Spenseweed is a plant of unspecified nature.
According to The Lore and Legends of Quendor, it is a common roadside plant with healing virtues. Although safe to eat, spenseweed is most effective when applied directly to wounds as a salve. This publication also advises us to avoid the cheap commercial preparations, which may contain artificial coloring and preservatives. A plant matching this description has been found in the Westlands, growing among the ruins of Pheebor, and near the village of Grubbo-by-the-Sea.
Early editions of the Encyclopedia Frobozzica called spenseweed "a deep-rooting underwater plant." Once established, it can usually only be removed by using certain spells of wilting. The 883 GUE edition stated that the misconception of spenseweed as a roadside weed is grossly inaccurate.
The underwater spenseweed might simply be another name for the morgia plant.