Giant seaweed can be harvested from a seaweed patch at level 23 Farming after planting a seaweed spore. Planting gives 19 Farming experience and harvesting yields 21 experience for each giant seaweed picked. A minimum of three can be collected per patch, with the average being around 15–30 (if super or ultracompost is used) depending on one’s Farming level.
They can also be obtained by killing Lobstrosities.
When used on a fire, giant seaweed turns into six soda ash, rather than just one given by normal seaweed. When used in conjunction with the Superglass Make spell, it similarly acts as six regular seaweed and gives 60 Crafting experience per seaweed when cast. The combination of 12 buckets of sand and two giant seaweed yields 12–27 molten glass. Players may also cast the spell with three giant seaweed and 18 buckets of sand, but this frequently gives more than 27 molten glass in return, dropping the excess on the ground which makes it generally less efficient.