Spiraea

A dense, mounded deciduous shrub from Japan and China, grown for flat-topped corymbs of tiny pink flowers that cover the foliage from June into July above oval, sharply-toothed leaves. Easy and low-maintenance in full sun, it tolerates clay, deer, erosion, and urban conditions. It is also an aggressive self-seeder that has escaped gardens and naturalized across the eastern U.S., where several states list it as invasive.

Thunberg spirea is a fine-textured, arching deciduous shrub native to East China and Japan, among the earliest spireas to flower — its slender stems are smothered in clusters of small white flowers in late winter to early spring, often before the narrow willow-like leaves fully emerge. In a sunny, well-drained border it is tough, fast to establish, and carries RHS Award of Garden Merit status. The honest catch is allelopathy: the roots and litter release cis-cinnamoyl glucosides and cis-cinnamic acid, compounds that measurably suppress germination and growth of nearby plants — avoid planting into a densely seeded wildflower mix or close-spacing with shallow-rooted perennials.