Almost always dumped pets. Report unusual sightings, particularly if they appear to be breeding.

What to note before you report

A record without a location is close to useless. Before you leave the spot, note down:

• The grid reference or a what3words location, plus the name of the drain, river or lode and the nearest bridge, sluice or lock.
• The date and roughly how much there was: one plant, a patch a few metres across, or a stretch of bank.
• Whether it is in the water, on the bank, or on structures such as sluice gates and pilings.
• One wide photograph showing the surroundings and one close-up showing leaves, flowers or shells clearly. Put something in frame for scale, such as a boot or a coin.
• Whether the site is publicly accessible, and whether you saw it from a footpath, a boat or a road bridge.

How to report it

1. Environment Agency incident hotline: 0800 80 70 60. Use it for anything in or beside a main river, or where a species is blocking a channel, threatening a structure or causing harm. Check the current number on the Environment Agency website before you rely on it.
2. iRecord. The national recording app and website pass your sighting to the local environmental records centre and the relevant national scheme. This is the best route for general sightings.
3. The internal drainage board or riparian owner. If it is in a managed drain, they maintain that channel and will want to know. Parish councils can usually point you to the right board.
4. Reserve staff. If you are on an RSPB, Wildlife Trust or National Trust site, tell the wardens directly. They often have a rapid response plan.
5. Your angling club or rivers trust. They know their stretch better than anyone and can follow up with the right authority.

If giant hogweed is growing beside a footpath, a play area or a fishing peg, say so when you report it. Invasive plants are not a matter for the emergency services, but a health risk beside a public route deserves a prompt call.

Do no harm while you are at it

Good intentions spread problems. Seeds travel on boot treads, mud, tyre treads, nets, waders and boat hulls, and a single fragment of knotweed or pennywort can start a new colony downstream. Anglers, paddlers and birdwatchers should follow the Check, Clean, Dry routine: check kit and clothing for plant fragments and mud, clean everything thoroughly, and dry it properly before moving to another water body.

• Do not pull up, cut or take cuttings of anything you are not sure about. Report it instead.
• Never tip pond or aquarium plants, water or fish into a drain, river or ditch.
• Do not release unwanted pets, including terrapins and goldfish.
• Avoid wading in Fenland drains. They are deep, steep-sided and often colder and faster than they look.
• If sap from giant hogweed touches your skin, wash it off, keep the area out of sunlight, and seek medical advice if a reaction develops.

Start with one record

Pick the stretch you walk or fish most often and learn it properly. Once you know what belongs there, the odd one out stands out immediately. Photograph it, pin the location, and send the record in. Then do it again next month, because repeat sightings at the same spot tell the people managing that drain whether a problem is spreading or has been dealt with.

Fenland's waterways are managed by a patchwork of boards, trusts, councils and landowners, and none of them can be everywhere. Thousands of pairs of eyes along the banks, bridges and towpaths are the best early warning system the Fens have. Use yours.

Photo: apnear40 / Pixabay