Old School RuneScape Wiki

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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 20–30 depending on one's Farming level if super or ultracompost is used.

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, you should use 1 giant seaweed per 6 buckets of sand. 18 sand and 3 seaweed yields between 18 and 38 molten glass, with an average return of 1.3 molten glass per bucket of sand. The excess molten glass is dropped on the ground.