Monument to a Happy Immigrant
This structure displays three Japanese Knotweed plants. Imported by Victorian horticulturalists as an ornamental species, Japanese Knotweed is now regarded as a troublesome pest because of its rapid invasion and domination of habitats, which results in the exclusion of other plants.
They are smothering the 'Monument to Fame'; an original feature of the Tower garden (above)

images: Site before and after installation