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How Japanese knotweed affects your home insurance


If you’re a homeowner in the UK, you’ve likely heard of Japanese knotweed. But many underestimate the damage and destruction it can cause.

Whether you own your property outright or have a mortgage, it’s worth making sure you can recognise this weed and find out if your home insurance covers any damage it causes.

What is Japanese knotweed?

Japanese knotweed is a fast growing, invasive plant that can cause damage to your property’s foundations, drainage systems, and walls.

Originally brought to the UK in 1825 as an ornamental garden plant, it soon spread out of control and now dominates gardens up and down the country.

It can grow more than 20cm a day and can reach up to 10ft high, with its deep roots able to stretch more than seven metres wide in all directions.

The rate at which it grows means it can cause a host of problems – from damaging tarmac, pipes, paving, and drains, to making the walls of your home unstable.

How can I identify it?

Japanese knotweed can be difficult to identify if you don’t know what it looks like. Like many plants, it undergoes growth cycles that causes its appearance to change depending on the time of year.

To help you identify it correctly, below is a description of how the plant grows and changes in appearance throughout the year.

  1. Spring: Appears with bright red buds in Spring. The shoots have a reddish-purple appearance that sprout from the ground.
  2. Summer: By summer, knotweed stems will grow into hollow reddish, green stems with deep green leaves. These stems can grow up to three metres in height. During this time, they are often mistaken for bamboo as the height and colour can be similar in appearance.
  3. Summer through to Autumn: Bright green heart-shaped leaves form over the plant in Summer. Around August and September these leaves are topped with small white flowers that cluster together. The leaves will die in Autumn.
  4. Winter: The foliage dies back in the winter, leaving brown, brittle canes above the ground.

What should I do if I find Japanese knotweed?

If you find this weed on your property, you don’t have to declare it or even treat it. However, you are legally responsible for ensuring it does not spread onto any neighbouring land.

Additionally, if it is found to spread out into the wild, you could face a fine of up to £5,000 or be sent to prison for up to two years.

Acting fast can help minimise the damage and if you’re selling your property, you will need to have a treatment plan in place.

Can Japanese knotweed cause damage to my property?

Yes. Japanese knotweed seeks moisture, and in doing so, its roots can get into and expand in small cracks in your brickwork, pipes and masonry.

By targeting weak points in your home, the plant can cause damage to your home’s foundations and retaining walls, making them unstable.

However, in January 2022, the Royal Institution of Chartered Surveyors (RICS) declared that Japanese knotweed posed little to no threat to structurally sound properties.

It’s rapid rate of growth also means that it can quickly take over other plants in your garden, reducing the usability of any outside area on your property.

Can Japanese knotweed damage foundations?

If your property has solid foundations, it is unlikely Japanese knotweed will cause damage. However, if these foundations have been previously cracked, weakened or damaged, the plant could exploit these weaknesses if left untreated.

Can Japanese knotweed grow through concrete?

Although Japanese knotweed can grow at an alarming rate, it does not have the force to break through bricks or concrete.

However, the plant seeks moisture to fuel its growth and in doing so, will exploit cracks and weaknesses in structures. Once structural damage has occurred, any surrounding concrete could shatter.

Can it damage drains?

It is important that property drains remain clear that water can be flushed through and avoid blockages.

In its quest to find a water source, Japanese knotweed can start growing in and around property drains to the point where water supply becomes affected and pipes need replacing altogether.

Can Japanese knotweed cause subsidence?

Japanese knotweed will not directly cause subsidence on your property. However, it can help subsidence along by taking advantage of already-weak structures.

Does home insurance cover damage from Japanese knotweed?

Standard home insurance policies won’t typically cover the costs of repairing damage caused by Japanese knotweed.

Although you may be covered if you have a comprehensive buildings insurance policy and there’s damage to the structure of your building. For example, if it causes subsidence.

However, if it came from your garden, it’s unlikely you’ll have a valid claim if you hadn’t tried to treat and remove the Japanese knotweed before the subsidence happened.

Will my insurer pay for treatment of Japanese knotweed?

It depends on your policy, but most home insurance policies don’t cover the costs of treating or removing Japanese knotweed.

Typically, it will be your responsibility to organise and pay for the plant to be dealt with.

Do I need to tell my insurer?

You do not need to tell your insurer you have Japanese knotweed in your garden unless they specifically ask. If they do ask, it is important to tell the truth so you don’t risk invalidating your policy.

What type of home insurance do I need to cover me?

If Japanese knotweed has not been discovered at your property before, it is possible to take out indemnity insurance.

This type of insurance can provide protection against future infestation of the plant, and can include cover for the costs of a specialist survey and treatment.

What if the plant extends from my property and damages my neighbour's home?

You are legally responsible for preventing the spread of Japanese knotweed to any neighbouring properties. This means you are also responsible for the cost of its removal and any repairs that may need to happen as a result of the plant extending beyond the boundaries of your property.

These costs could add up to thousands of pounds, so it is important to learn how to recognise this weed, so you can act quickly in the event you discover it on your property.

Will it affect my premiums?

If you have the plant on your property, it shouldn’t affect your premiums. This is because most insurers will not cover costs associated with removing the plant or repairs for any damage it's caused.

But if you do have Japanese knotweed on your property, your insurer will expect you to take reasonable measures to protect your home and neighbouring properties by keeping the plant under control. For this reason, removing the weed entirely is often the most suitable option.

Will it affect my mortgage?

If you're getting a mortgage on a property affected by Japanese knotweed, you may experience problems. For example, if the plant appears on a survey during the mortgage application process, your bank or building society could insist it be professionally removed before the mortgage can proceed.

If the property you're looking at buying has Japanese knotweed, you could use this as leverage to negotiate a lower price. But before going ahead with the purchase, make sure the plant has been completely removed.

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