Short answer: Japanese knotweed near a house is a lending and valuation issue in Ireland, not only a garden one. Irish lenders publish no rule, but valuers note it and some follow the UK practice of asking for a treatment plan when it is within seven metres of the building. Identify it by hollow bamboo-like stems with purple speckles and shield-shaped leaves, walk the back boundary at the viewing, and ask the seller in writing whether it is or was on the property. Treatment takes three to five seasons; excavation runs into five figures.
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.
Questions people ask
- Is it illegal to have knotweed on my land?
- Having it is not an offence. Spreading it is: under the European Communities (Birds and Natural Habitats) Regulations 2011 it is an offence to disperse or cause the spread of a listed invasive species, which covers cutting and dumping it. Treatment contractors handle the waste under licence.
- Does a seller have to disclose knotweed?
- Not unless asked. Ask in writing before contracts, and have your solicitor include it in the pre-contract enquiries. A false written answer is a misrepresentation.
- Can I insure against it?
- Specialist treatment firms sell insurance-backed guarantees with their management plans, which is what UK lenders look for. Ordinary home insurance does not cover knotweed damage.