Must authorities be informed over Japanese knotweed infestation in French garden?

Reader Question: I think I have a large patch of Japanese knotweed growing in my garden in the Hérault. Do I need to inform any local authorities? 

Japanese knotweed (renouée du Japon) is an extremely invasive plant species not just in France but across Europe.

It can survive in high temperatures, drought conditions, high salinity, is largely unaffected by disease and even if cut down can often quickly grow back due to entrenched underground roots. 

In some cases, shoots can grow up to 30cm per day, up to a maximum height of over four metres.

Although it can be eaten, and is a component of some Japanese dishes, it is not cultivated.

The weed is known primarily in France for its rapid destruction of local ecosystems, as it aggressively takes over spaces formerly occupied by other plants and flowers, which in turn can have a knock-on effect on local wildlife. 

‘Invasive’ plants may be listed as such in regional rules or across the whole territory. In the latter case they are usually listed in this 2018 decree. 

Japanese knotweed is not so listed, but it was, nonetheless, recently described as “an invasive exotic species which has a negative impact on biodiversity in France” in a ministerial response to an MP’s question, from the Ecology Ministry.

It means Japanese knotweed should not be introduced into the French environment, either intentionally or by negligence, nor can it be planted, bought, or sold, as stated in Article L411-4 of France’s Environmental Code. 

This includes deliberately planting it in fully private locations. 

What should I do if I have Japanese knotweed in my garden? 

Despite it being considered an invasive species, unlike for certain animals, you do not need to inform local authorities of the presence of the plant. 

You can contact your local mairie or local experts if you are unsure whether you have the plant in your garden, as they may be able to confirm it for you, however this is not mandatory. 

However, you should do your best to remove the weed as much as possible and prevent it from spreading, either inside your garden or to neighbouring areas. 

Below are some tips from the Feyzin mairie (a suburb of Lyon) about how to deal with Japanese knotweed: 

• Do not simply clear and throw it away without care, as it spreads easily – 1cm of a root or stem is enough for a new plant to grow back from

• Cut it back to its base at least seven times a year, then put it in a sealed bag to prevent it any spreading of material during transport

• Do not use chemical weedkillers as they are ineffective and pollute soil and groundwater

• Once cut, it should not be treated as green waste.(déchets verts) but treated separately

• Put the waste in household rubbish bags to be collected at the same time as usual household waste, and not for déchets verts collection

• If you are transporting it yourself, make sure you do so in closed bags or a tarpaulin-covered trailer to avoid dispersal along the route