noun
mountain hydrangea; tea-of-heaven
A species of hydrangea native to Japan and Korea, Hydrangea macrophylla subsp. serrata (syn. Hydrangea serrata).
サワアジサイは山地の湿った場所に自生する。
Mountain hydrangeas grow wild in moist mountainous areas.
庭にサワアジサイを植えた。
I planted a mountain hydrangea in my garden.
アジサイ is the common hydrangea (Hydrangea macrophylla), while サワアジサイ is a related species with smaller, more delicate flower clusters.
From 沢 (sawa, 'mountain stream') + 紫陽花 (ajisai, 'hydrangea'), referring to its natural habitat.