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Japanese Kelp - Invasive Species Information

Reproduction: prolific spore production and its ability to foul structures both in its macroscopic and microscopic forms. It spreads rapidly and geographically distant populations can appear in a short space of time. It is transported to new areas by human activities in both its highly visible sporophyte, or as the microscopic gametophyte and spore stages in its life cycle.

Wakame - Undaria pinnatifida

Japanese Kelp stand

Japanese Kelp, is a highly invasive seaweed that can grow to 3m in length and has green-brown fronds. It grows in sheltered temperate waters, forming dense forests at depths of up to 15m and can quickly displace native habitats.

This seaweed is green and golden-brown in colour with a lighter coloured stem and a branched holdfast (for attachment to the substrate). The stipe has very wavy edges, giving it a ridged appearance. Undaria has a broad, attened leaf shaped blade with a distinct midrib and wavy margins. 

Wakame - Undaria pinnatifida
Wakame - Undaria pinnatifida

How To Identify Japanese Kelp?

Fonds: Green-brown fonds can grow up to 1-3m tall that stop short of base, Midrib up to 3cm wide with frilly hold-fast base

Japanese Kelp stand

Japanese Kelp leaves

Japanese Kelp - Undaria pinnatifida ID Guide

Japanese Kelp - Undaria pinnatifida ID Guide

Why Is Japanese Kelp A Problem?

Can be highly invasive and grow rapidly into dense beds.

Overgrows and excludes native algal species preventing biodiversity and altering marine food chain. as an opportunistic species, takes advantage of areas disturbed by human activities or grazing by herbivores such as urchins and is also known to grow in areas not normally occupied by native seaweeds where it may have a high degree of impact. 

What Is Japanese Kelp - (Undaria pinnatifida)?

HabitatAquatic, marine (costal areas).
Distribution in Ireland: Present around coastal waters

Family name: Alariaceae

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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