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Japanese Knotweed

Life Cycle & Ecology

Japanese Knotweed in Europe occupies two main types of habitat, one natural and one man-affected.  It is particularly well-suited to growth along riversides, where it gives every appearance of being native, and along which it is able to spread naturally by water-borne rhizome or stem fragments.  The other main habitat is in man-managed areas such as roadsides, railways, derelict industrial land and anywhere else it has been discarded.  It also persists in gardens where  it was initially planted in Victorian times for its perceived horticultural value.

This is the typical appearance of Japanese Knotweed in Britain. Large monocultures spreading laterally along water courses by rhizome extension. The leaves of such plants are normally untouched by our native invertebrates This shows Japanese Knotweed in its natural habitat – but this is actually Wales!
 It is well known that Japanese Knotweed can spread from rhizome fragments, but the role of regeneration from stems in water is less well-known - see Brock and Child references  Japanese Knotweed is commonly found along railway lines - this is F.x bohemica in Bohemia!  Does your garden look like this? - if so hard luck! One of the main problems of control is that in such circumstances the plant is invading from elsewhere making control complicated

Stout green annual stems appear in the spring and grow rapidly. In colonies not subject to scouring by water, an appreciable layer of dry dead leaves can accumulate. In areas scoured by water, colonisation by woodland species such as bluebell can occurIn the spring the new stems elongate rapidly, and in a matter of weeks they have put out a green canopy that excludes most of the light.  Flowering in Britain, occurs in late August and September, and in places where there is an adjacent pollen source, large amounts of seed may be produced.  Such seed is inevitably hybrid, and although viable under controlled conditions, it rarely germinates in situ.

Flowering occurs in the late Summer and Autumn. This is a male plant in JapanThe above ground growth is very frost sensitive, and is killed by the first autumn frost, the characteristic reddish-brown stems persist throughout the winter, and provide some protection for the emergent shoots in spring.

In appropriate conditions huge amounts of viable seed may be found on Japanese Knotweed plants in Britain, this example is from Dolgellau, Wales Although much viable seed is found in Britain - it has rarely been seen to germinate in situ - here is an F1 F. x bohemica seedling in Wales in early May The green stems are killed by the first frost, but the distinctive reddish brown dead canes are quite persistent

 

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