The plant is a slow-growing perennial with dark green or brown fronds. The fronds have bubbles (technically bladders) in them. The air bladders are in pairs, and serve to keep the seaweed afloat closer to the surface of the water where there is more sunlight. They appear as brown, slippery seaweed on rocks at the ocean edge in New England, where they wash up on shore after a storm.
When heated, the bubbles burst, releasing steam.
In a pot, put first 1 inch (2 1/2 cm) of water, then put in a 5 inch (12 1/2 cm) layer of the seaweed. Or, when cooking directly over hot coals in a firepit, put a 5 inch (12 1/2 cm) layer of the seaweed on the hot coals. Whether using the pot or direct on coals method, after the first layer of seaweed, put the first layer of food, then top with another 2 inches (5 cm) of seaweed. Repeat layers of food and seaweed as needed, finishing with a 2 inch (5 cm) layer of seaweed.
For a fire-pit, you need great quantities of it, about 20 pounds (9 kg) in total. Some people like to (carefully) cover the hot coal pit with a wet canvas to keep the steam in.
Steam the food for about 1 to 2 hours.
Rockweed can be added to poaching water, then discarded when serving. In stews, it can be put in a bouquet garni for easy removal and discarding after cooking.
Tea can be made from fresh or dried Rockweed.
Rockweed can also be used as a fertilizer by just being ploughed into fields.