Japanese knotweed in Pembrokeshire: Where it is thriving

Fishguard and Pembroke Coast National have the largest number of occurrences in a 4km radius with more than 220.

This is significantly more than other areas in the county, with a breakdown of occurrences per major settlement as follows:

• Newport - 106
• Saundersfoot - 68
• Tenby - 44
• Pembroke - 41 
• Milford Haven - 40
• Pembroke Dock - 40
• Narberth - 18 
• St Davids - 5 

Cardigan, which is slightly outside Pembrokeshire in Ceredigion, has also had ten occurrences.

Heat map of Japanese Knotweed in Pembrokeshire from Environet (Image: Environet)

Knotweed is a non-native invasive plant that was brought to the UK during Victorian times, and it is seen as a pest species which causes damage to its environment.

The characterisation of Knotweed differs season to season, but it can be identified by leaves which are shield-shaped, stems that look like bamboo canes with purple speckles, red shoots in Spring and cream-coloured flowers in Summer.

The plant is most commonly found near railway locations as it was traditionally used to support railway embankments and their surroundings.

If the plant has grown up to a metre onto a property or piece of land, owners could be entitled to make a claim.

There are currently almost 55,500 known occurrences of Japanese knotweed in the UK but how did a plant originating from Japan end up in the country?

It was largely believed European adventurer Philipp Franz von Siebold transported Japanese knotweed over to Europe from a Japanese volcano.

In a 1850, a specimen from the plant was donated to the Royal Botanic Gardens in Kew, London.

It was favoured by gardeners because it looked like bamboo and grew everywhere.

It is largely thought the weed travelled rapidly across the UK, aided by rail and water networks.

How to spot Japanese knotweed

Knotweed hibernates over winter but in March or April it begins to grow, with red or purple spear-like shoots emerging from the ground which quickly grow into lush green shrubs with pink-flecked stems and bamboo-like canes.

For homeowners, the plant can pose serious problems if left unchecked, with the potential to grow up through cracks in concrete, tarmac driveways, pathways, drains and cavity walls.

The roots can grow as deep as three metres and spread up to seven metres horizontally. While serious damage to property is rare thanks to regulation which requires knotweed to be dealt with, it commonly impacts use of the garden, causes legal disputes between neighbours and can impact a property’s value by around five per cent.

Why is Japanese knotweed so notorious?

Japanese knotweed has acquired a reputation as one of the most invasive plants which is known to cause damage to properties.

It is also notoriously difficult to kill – root systems can extend up to three metres deep and leaving even just a few centimetres of root behind will result in the plant quickly growing back.

Some argue that Japanese knotweed is no more dangerous to structures than having a tree or shrub growing next to a property.

What to do if you have Japanese knotweed

The first step to tackling the plant is to commission a professional Japanese Knotweed survey and find out the extent of the infestation. They will be able to tell you where it originated, and the best way to tackle it.

Professional treatment should then be organised, which usually involves the use of herbicide for excavation of the infestation.

To report further sightings of Japanese Knotweed in your area, use the heat map feature on Environet.