Ascophyllum nodosum

This species of rockweed is a long-lived perennial plant, and studies have documented that individuals can live for decades.
Photo Credit: Jessica Muhlin
Ascophyllum nodosum
Common Name: rockweed
Other Common Names: knotted wrack
Plant Functional Group: Algae
Class > Order > Family: Phaeophyceae > Fucales > Fucaceae
What does the species look like?

Ascophyllum nodosum, sometimes called knotted wrack, is a member of a brown seaweed group called “rockweeds”. It forms single bladders in long, strap-like fronds, which hang downwards, draping sheltered intertidal rocks. A number of fronds grow from each holdfast (where the seaweed is attached to the rock).
Where is the species found?
States & Provinces
CA, CT, DE, MA, MD, ME, NC, NH, NY, RI, VA
Distribution

Abundant in the mid-intertidal zone on sheltered, hard substrate. A hardy organism, A. nodosum can be found in varying environments and is tolerant of fluctuating temperature and salinity levels.
General Phenology and Life History

Each individual is either male or female. Reproduction appears to be correlated with temperature—the onset of reproduction generally occurs when water temperatures are 6?C (43?F), and peak reproduction at 10?C (50?F). Like trees and other organisms, A. nodosum provides a record of its annual growth, which can be used to see which years were good or bad for growth. Each spring, individuals lay down a new vesicle (airbladder) at the tip of each frond. Measuring the distance between vesicles provides what the growth rate is from one growing season to the next.
Which phenophases should I observe?
Do you see...?
Leaves New side branches
One or more new sides branches with smooth tips are visible on the plant. Side branches are considered "new" until they develop a receptacle, or until they reach a similar width and tough texture as the mature shoot on which they are growing. Do not include side branches with ragged tips, where a receptacle has recently detached.

How many new side branches are present?

  • Less than 3
  • 3 to 10
  • 11 to 100
  • 101 to 1,000
  • More than 1,000
Flowers Receptacles
One or more receptacles are visible on the plant. Receptacles are oval pods that are initially flat and become inflated, changing from olive green to orange or dark brown, sometimes becoming torn.

How many receptacles are present?

  • Less than 3
  • 3 to 10
  • 11 to 100
  • 101 to 1,000
  • More than 1,000
Flat receptacles
One or more flat, olive-green receptacles are visible on the plant.

What percent of all receptacles on the plant are in this stage?

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
Smooth inflated receptacles
One or more inflated, smooth, olive-green receptacles are visible on the plant.

What percent of all receptacles on the plant are in this stage?

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
Dotted inflated receptacles
One or more inflated, yellow or orange receptacles studded with small, raised dots are visible on the plant. Do not include receptacles that are torn or ruptured.

What percent of all receptacles on the plant are in this stage?

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
Torn receptacles
One or more torn or ruptured, orange or dark brown receptacles are visible on the plant.

What percent of all receptacles on the plant are in this stage?

  • Less than 5%
  • 5-24%
  • 25-49%
  • 50-74%
  • 75-94%
  • 95% or more
What do these phenophases look like?

There is currently no photoguide available for this species. If you'd like help us create one, use the guidance document and species template provided here. Then send it via email to education@usanpn.org when it is complete.

ITIS information for rockweed (ITIS TSN: 11331 )