Japanese knotweed (Fallopia japonica) is an invasive non-native species that is highly regenerative and can cause both ecological and structural damage. Japanese knotweed is an impressive species that grows rapidly to a height of 2 – 3 metres (6-10 feet) in a single season. In the spring, new growth emerges as rapidly growing soft red/purple shoots reminiscent of asparagus spears. The stems are hollow and bamboo-like and can grow as much as 2 cm per day. As the shoots grow into canes the leaves unfurl and the plant turns green, with stem diameters reaching widths of up to 40 mm (1.6 inches). The petioles, or leaf stems, are distinctive, with leaf growth on alternate sides producing a zigzag pattern in the stem.
The extensive rhizome system allows new shoots to be produced from any part of the rooting system in the spring, with a 1m² stand of knotweed capable of producing as many as 238 new shoots. This new growth forms dense thickets (which are known as ‘stands’) with new shoots remaining interlinked back to the parent plant. This means the shoots have access to the stored energy of the parent plant, allowing the new growth to possess sufficient energy to break through hard paved material. Established stands of knotweed with a large reserve of stored energy contained within the rhizome system can be vigorous enough to penetrate hard surfacing such as bitumen macadam and concrete. Rhizomes have a dark brown ‘knotty’ appearance and are bright orange inside. Snapping a fresh rhizome can be quite reminiscent in both sound and colour of snapping a carrot. The actual roots produced by the rhizome are fine, white and threadlike.
Elongated clusters of creamy-white flowers appear towards the end of August and results in the production of seeds, which are currently non-viable in the UK. During late autumn/winter senescence occurs following pollination, when all of the moisture and nutrients are pulled down by the plant from the top growth into the crowns and rhizomes. Consequently, the canes die off and turn brittle. The crown, rhizome and root below ground remain dormant but alive throughout the winter. Rhizomes have been recorded surviving in frozen soil for a number of months. The crowns are hardier still and can survive composting and drying out. The stems die quickly after the first frost, leaving an unsightly mass of dead vegetation. The orange to brown coloured, woody, stems persist erect throughout the winter and the slow decomposition of leaf litter forms a deep organic layer preventing native seeds from germinating.
Reproduction is primarily by vegetative regeneration of rhizomes and fresh stems. In some cases the rhizome system could extend from a parent plant up to 7 metres (23 feet) laterally and up to 2-3 metres (6-10 feet) deep, although it is more common to find much smaller distances – unless particularly soft soil conditions or high bedrock encourages greater spread. Very small fragments of rhizome (as little as 0.7 gram – about the size of a small fingernail) can give rise to new plants. Fresh stems produce shoots and roots when buried in a soil medium or floated in water. The plant thrives where there is an available water supply for it to tap into and will be at its most prolific along open stream courses. Stem fragments in water may produce viable plants within 6 days. Japanese knotweeds’ ideal environment is in open damp positions with full sun, but it can grow with no discernable signs of stress in a variety of soil types, no matter how poor, with pH values ranging from 4.5 to 7.4. Common locations to find knotweed are railway lines, riverbanks, roads, footpaths, graveyards, disturbed/derelict sites or anywhere it has been dumped, dropped or deposited.
It 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. As landfill charges continue to increase, it is a sad thing to acknowledge that fly-tipping of knotweed is likely to continue.
It has been estimated that the discovery of Japanese knotweed on a development site can add around 10% to the development budget due to the cost of treatment and disposal. Left uncontrolled, it can quickly dominate a garden and spread to adjoining properties. It can also spread into foundations and boundary walls. Common problems caused by knotweed are:
– Damage to paving, tarmac, driveways, etc
– Damage to walls, foundations, etc. (knotweed has been discovered growing under suspended floors with minimal light)
– Shading of light to windows and gardens due to height of growth
– Damage to flood defence structures
– Damage to archaeological sites
– Displacing of native flora and fauna
– Reduction in land and property values
– Increased risk of flooding through dead stems washing into river and stream channels
– Increased risk of soil erosion and bank instability following removal of established stands in riparian areas
– Restriction of access to riverbanks for bank inspection and amenity use
– Accumulation of litter in mature stands
– Aesthetically displeasing
To ignore Japanese knotweed can prove costly in the long run and incorrect treatment or allowing it to spread beyond site boundaries – risks putting you in breach of the law.