Astilbe, commonly known as False Spirea, is a popular perennial valued for its feathery plumes of colorful flowers and attractive, fern-like foliage that thrives in shaded garden spaces. Although often sold as bare-root material, Astilbe does not grow from a true bulb, such as a tulip or daffodil. Instead, they are propagated from crowns or bare-root divisions, which are rhizomes containing the root system and dormant growth buds. This distinction is important because the planting technique for a crown differs significantly from that of a bulb. Understanding the correct planting depth for these crowns is paramount for successful establishment and preventing issues like rot or desiccation.
Preparing the Planting Site
Astilbe plants flourish in an environment that mimics the forest floor, preferring light to moderate shade where they are shielded from intense afternoon sun. Direct midday heat can scorch the foliage. The ideal soil must be consistently moist yet possess excellent drainage, preventing the roots from sitting in standing water which leads to rot.
Amending the planting area with rich organic material, such as compost or aged manure, is beneficial before planting. Astilbe performs best in fertile, humus-rich soil, and this amendment helps retain moisture while improving soil structure and drainage.
The best time to introduce Astilbe divisions is during the spring or fall, allowing the plant to establish its root system before summer heat or winter dormancy. The planting hole should be dug wide enough to easily accommodate the entire root mass, typically two to three times the width of the root ball. Loosening the surrounding soil is necessary to encourage the roots to spread outward, which is more important than digging a deep hole.
Proper Planting Depth and Technique
Ensuring the crown, the point where the roots meet the emerging stems or buds, is positioned correctly is key for Astilbe survival. When planting bare-root divisions, the crown should be placed level with the surrounding soil surface or covered with no more than one inch of soil. Planting too deeply can lead to the crown rotting, as new growth buds become smothered. Conversely, planting too shallowly leaves the crown and upper roots exposed, making them vulnerable to drying out.
To plant, gently spread the roots out in the prepared hole, fanning them downward and outward to encourage rapid establishment. The small growth buds, often referred to as “eyes,” should face upward toward the sky. After positioning the crown, backfill the hole with the amended soil, carefully working it in around the roots to eliminate air pockets. Firm the soil gently with your hands; avoid heavy compaction, which would impede root growth and drainage.
Essential Steps for Establishment
Immediate and consistent watering is necessary following planting to settle the soil around the roots and begin the establishment process. Astilbe has a low tolerance for drying out, so the soil must be kept uniformly moist, especially throughout the first growing season. Deep watering is more effective than shallow sprinkling, encouraging the development of a robust, deeper root system.
Applying a two-inch layer of organic mulch, such as shredded bark or leaf mold, immediately after planting is an effective practice. Mulch serves multiple functions, primarily conserving soil moisture by reducing evaporation and moderating the soil temperature. This layer also protects the shallowly planted crowns from temperature fluctuations and helps prevent them from being pushed out of the soil during winter freeze-thaw cycles. Spacing the plants between 18 inches and three feet apart, depending on the variety, will ensure adequate air circulation and prevent overcrowding as the clumps expand.