Japanese Knotweed

Paul Vernon

Paul Vernon · Surveyor (retired)

When you are looking for a new home, you tend to spend a lot of time looking at the front (waiting for the agent to show up) and then really concentrate on the inside of the house, but how many of us have more than a cursory look at the garden? Is it big enough? What direction does it face? But do you really have a look at what is growing in it? I suspect not.

We all like colour in our gardens, and a lot of that is down to the plant hunters of the Victorian era who transformed British gardens. Setting out on dangerous missions to track down specimens, they brought back much-loved plants like camellias and rhododendrons.

However, not everything that they bought back turned out to be as good as it looked. They also introduced plants that have proved to be a curse for a variety of reasons. There are quite a few, but the worst of these are Japanese knotweed, Giant hogweed, Himalayan balsam, Rhododendron and New Zealand pigmyweed . These are known as Invasive non-native plants and many of these have spread from gardens into the wild. There are more than 1,400 non-native plants now established in the wild in Great Britain, of which 8% are stated to have a negative impact.

In this article, I will make use of data sheets provided by the Royal Horticultural Society (RHS) and the Government.

What are non-native invasive plants?

Non-native plants are those that occur outside their natural range due to direct or indirect introduction by humans. If the introduced plants persist in natural or unmanaged habitats, they are termed ‘naturalised’.

Many naturalised species do not present a problem but some that spread and outcompete native species can threaten ecosystems, habitats or native species. Only where this occurs are the plants termed invasive non-native species. These are considered to be invasive either due to lack of natural control mechanisms (such as herbivores); rapid rate of spread (by seed or vegetatively) or suppression of other species (such as allelopathy or competition for resources).

The Law

There are a number of different regulations in place to help protect our environment from invasive non-native plants.

In Scotland the Wildlife and Natural Environment Act (Scotland) 2011 is now in force making it illegal to plant any non-native plant in the wild in Scotland.

Elsewhere in the UK it is an offence to plant or cause to grow in the wild plants listed on Schedule 9 of the Wildlife and Countryside Act (1981). For some of these, there is even a ban on their sale in the UK.

EU Regulations on Invasive Alien Species lists 36 plants and still applies in the UK. These plants should not be planted or caused to grow in the wild but in addition are banned from sale and gardeners possessing them should undertake measures to control them.

If you already have these species in your garden or on your land, you are not likely to be prosecuted simply for having them. However, you are advised to control them and, for those 36 plants listed by the EU, you are required to take all possible steps to remove them, even if you didn’t plant them. It is also your responsibility to ensure that they are not allowed to spread.

Non-native invasive plants can:

  • Change ecosystems and habitats and have non-biotic effects, such as reducing or impeding water flow leading to flooding, or changing the acidity or the chemical composition of the soil, or lock up nutrients
  • Outcompete native plants either by habitat change or by spreading so rapidly as to crowd out slower growing species, threatening the long-term survival of species
  • Take a long time to become invasive. Many of the plants now considered invasive have been growing in the UK for over 100 years and for much of that time showed no sign of becoming a problem
  • Be expensive to eradicate. It is also very costly to restore degraded habitat, if it can be done at all.

I’ve singled out 5 plants in my introduction. These present problems as follows:

  • Giant hogweed. This can cause burns and irritation when you come into contact with it.
  • Himalayan balsam. This is an invasive weed of riverbanks and ditches, where it prevents native species from growing.
  • Rhododendron. Whilst it is a popular garden plant, in the wild, it is responsible for the destruction of many native habitats and the abandonment of land throughout the British Isles because where conditions are suitable, it will out compete most native plants. It will grow to many times the height of a person, allowing very little light to penetrate through its thick leaf canopy. This effectively eliminates other competing native plant species which are unable to grow due to insufficient light. This in turn leads to the consequent loss of the associated native animals.
  • New Zealand pigmyweed. This is an alien invasive species that thrives in waterways. It forms thick mats of vegetation across the river and lake beds, out-competing the native species, some of which are under threat. It’s also covering up the spawning gravels for freshwater fish.
  • Japanese knotweed. This is not usually considered harmful to humans. However, it’s interest to us here is that it has acquired a reputation as one of the most invasive plants, and has been blamed for causing damage to properties. This, combined with its refusal to die, has made it a serious concern for the housing industry.

If you want to know more about invasive non-native plants, then I suggest that you refer to the RHS website, but I propose to concentrate on Japanese Knotweed because of it’s impact on properties.

Japanese Knotweed

Japanese knotweed (Fallopia japonica) has become a major issue for many UK property owners. It has been described by the Environment Agency as “indisputably the UK’s most aggressive and invasive plant”. In winter the plant dies back to ground level but by early summer the bamboo-like stems emerge from rhizomes deep underground to shoot to over 2.1m (7ft), suppressing all other plant growth,. Eradication requires determination as it is very hard to remove by hand or with chemicals.

