Knotweed is the one garden plant that turns up in mortgage conditions. Roots go three metres down and seven metres out, it comes up through tarmac, drains and floor slabs, and a piece the size of a fingernail regrows. It spreads along rivers and railway lines and from dumped garden waste, which is why old railway towns and riverside estates score high in the records.
What lenders do
UK lenders have a formal framework: within seven metres of the house, decline or demand a management plan with an insurance-backed guarantee. Irish lenders have no published rule, but valuers now note it and some banks follow the UK line in practice. A valuation that says knotweed on or adjoining the site is a conversation with the bank you want to have before, not after, you pay the booking deposit.
Spotting it
Hollow, bamboo-like stems with purple speckles, growing in dense stands up to three metres. Shield-shaped leaves in a zigzag along the stem. Creamy flower tassels in late summer. In winter, dead brown canes standing in a clump. Walk the back boundary and any lane or stream behind the garden at the viewing. Ask the seller, in writing, whether it is or was on the property and what treatment records exist.
Cost
Herbicide treatment takes three to five growing seasons. A small stand costs a few hundred euro a year with a contractor; a large one, or excavation with licensed disposal of the soil, runs into five figures. Never strim it or put it in the brown bin. It is an offence to spread it.
Records
Sightings logged with the National Biodiversity Data Centre are public through GBIF. The report lists knotweed and giant hogweed records within 250 m of the house with the year and distance; our sample house has a 2017 record 80 m away, which is exactly the kind of thing a seller does not mention. County figures: knotweed by county.