Japanese Knotweed Myths

Breaking down the myths and facts surrounding Japanese Knotweed

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The Myths of Japanese Knotweed – Most of Us Have Seen the Hype!


Spreads like wildfire

Regenerates from 0.7g

Grows through concrete

Japanese Knotweed Myths – Triffid

The truth is that Japanese Knotweed (also known as Asian Knotweed or Fallopia Japonica) obeys the laws of science and creation, not science fiction! As with any plant, it must have water, nutrients and must open up its leaves to carry out photosynthesis to survive and grow.

The Japanese Knotweed plant, if not controlled, can create a dense blanket effect choking out other native plant species.

Japanese Knotweed Spreads Like Wildfire

The impressive upward Spring growth of Japanese Knotweed, 0-3 meters high in 6 weeks is supported by a large mass of below-ground rhizome that has probably taken 20, 50 or more years to form. Questioning older residents neighbouring a Knotweed infested site often confirms this with comments such as “Our children played in that stuff 30 years ago. It was pretty much the same then as it is now.”

Rapid spread over a large area is, almost always, man-made by the movement of material containing Knotweed.

3 meters deep? Yes, occasionally, but generally Knotweed has proved to an average depth, in natural ground, of 1 – 2 meters deep with occasional deeper penetration by single rhizomes.

In very well established Knotweed, the rhizome may sometimes form a longitudinal matrix at the deeper points (we have found this on about 1 in 100 sites we have dug).

Either way, Knotweed clearly demonstrates a disinclination to penetrate through certain types of clay or below ground strata’s. This seems to be the governing factor for the depth penetration, Knotweed is not found below a water table. An upward variation in a water table will cause rhizome to rot.

In made up ground, Japanese Knotweed can often be found at greater than normal depths, probably introduced to the deeper locations with contaminated fill material and then grown up through successive layers placed on top.

Japanese Knotweed can regenerate from 0.7 of a gram

Amazing? Not really.

Millions of new ornamental plants are commercially produced each year by micropropagation from very small sections of plants of far less vigour than Japanese Knotweed. There are many other rhizomatous plants and weeds which in Nature will produce new plants from small fragments in broken up soils.

A piece of buried Knotweed rhizome must from the finite energy stored in this piece, produce a stem that will reach to the ground surface. Then, as seen above, open up leaf and photosynthesis before it runs out of stored energy, where if it does, there can be no more cell division, no more growth, and imminent death of the entire entity will result.

Japanese Knotweed Myths – It can grow through concrete and foundations

It does not grow through concrete!

It does grow through existing fractures or voids in concrete and will, if ignored for years, cause heave and add to fractures. Likewise, it will take advantage of voids formed by contraction of soil and subsoil away from retaining structures in dry conditions and will then add to the pressure when wet soil expands.

Thought…how could a root barrier membrane that you can drive a garden fork through, if you tried reasonably hard, stop a plant that is claimed can grow through concrete?

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