Geotechnical and
Environmental Services

Specialists in the investigation, assessment and reclamation
of development land.

Myths and facts

The subject of Japanese Knotweed is surrounded by misunderstanding and nonsense. On this page, BRD debunk a few of the extravagant claims and present some truths.

It is illegal to have knotweed on your site – Myth

The criminal offence is committed when you cause the plant to grow in a new area.  On a development site this is a very real possibility as even slight disturbance can cause the plant to spread.  For example, BRD attended a site where six months previously an excavator had been driven, in ignorance, once through a clone of knotweed.  The result was that for 150m, the route the excavator had taken was clearly seen as two lines of new knotweed plants shooting up.

You only need a fragment of root the size of the tip of your finger to start a new plant – Fact

It never ceases to amaze us just how small fragments of knotweed rhizome (root like structure) can speedily generate a new plant.  It is this fact that is one of the reasons why the plant is so difficult to eradicate.

Knotweed can grow through concrete – Myth

Absolute rubbish, but knotweed is more than capable of growing through any cracks or joints within the concrete.  A tarmac paved surface is certainly no obstruction to knotweed growth particularly if it is in a poor condition.  Likewise brick set paving is easily disrupted by knotweed growth.  A Client of ours chose to ignore our advice and three years later was complaining about the costs of renewing a private road that knotweed had lifted up and disrupted just as we had predicted.

Japanese Knotweed is a dangerous plant – Myth

This myth probably arose from confusion with another invasive plant Giant Hogweed which does present a real health risk (see our separate page for more details).  Knotweed has no toxic effect on humans.  In fact there are recipes on the internet for using knotweed shoots a bit like rhubarb – not that any of us at BRD have tried this form of eradication yet!

To dig out knotweed you need to excavate 7m around the plants and 3m deep – Myth

This hearsay advice stems from the early days of knotweed management when its eradication was not as well understood.  The extent of the knotweed rhizome (root like structure) varies with many factors including length of time the plant has been established, soil conditions, other vegetation and presence of nearby structures.  At the extremes, BRD have removed knotweed plants from small hand dug excavations (0.3m square) to an excavation that extended 14m to remove a single length of rhizome.  Fortunately knotweed rhizome is a distinctive and with vocational training can be readily identified.  BRD specialists can therefore supervise excavations to ensure only knotweed contaminated soil is removed.

It can grow a foot in a week – Fact

In ideal growing conditions and early in the season (April/May), growth of 300mm/12 inches per week can be recorded.  Indeed, BRD have even observed measurable growth in a single day!