Physical Description

  • Not a plant, but a species of brown algae.
  • Tall vertical alga/seaweed, green-brown in coloration.
  • Each ridged leaf blade has a gas-filled pod at the base that floats, helping keep the kelp vertical.
  • No roots, instead attached to the rocks by a structure called a holdfast.
  • Can reach heights of 100 feet tall.
  • When it reaches the sea surface, it continues to grow horizontally.


  • From Santa Cruz, California to Turtle Bay, Mexico.
  • Along the temperate coasts of South America, New Zealand, and Australia.


  • Found in cold, clear saltwater.
  • Found on rocky reefs up to 100 feet below the sea surface.


  • Reproduction occurs through alternation of generations. 
  • The large kelp forms spores that are released into the water. 
  • These spores become microscopic female and male stages of kelp. 
  • When the male stage (sperm) fertilizes the female stage (egg) in the water, it becomes a tiny plant called a sporophyte. 
  • This sporophyte anchors itself to the bottom using its holdfast. 
  • This sporophyte grows into the large kelp frond that can produce spores again. 


  • Uses the sun to get its energy through a process called photosynthesis. 


  • Bristle worms, scud, prawns, snails, and brittles stars eat the holdfasts.
  • Opaleye and halfmoon fish eat kelp blades.
  • Purple sea urchins can devastate a kelp forest if left unchecked.

Interesting Facts

  • Giant kelp is the largest seaweed in the world and the largest of all marine algae..
  • It is one of the fastest-growing species in the world, and can grow up to two feet in a day! 
  • When a lot of giant kelp is together, it creates a dense forest that is home to many species of fish, sharks, lobsters, squids, otters, sea stars, and other creatures. 

Sources: Oceana; Monterey Bay Aquarium; National Ocean Service; California Department of Fish and Wildlife; US National Park Service; University of Southern Florida

Photo: Howard Hall

Look at live kelp in person anytime on Birch Aquarium’s Kelp Cam!