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Management and control of Japanese Knotweed

Photo of japanese knotweedJapanese Knotweed (Fallopia japonica) first introduced in around 1825 is a highly invasive, perennial plant. It forms dense stands 2-3m tall which can damage paths, roads and buildings, smothering all other plant life. It spreads by its extensive underground stem (rhizome) network.

The invasive stem network can reach depths of two metres and grow more than seven metres away from the parent plant. As little as 0.7g of rhizome or freshly cut stem material can produce a viable plant. Although Japanese Knotweed does not set seed, some hybrid plants have produced viable seed in laboratory conditions.

It is an offence to cause Japanese Knotweed to spread under the Wildlife and Countryside Act 1981. Any waste material from cuttings is classified as controlled waste and should be disposed of according to the Environmental Protection Act (Duty of Care) Regulations 1991.

Frequent cutting can slow down encroachment but will not eradicate the weed. Deep roots and rhizomes make it impractical to remove the weed by excavating affected soil. Systemic herbicides have been found to offer the most effective method of control.

For more information on this plant, visit the CABI website.

How to control it

In October to late February, cut dead stems of the weed and dispose of them by drying and burning. The weed can regrow from a tiny fraction of the plant and spreading it about with a strimmer would only heighten the problem as the weed could regenerate in a number of locations. In late July to September, apply approved glyphosate-based herbicide to all shoots when the weed has reached its maximum size (2-3m tall). To get rid of Japanese Knotweed entirely, a number of treatments of herbicide will be required.