Japanese knotweed in County Mayo: what the records show

The National Invasive Species Database holds 35 knotweed records for County Mayo — the 18th most of Ireland's 26 counties — with the most recent from 2025. Here is what those records can and cannot tell you about a house you are buying.

Reading the records

Every record is a sighting someone reported to the National Biodiversity Data Centre, with where and when it was seen. The records are public and they are the best national picture there is, but they are a picture of where people looked: a stretch of road surveyed by the council fills up with records, and a garden nobody reported stays blank. A low count in County Mayo does not mean knotweed is rare here, and a high count does not mean any particular house is affected.

What matters to a buyer is distance. Knotweed on or right beside a property is a mortgage and structural question; a record half a kilometre away is context about the area. Knotweed spreads along rivers, roads and railway lines, so the route matters as much as the straight-line distance.

Records by county

• Cork895
• Kerry805
• Limerick377
• Dublin234
• Clare226
• Wexford140
• Leitrim115
• Galway110
• Donegal107
• Roscommon106
• Waterford104
• Sligo86
• Tipperary80
• Laois77
• Louth77
• Kilkenny48
• Kildare42
• Mayo35
• Meath32
• Westmeath31
• Wicklow29
• Cavan26
• Monaghan18
• Longford16
• Carlow13
• Offaly11

What it means for a purchase

A sighting close to a house is a reason for a specialist survey before you commit, not necessarily a reason to walk away. Treatment is a programme over several growing seasons, and lenders generally want to see a plan from a specialist. Get the survey and the quote before you make an offer, because a seller's price rarely moves after sale agreed.

Figures: knotweed (genus Reynoutria, including records filed as Fallopia) in the National Biodiversity Data Centre's National Invasive Species Database (CC BY 4.0), via GBIF, placed in counties by their recorded coordinates. 3,840 records nationally.

How a VeriSale report checks it for one address

VeriSale searches the invasive-species records within 1 km of the property, finds the nearest Japanese knotweed sighting and grades it by distance — so a record a kilometre away reads as wider-area context, not a red flag on the house.

• Within about 10 mGraded low: “on or right beside the property, a real mortgage and structural concern.”
• Within about 50 mGraded low-to-mid: “close enough to matter; have it inspected before you commit.”
• Within about 250 mGraded mid: “nearby but not adjacent; worth a surveyor’s eye on the garden and boundaries.”
• Further away, or no knotweed recordedContext only, graded well — with the reminder that records are incomplete.

This is one of 39 checks in the report, each graded A–F with a plain-language summary. The report compiles public data; it is not a survey or legal advice. More about japanese knotweed and how to read it.

What to ask before you bid

• Has knotweed ever been identified on the property or on a neighbouring boundary?
• Is there a treatment plan in place, who is carrying it out, and is it insurance-backed?
• Will the lender’s valuer need a specialist report before drawdown?

Frequently asked questions

More on County Mayo

Buying a house in Mayo: the full county guide

Japanese knotweed in other counties