Episode 25 of The Trigan Empire
The Giant Plant
aka The Giant Rallus
The Rallu Invasion (the Collection)
De reuzen-ibisplant (Dutch title)
The giant ibis plant (Dutch title, translated)
The Giant Plant was originally published in 7 installments in issues 474 (13 February 1971) to 480 (27 March 1971) of Look and Learn magazine.
It is reprinted in the Don Lawrence Collection Volume 7, The Rallu Invasion.