Japanese kelp

Body contents

Japanese kelp (Undaria pinnatifida) can form dense forests on any available space. It can quickly overgrow native species.

What to look for


  • green to golden-brown colour
  • blades are smooth and thin, stop well short of base
  • claw-like attachment
  • obvious strap-like midrib along length of blade
  • ruffle near base (mature plant)
  • up to 1m long, but can be 3m.
Japanese kelp
Japanese kelp

Where to look

Look in or around:

  • artificial structures, including aquaculture equipment
  • rocks
  • rocky reefs
  • vessels.

Likely habitats include:

  • shores and shallow waters, up to 20m deep.

Established pest along the east coast of Tasmania, and in Port Phillip Bay, Victoria. Mature plants found from early winter to late summer.

Similar native species

These native species look similar to Japanese kelp. They do not need to be reported.

Common kelp (Ecklonia radiata)

Japanese kelp


  • blades are rough
  • no midrib
  • no ruffle near base.


  • moderately exposed coasts
  • rocky shores and shallow waters, up to 44m deep.

Known to occur in southern Australia, from Caloundra, Queensland to Kalbarri, Western Australia

Phyllospora comosa

Japanese kelp


  • blades terminate at base
  • branches close together
  • no midrib
  • no ruffle near base
  • sawtooth edged fronds.


  • hard surfaces
  • exposed coasts
  • shallow waters, up to 20m deep.

Known to occur from Port Macquarie, New South Wales to Robe, South Australia, and Tasmania.


Report it

See something unusual? Report it. Even if you’re not sure.

If you see something you think is a pest:

  • note the exact location (screenshot your map app or enable photo geotagging on your phone)
  • take a photo (use something for size reference, like a coin or note)
  • contact your state or territory authority.


Stop the spread of marine pests

Start with these simple steps:

  • Check anchors and other equipment for tangled algae.
  • Inspect and clean your boat or yacht. Make sure you check hard to reach areas.
  • Treat the hull of your boat or yacht regularly.
  • Clean and dry your fishing and diving gear after every use.

How you can stop the spread of marine pests.

Your location

Check our marine pests map.

The map shows known pests and pests to look for around Australia.

Japanese kelp

Japanese kelp (Undaria pinnatifida)

In Australia

Features: Green to golden-brown. Blades smooth and thin, stop short of base. Ruffle near base. Strap-like midrib.
Habitat: Lower shoreline, cold ocean waters.
Affects: Native species, aquaculture.
Movement: Vessels, fisheries and aquaculture equipment.