The online map, created by Environet, pinpoints thousands of infestations of the notoriously invasive weed up and down the country.

The plant has a vigorous growth, advancing up to four inches a day in the summer, and its roots or rhizomes spread far underground causing structural damage to buildings.

If the plant is found growing within 23 feet of a property, mortgage applications can be refused unless there is a plan in place to eradicate it.

The plant has a vigorous growth, advancing up to four inches a day in the summer, and its roots or rhizomes spread far underground causing structural damage to buildings (Pictured, growth in south Wales)

Even if the seller carries out the expensive work, the stigma associated with the plant means property values can decrease by up to 10 per cent.

And in Cardiff, there are 414 occurrences within a 2.5 square mile area of the city centre.

The tool was created by Nic Seal of Environet UK, who said: ‘This heatmap will enable us to build a nationwide picture of the Japanese knotweed problem and give the general public the information they need to assess the risk locally, particularly when buying a property.

‘It will also be a useful tool for homeowners who want to be aware of infestations near their property which could spread, putting their home at risk.

‘The site is already well populated, but this is an ongoing project. The more people who report sightings, the more effective it will become.

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