Western hydrangea; Hydrangea macrophylla
Common ornamental shrub with large rounded flower clusters; often called 'hortensia' in some regions. The Japanese native hydrangea is 紫陽花 (あじさい), but セイヨウアジサイ specifically refers to the Western species.
Western hydrangeas are blooming in the garden.
The flower color of Western hydrangeas changes depending on soil acidity.
Refers to hydrangeas in general, especially the Japanese native species. セイヨウアジサイ is a specific Western variety.
Katakana spelling is standard for this plant name; usually written in kana.
Kanji spelling is possible but rarely used; learners will mostly encounter the katakana form.
From 西洋 (せいよう, 'Western') + 紫陽花 (あじさい, 'hydrangea'). The spelling reflects the plant's introduction from the West.