Its an amazing plant. It spreads rapidly, its been known to grow 10cm in a day, The root stalks of the plant are extremely tough and re-grow readily and it can grow from a cutting the size of a finger nail, or from shoots that grow from it’s roots. It is extremely durable and able to grow practically anywhere, which is why it has spread like wildfire across the country.

You will have seen this plant, even if you don’t know it. It’s common on many roadsides and railway corridors, where it spreads with little or no attempt at control by the authorities.

Why should I worry?

The persistent nature of the plant and its thick root system means that it’s able to grow through weaknesses in pavements, pipes and even concrete. It can grow through cracks in cement, between floorboards, and out from the joints in a stone wall and so is capable of damaging the foundations of houses, inflicting thousands of pounds worth of damage. Even the smallest fragment of the plant that has been discarded in the soil can mean that it is contaminated and the knotweed is likely to regrow. It might seem that emerging shoots are not much of a nuisance at first. However, since the plant can grow so quickly and without warning, it will get out of control quickly and cause plenty of damage if left untreated. Its strong roots can grow down to 3 metres deep, thus causing damage to buildings. It’s not the same problem in Japan, but in Europe, there is no insect, pest, or disease in the ecosystem that can control it and so it grows unhindered.

If it isn’t a worry to you, then it certainly should be. Few, if any lenders will give a mortgage on a property where this is present. A seller is required to declare whether they know of it’s presence and any competent surveyor should be looking for it when they inspect the property for you or the lender. If you already own the property, then you should address it without delay, because this is one problem that certainly will not go away, and a successful removal process takes years, not months.

What do I do if I have Japanese Knotweed?

Simple – get rid of it, and start as soon as possible. If you go onto google, you’ll find lots of pages telling you how to do it yourself, but this is one of those things where I wouldn’t take the chance. There are plenty of experienced professional companies out there who know what they are doing. A quick search reveals typical prices for residential properties between £1,500 and £5,000 (but I have even seen £15,000 mentioned) but obviously it will all depend on the nature and extent of the infestation. Any waste from the operation is classified as a controlled waste and can only be disposed of at specialist waste sites, or burnt. You must not put it into the compost or any form of household waste collection. If you use a professional, you’ll have the comfort of knowing that they know what they are doing and you should get a warranty on the effectiveness of removal. You’ll hope that you won’t need it!

Could I get a mortgage?

Each lender is different and may have their own criteria. There is no industry-wide standard, though the Royal Institution of Chartered Surveyors (RICS) has four categories, and these broadly fit into how lenders view your knotweed problem, each rising in severity.

Category 1 - Knotweed is found in a neighbouring property, more than seven metres, or an empty space like a railway bank or wasteland. Category 2 - Knotweed is within seven metres of your property but not within it. Category 3 - It is within the boundaries of the property but is more than seven metres from a living space. You will need a professional opinion. Category 4 - The worst. It is within seven metres of the property and causing damage to walls, paths or foundations. This needs immediate professional intervention.

Whether or not you can find a lender depends how your property rates against the categories above.

  • Category 1 equals no knotweed on the property. If your survey shows up Japanese knotweed, this will require another survey by a knotweed expert. Some lenders may already decline you at this stage.
  • If your knotweed exposure hits Category 2 or 3, as described above, you may be asked to provide a bigger mortgage deposit, or could even be charged a higher interest rate.
  • If your proposed property is at Category 4, many mortgage lenders will refuse you on this basis. If they do accept you for a mortgage, they will expect an independent examination and a treatment plan to be put into action.

A lender may expect the knotweed to be completely removed and any damage is repaired - complete with a guarantee from a treatment expert that the weed has gone - before a mortgage can be completed, or you may be lucky and get them to agree to lend on the understanding that an approved contractor has been appointed.

Obviously, as a buyer, you’d be in a position to require the vendor to bear the cost of these works.

Would I buy?

Before I started the research, I’d have said an emphatic no, but as I looked into it more, I started to soften my stance as I read about the success of treatments. However, on balance, I’d still be very wary. I’d ask myself 2 questions:

  1. If I could get a mortgage, what confidence would I have that I could actually sell when I wanted to?
  2. Where has it come from? If you’ve got it, then it’s likely to have come from a neighbour. If they haven’t any plans to deal with their infestation, then you’ll only have to deal with it again, and possibly again. If your neighbours have got it, then the law doesn’t require them to eradicate it, although they must take steps to stop it spreading. But if they refuse to deal with it, then neighbour relations will suffer. Imagine introducing yourself to your new neighbour by saying “you are infesting my property”!

About the author

Now retired after 40 years in the energy sector, Paul qualified as a Mining Surveyor and worked in a variety of roles involved in coal mining and land reclamation before transferring to the renewable energy sector where he developed renewable energy schemes. Key to this was an awareness of the issues and challenges that face development schemes of any nature. Particular skills range from site identification through property negotiations, project design and consenting to pre- construction Condition Discharge and design refinement of consented projects.