Author: Mike Clough

Date Posted: Wednesday 29th April 2015

I’m often asked about sex, generally it’s me… asking myself when I might get some…but on other occasions its people asking me about the reproductive capabilities of Japanese Knotweed?

How can all of these plants have spread across the UK without the ability to produce viable seed?

They produce flowers, they seem to produce seed…what’s going on?

Japanese Knotweed, Giant Knotweed and its hybrids and back crosses have two different sexes, however it is a rather unusual sex system – known as gynodiecy – where you have female and hermaphrodite individuals.

The hermaphrodite individuals produce good pollen and can also produce small quantities of viable seed when cross pollinated.

This sex system is thought to have originated from a mutation in a hermaphrodite species which knocks out the pollen production in affected individuals to produce female individuals.

It is accepted that only a female clone of Japanese Knotweed is found in Britain and thus the plant is unable to reproduce itself by seed. It is accepted that any seed found on these plants is the result of pollination by related species.

It has also been found (Bailey 1989) that the hermaphrodite plant of F.sachalinensis and F. x bohemica are self – incompatible, that is they are unable to form seed without an additional source of pollen.

Japanese Knotweed in the UK occupies two main types of habitat – one natural and one created by man’s intervention.

It establishes well along river banks and water bodies where it shows the characteristics of being native – the plant spreads by propagules, water borne rhizome or viable stem fragments. During flooding events the plant spreads by fragments being carried downstream by the water, then being deposited lower down the watercourse where it establishes and spreads creating new infestations.

The other main area where growth occurs are the man-made habitats of our poorly managed roadsides, railways and areas of derelict industrial land. There are also large areas where Japanese Knotweed has been planted on purpose, the Victorian gardeners also often used the plant for its perceived horticultural value. There is also some evidence of railway and river authorities using the plant to stabilise embankments.

Where Japanese Knotweed grows adjacent to residential areas – the plant is often cut back by home owners who do not realise that the cuttings they throw onto the compost have the ability to grow into new plants – thus inadvertently making the problem worse.

Once the plant becomes established the new stems elongate rapidly and in a few weeks will have produced a dense canopy of green which excludes any light from reaching any of our native flaura. The plant will flower in late August and September – and in areas where there is a pollen source – large amounts of seed may be produced. These seeds are inevitably hybrid and although they can be viable under laboratory conditions – they only rarely germinate in –situ.

So whilst it doesn’t have a fantastic sex life, the plants reproductive capabilities are second to none…

Mike C

With thanks to John Bailey, University of Leicester.

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Mike Clough

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