Byron often sneered at ‘S[outhey]’s unsaleables’,1 particularly Thalaba the Destroyer. Indeed, he states in one footnote that Thalaba will never be read until all great works are gone and forgotten. However, despite the frequent animadversions upon Thalaba and its author, Byron was not averse to appropriating ideas and images from the poem. This article examines a previously unconsidered instance of this, discussing how Byron’s presentation of the Upas Tree in Childe Harold’s Pilgrimage owes far more to Southey’s depiction of the myth than it does to Erasmus Darwin’s ‘Loves of the Plants’ (as has been previously supposed). Southey could, in turn, have been influenced by Byron’s reconfiguration of his own style and imagery, and the parallels between Childe Harold Canto IV, 126 and Southey’s later use of the Upas motif in A Tale of Paraguay Canto 3, 2 are striking.