Astilbe is a genus of perennial plants, native to North America, and some parts of Asia. Their dense blooms and attractive foliage provide a bright, colorful appeal to shaded locations in many gardens. