Seaweed spores are used in the Farming skill and require level 23 in the skill to grow them. They are planted at the seaweed patch in the underwater area of Fossil Island. Players can pay Mernia to protect this patch by giving her 200 numulites.
Once fully grown, which takes approximately 35–40 minutes, players will be able to harvest giant seaweeds. 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. Without compost, the average is around 8–16. Magic secateurs and the farming cape do not have an effect1.
Specifically, the patch has three “lives” by default, and each tier of compost adds one extra. The chance to not consume a life upon harvest is 64.1% at level 23 and 82% at level 99, using linear interpolation for other levels2.
Seaweed spores can be obtained via the following methods: