The answer is when it is ‘knotweed’; of which there are various different species but one in particular that causes angst. Japanese knotweed is an amazing plant that can grow through concrete and burrow through buildings.
It strikes fear into the heart of developers and land managers and is covered by regulations and legislation – though my experience is that local authorities and utilities such as Npower with their pipework along Jordanthorpe Parkway a
couple of years back, simply ignore.
In the latter case, the contractor for re-piping work took vehicles though the heavy clay soil and crossed a massive Victorian planting of knotweed at Jordanthorpe House and then cut a trench about a half-mile long to near Coal Aston.
This is clearly against both guidance and legislation and potentially spread knotweed along the entire length of the cut.
A little like dandelion roots, tiny fragments of knotweed rhizome can form new plants – though of course you won’t become aware of this for several years.
The remediation of knotweed infestation can cost anything from a few hundred pounds, to in the case of the new Wembley Stadium, over a million. It is illegal to knowingly allow the plant to escape into the wild or to grow in the wild; which in practice means outside a garden.
There are potential fines and legal actions that can be taken but most local authorities and the Environment Agency no longer have the resources (or skills) to deal with the problem and so don’t expect support from them if you find knotweed.
House-holders can come up against this issue when either trying to buy or sell a property since if you have knotweed in your garden or even nearby you won’t get a mortgage.
It should be picked up by property surveys but sometimes is overlooked as most surveyors are not field botanists!
I have been called out to help a knotweed-stricken householder only to find in one case it was wild clematis, and in another herb Robert! The plant can cause Serious problems but these are solvable if dealt with correctly and with time.