Japanese knotweed (Fallopia japonica)

Japanese knotweed is an introduced weed which is aggressive and invasive, rapidly colonising river banks and areas of open ground. It is a perennial plant which grows from rhizomes in the spring to a height of about 3 metres in mid summer with stiff bamboo-like stems. In the autumn, the leaves and stems die back but remain upright. The plant produces masses of white flowers in summer but is not thought to produce viable seeds. The plant overwinters as rhizomes which grow rapidly and have been known to grow through cracks in rock, concrete and tarmac. Cut or broken stems lying on damp soil can root and form new plants. This plant can spread to new sites by the transport of topsoil, by floating downstream onto river banks and by being caught on footwear and walked to new areas.

Under the 1981 Wildlife and Countryside Act it is an offence to allow Japanese knotweed to grow in the wild. Waste material when digging should be disposed of according to the Environmental Protection Act 1990 (Duty of Care) Regulations, and a licence is required.

Giant hogweed (Heracleum mantegazzianum)

Giant hogweed is characterised by its size and may grow to between three and five metres in height. Individual plants can take up to four years to reach this height and flower. Other than in size, it bears a close resemblance to Cow Parsley and Hogweed. Giant hogweed has a reddish purple stem and spotted leaf stalks, with fine spines that make it appear furry. The leaves may expand to 1.5 metres in width with flower heads commonly 250mm in width.

Giant hogweed contains a substance within its sap that makes the skin sensitive to ultra violet light. This can result in severe burns to the affected areas, producing swelling and severe, painful blistering. Large, watery blisters usually appear 15 to 20 hours after contact with the sap and exposure to sunlight. Damaged skin will heal very slowly, leaving residual pigmentation that can develop into Phytophotodermatitis - a type of dermatitis that flares up in sunlight for which there is no straightforward treatment.

Contact between the skin and the sap of this species occurs either through brushing against the bristles on the stem (as with a stinging nettle) or breaking the stem/leaves. It is essential to wear long sleeved clothing when working in areas where giant hogweed is present. In the event of contact with the sap of this plant, the skin should be covered to reduce the exposure to sunlight and washed immediately with soap and water.

Giant hogweed is an aggressive competitor, which is able to out-compete native plant species, reducing the amount of suitable habitat available for insects, birds and mammals. Giant hogweed produces approximately 1500 seeds per flower head in late summer. These seeds can remain inactive in the soil for several years. The movement of soil polluted with giant hogweed seeds must be carefully controlled to prevent the spread of the plant.

Due to the density of stands of giant hogweed and the safety implications, footpaths and other amenity areas can be rendered unusable by the presence of this species.

When giant hogweed dies back during the winter months, it leaves bare ground. This can lead to an increase in soil erosion on riverbanks and steep slopes.

Other invasive species

These include Himalayan balsam, ragwort, bracken and others.

Japanese knotweed and giant hogweed are the only two flowering plants currently listed in Schedule 9, Part II of the Wildlife & Countryside Act 1981 under which it is an offence to "plant or otherwise cause to grow in the wild."