Japanese knotweed

Japanese knotweed and other knotweeds

Japanese knotweed in flower
Elaine O'Riordan
Japanese knotweed flowers and leaves
Elaine O'Riordan
Knotweed canes in winter
Elaine O'Riordan
Sprouting rhizome
Elaine O'Riordan
Knotweed species leaves compared
National Biodiversity Data Centre

About

Fallopia japonica or Japanese knotweed is known as one of the 100 worst invasive species in the world. It was first introduced to Ireland in the late 19th century for ornamental use. However, it escaped into the wild, and has spread throughout the country. This weed is now causing problems in many parts of County Galway.

How to recognise it?

The hollow stems of the plant are purple or green, and resemble bamboo canes. The leaves are bright green, shield or heart shaped and grow in zig-zags along the branches. It produces long strings of creamy-white flowers late in the summer. Underground the extensive rhizomes (underground stems resembling roots) are bright orange when cut open. In winter the plant dies back leaving distinctive brown stems.

There are four species of invasive knotweeds common in County Galway. Japanese, Giant and Bohemian knotweed are all very similar in appearance with the main difference evident in the size of the leaf and the plant itself. Himalayan knotweed is more distinguished in having a longer leaf shape and straighter stems.

All the invasive knotweed species are similar in behaviour and require the same treatment.

Why is it a problem?

Knotweed grows very vigorously and quickly forms dense thickets that can take over gardens, and wildlife habitats. It can rapidly spread along roadsides, riverbanks, railway lines and derelict sites.  When the plant dies back in winter it leaves bare ground which is more susceptible to erosion. In urban environments, it has even been known to grow through walls, tarmac and concrete!

How does it spread?

Although Japanese knotweed produces flowers, the plants in Ireland are infertile, and cannot produce seed. Instead, the plant spreads by growth of its rhizomes and by fragmentation. The rhizomes can grow up to 7 m out from the parent plant and up to 3 m deep. In addition, any cut or broken fragments of the root or stem will sprout to form new plants. The plant is most commonly spread by the movement of soil from infected areas, or by the mowing or flailing of plants in lawns or hedgerows.

Legislation

Japanese knotweed and its relatives are listed on the Third Schedule of the European Communities (Birds and Natural Habitats) Regulations, 2011 (S.I. No. 477 of 2011) which makes it an offence under Regulation 49 to plant, disperse, allow or cause to grow these plants in the Republic of Ireland.

Regulation 50 makes it an offence to import, buy, sell, breed, reproduce or propagate, advertise, offer or export for sale, publish a price list, transport or distribute any species on the Third schedule. (This regulation is not in effect pending Ministerial notice)

A licence may be obtained in certain circumstances to permit these activities.

What can you do?

  • Report the location of the weed to Galway County Council (see below) or the National Biodiversity Data Centre http://www.biodiversityireland.ie/projects/invasive-species/submit-sightings/
  • Inform landowners that have knotweed on their property.
  • Do not move or dig soil within 7m of an infestation until sure the plant is dead for some time.
  • Knotweed should never, ever be cut, flailed, mulched or strimmed.
  • Do not dump or compost living remains of the plant, instead can kill them using fire, sunlight or by wrapping them in plastic.
  • Control: Use the most suitable method to remove the infestation from your property (see below).

Control

Manual Control

This is not generally recommended for Japanese knotweed, as cutting or disposing of the plant may cause it to spread further. It may be necessary on development sites to excavate the knotweed completely and dispose of by bunding and deep burial onsite or offsite at a licenced waste facility.

Chemical Control

Herbicides usually provide the best and most cost effective method for Japanese knotweed control. A systemic herbicide that will kill the roots of the plant is needed and a Glyphosate based formula is recommended. The weedkiller can be applied in 2 ways:

Foliar spray: Using the manufacturers recommended concentration, leaves should be sprayed until the point of ‘run-off’ i.e well coated but not dripping. A long lance sprayer is useful to ensure coverage of large plants. Spraying should only be carried out in cool, dry, calm weather.

Stem injection: The hollow stems are injected with a small amount of concentrated herbicide using a specialised injector gun. This method is recommended for sensitive sites such as around waterbodies.

Please take care when using herbicides, and carefully consider your impacts on nearby land or waterways. Read the label carefully, and always use appropriate safety equipment.

Timing

In Ireland, herbicide treatment of knotweed is most effective when applied in late summer to autumn, after flowering. Very large stands which would be difficult to cover when fully grown may need two treatments in the season. The first spray should be in May which will make the area accessible for a second spraying in September.

It should take about 3-4 years of annual treatments to clear the worst of the infestation, after which you should perform annual checks to control any new growth.

Pesticide regulations

All Plant Protection Products should be used in accordance with the product label and with Good Plant Protection Practice as prescribed in the European Communities (Authorization,Placing on the Market, Use and Control of Plant Protection Products) Regulations, 2003 (S.I. No. 83 of 2003). Herbicides/pesticides authorised for professional use may only be used by persons who have completed the prescribed training and hold the relevant specified certificate(s). http://www.pcs.agriculture.gov.ie/sud/

Further Information

Further Information about Japanese knotweed and other invasive species can be found at:

http://biodiversity.galwaycommunityheritage.org/

http://www.biodiversityireland.ie/projects/invasive-species/

Contact Details

To report Japanese knotweed or other problem species, contact Galway County Council

Phone: 091-509309

Email: invasivespecies@galwaycoco.ie

This page was added on 01/11/2016.

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