Types of Invasive Species

Giant hogweed is a tall (usually 3 – 5m), biennial or perennial herbaceous plant with several hundred small white flowers in large umbrella-like flower heads up to 500mm across. Stems are green with dark-red or purple blotches and are hollow; they can be up to 100mm in diameter. Leaves are dark green in colour and grow in a rosette formation. They are jagged in appearance and spiky are the ends. Lower leaves can be up to 1.5m long. Giant hogweed looks like very large cow parsley, with a pale, swollen rootstock. It produces 20-50,000 viable seeds a year, which are penny sized and paper thin.

Spreads solely by seeds, mainly through deliberate planting, wind dispersal and in water courses.

Warning: contact with any part of this plant must be avoided as even minute amounts of sap can cause blistering of the skin following exposure to sunlight. This plant should not be touched without protective clothing.

Other negative impacts include out-competing native flora, river bank erosion and increase in flood risk

It seeds readily and seeds are buoyant so easily spreads and germinates along watercourses. It is often spread by humans sharing and dispersing seed. It out-competes native species in ecologically sensitive areas, particularly river banks. Where it grows in dense stands along river banks it can impede flow at times of high rainfall, increasing the likelihood of flooding. Die back of extensive stands over winter can leave river banks bare and exposed to erosion.


Height: 50cm to 250cm

Stems: Hairless, green to red, hollow, translucent.

Leaves: To 25cm long and to 7cm wide. Hairless, with distinctly toothed margins.

Stems and leaves, although much larger, are very reminiscent of the popular ‘Busy Lizzy’ houseplants to which it is related. The flower shape in Himalayan balsam is much more complex that that seen in ‘Busy Lizzie’.

Flowers: Large (to 4cm long) flowers varying in colour from purple to pale pink are produced between July and October. Flowers with distinct lip and spur.

Growth form: Annual, dying back after seed dispersal. Germinates in early to mid-spring followed by rapid growth.

Spreads rapidly in the wild by natural means and as a result of spread by humans. Spread is solely by vegetative means, either fragments of rhizome or stem. Negative impacts include outcompeting native flora, contributing to river bank erosion and increasing the likelihood of flooding. Can also cause significant delays and cost to development as well as structural damage (it can grow through asphalt and some other surfaces).


Note on identification: Always examine mature leaves nearer the base of the plant.

Height: To 3 meters but more often c. 1.5-2 meters.

Stems: Mature stems are hollow, in growing season green-purple splotched. Reminiscent of bamboo due to to prominent nodes. Stems profusely branched.

Twigs: Frequently display a zig-zag patterns from node to node.

Leaves: Variable in size depending on situation but generally 10-15 cms long and to 6-10 cms wide at the widest point. Leaves are quite ‘leathery’

Mostly flat base and with an abruptly tapered, distinct point to the leaf tip.

Leaf trichomes: Hair-like projections on veins on the underside of the leaves are produced in some Fallopia species, however in Japanese knotweed these are absent or so small as not to be obvious or appearing as small bumps.

Flowers: Loose sprays of creamy white to greenish or pinky white small flowers produced in Autumn.

Growth form: Perennial, living for more than two years. Tall, and creeping by dense underground rhizomes. Rapid growth obtaining full height by early summer. Forms dense clumps. Foliage dies back in mid- to late Autumn.

Winter heliotrope (Petasites fragrans), is a low-growing herbaceous plant originating in North Africa. It is established widely in Ireland being frequent along roadsides, hedgerows, woodland edges and waste ground. As apparently only the male plant that is present in Ireland, its spread is confined to vegetative means.

Winter heliotrope produces large roundish leaves up to 30cm in diameter, which are downy underneath. Its pale pink flowers, which are amongst the earliest flowers of the year appearing in December and January, have a distinctive sweet smell.

The foliage appears later in spring (though last years foliage may not dieback completely) and forms a dense carpet at about 30cm in height. Winter heliotrope has a rhizomatous root system which enables it to spread vegetatively.

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