Nasturtiums are a type of flower that members can grow from their corresponding seeds. This requires level 24 Farming, and grants a total of 130.5 experience - 19.5 for planting, and 111 for harvesting. Only one bunch of nasturtiums is ever harvested, even if they grow without becoming diseased.
Players can pay a farmer one bunch of nasturtiums to look after growing wildblood hops.
Fully grown nasturtiums in a flower patch protects watermelons in a nearby allotment patch from disease.
Players can also use nasturtiums to make their own imp repellent by adding them to anchovy oil.
According to the book Gielinor's flora - flowers, the name nasturtium literally translates to nose-twist due to it being able to produce a pungent odor.
For an exhaustive list of all known sources for this item, see here