A new and distinct cultivar of Hydrangea plant named ‘；H218911’；, characterized by its upright and uniformly mounded plant habit； vigorous growth habit and rapid growth rate； freely branching habit with strong, thick and sturdy stems； freely and uniformly flowering habit； mophead-type inflorescences with numerous light red purple, yellow green and greenish white-colored sterile flowers； and good postproduction longevity.
