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Wireweed - Invasive Species Information

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Reproduction: Both sexual and asexual. Reaches sexual maturity in the summer.


The species is monoecious, meaning an individual is capable of producing male and female gametes. Its cycle of development is monogenic (i.e. only one generation is present during its life cycle).

Wire weed stand

Wireweed - Sargassum muticum

At the level of fertilization: male gametes are dispersed in seawater while female gametes remain in the receptacle where fertilization takes place.


Development is also done at the receptacle and then once at the stage of seedling, the latter is detached for fixation on a new support and form a new individual. This species can also reproduce asexually. But this has never been observed in temperate environments.

Wireweed is composed of two distinct parts:

  1. a perennial part, which contains the holdfast and one or more short main axes;

  2. an annual part: the secondary axes, which develop on the main axis, whose growth is unlimited and the size is variable.


There are three types of ramifications: laterals with foliaceous expansions called fronds, laterals with fronds and aerocysts and laterals with fronds, aerocysts and reproductive organs called receptacles.


In winter, only the perennial part persists (5 cm). In summer, the lateral part is in maximum development of 2–3 meters up to 10 meters.

How To Identify Wireweed?

Frond: Green/Brown

felty, fibrous discoid holdfast up to 1.5 cm in diameter

Blades: linear-lanceolate on the basal portion of stipe, to 10 cm long, toothed with the upper stipe narrow, often only 4 mm long

Wireweed - Sargassum muticum ID Guide
Wire weed

Wireweed - Sargassum muticum ID Guide

Wireweed - Sargassum muticum

Why Is Wireweed A Problem?

Wireweed's proliferation is significant, especially during the summer, which causes economical and ecological problems.

Ecological: Because of its large size and dense ramifications, Wireweed forms a screen within the water column preventing the penetration of light.


It also captures the nutrients depriving availability to other species such as phytoplankton. However, can also serves as shelter and protection for fish larva or crustaceans as well as laying support for cuttlefish.


Economical: Fixes itself to the shells of oysters creating problems in shellfish farming through increased manual work to eliminate the algae.


In addition, it can wrap around farming structures, requiring additional maintenance. It also gets entangled with the propellers of the boats.

What Is Wireweed - (Sargassum muticum)?

Habitat: Aquatic, marine
Distribution in Ireland: Abundant coastal areas

Status: Established
Family name: Sargassaceae

European Communities (Birds and Natural Habitats) Regulations 2011 non-native invasive plant species A-Z (Updated 2017)

There are currently 35 invasive plant species listed in the European Communities (Birds and Natural Habitats) Regulations (annex 2, Part 1)...


Click on a species from the following list to find out more regarding non-native species subject to restrictions under Regulations 49 and 50.

  1. American Skunk-CabbageLysichiton americanus

  2. Brazilian Giant-RhubarbGunnera manicata

  3. Broad-Leaved RushJuncus planifolius

  4. Cape PondweedAponogeton distachyos

  5. Cord-GrassesSpartina (all species and hybrids)

  6. Curly Waterweed - Lagarosiphon major

  7. Dwarf Eel-GrassZostera japonica

  8. FanwortCabomba caroliniana

  9. Floating PennywortHydrocotyle ranunculoides

  10. Fringed Water-LilyNymphoides peltata

  11. Giant HogweedHeracleum mantegazzianum

  12. Giant KnotweedFallopia sachalinensis

  13. Giant-RhubarbGunnera tinctoria

  14. Giant SalviniaSalvinia molesta

  15. Himalayan BalsamImpatiens glandulifera

  16. Himalayan KnotweedPersicaria wallichii

  17. Hottentot-FigCarpobrotus edulis

  18. Japanese KnotweedFallopia japonica

  19. Large-Flowered WaterweedEgeria densa

  20. Mile-a-Minute WeedPersicaria perfoliata

  21. New Zealand PigmyweedCrassula helmsii

  22. Parrots FeatherMyriophyllum aquaticum

  23. Red AlgaGrateloupia doryphora

  24. RhododendronRhododendron ponticum

  25. SalmonberryRubus spectabilis

  26. Sea-Buckthorn Hippophae rhamnoides

  27. Spanish Bluebell Hyacinthoides hispanica

  28. Three-Cornered LeekAllium triquetrum

  29. WakameUndaria pinnatifida

  30. Water ChestnutTrapa natans

  31. Water FernAzolla filiculoides

  32. Water LettucePistia stratiotes

  33. Water-PrimroseLudwigia (all species)

  34. WaterweedsElodea (all species)

  35. WireweedSargassum muticum

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