Astilbe arendsii is a widely cultivated group of hybrid astilbes valued for their balanced, compact-to-medium stature and showy plumes, typically reaching about 18 to 36 inches in height when in bloom, with some taller cultivars extending up to around 40 inches under ideal conditions, and forming clumps approximately 18 to 30 inches wide as they slowly expand. The foliage emerges in dense, mounded clumps, with compound leaves generally measuring 8 to 16 inches long overall, composed of serrated leaflets about 2 to 5 inches in length and 1 to 3 inches wide, giving a lush, fern-like texture. From this foliage rise upright flowering stems that are usually 16 to 30 inches tall, topped with feathery panicles ranging from about 6 to 14 inches long and 3 to 6 inches across, often fuller and more pyramidal than those of other astilbe species. Individual stems are typically around 1/4 to 1/2 inch thick and sturdy enough to hold the dense flower clusters upright. Mature clumps can gradually increase in diameter to around 2 to 2.5 feet over several growing seasons, supported by a fibrous root system with short, branching rhizomes that extend a few inches outward each year, contributing to the plant’s steady but controlled spread in garden settings.