The plant & the problem

Japanese knotweed, a rhizomatous perennial was introduced from Asia to Europe in the mid-nineteenth century as an ornamental and fodder plant. It is an impressive species which grows to a height of 2-3 meters, with bamboo like stems, arching branches and clusters of creamy white flowers appearing late in the season. The orange to brown coloured, woody, dead stems persist erect throughout the winter and new shoots, produced from the extensive rhizome system, grow up amongst these the following spring to form dense thickets. The dead stems and leaf litter decompose very slowly and form a deep organic layer which prevents native seeds from germinating. Once present at a site, Japanese knotweed increases in area very rapidly and soon forms monoculture stands.

Reproduction is primarily by vegetative regeneration of rhizomes and fresh stems. The rhizome system may extend from a parent plant up to 7 meters laterally and to a depth of 2 metres. Very small fragments of rhizome (as little as 0.7 gram) give rise to new plants. Fresh stems produce shoots and roots when buried in a soil medium or floated in water. Stems in water may produce viable plants within 6 days.

Japanese knotweed thrives on disturbance and has been spread by both natural means and by human activity. In riparian areas, high water flows disperse fragments of the plant downstream where new colonies form. In the past, fly-tipping and transportation of soil containing rhizome fragments have been a major cause of spread, particularly in the urban environment.

Specific problems caused by Japanese knotweed are:

* Damage to paving and tarmac areas
* Damage to flood defence structures
* Damage to archaeological sites
* Reduction of biodiversity through out-shading native vegetation
* Restriction of access to riverbanks for anglers, bank inspection and amenity use
* Reduction in land values
* Increased risk of flooding through dead stems washed into river and stream channels
* Increased risk of soil erosion and bank instability following removal of established stands in riparian areas
* Accumulation of litter in well established stands
* Aesthetically displeasing
* Expensive to treat

We here at The Invasive Plant Company are usually called upon to remediate sites from Japanese Knotweed, the most commonly known species of invasive plant.

But among other species, we are also called upon to remediate Giant Hogweed.

Below are two pictures of the same infestation on a site we have been employed to remediate.

With the onset of milder weather and longer days, the fitness regimes of some members of the LK Group staff have come out from the mothballs and the sweat has once again begun to bead on some foreheads!

Conor Leyden, Invasive Plant Company director is scheduled for a big day in May.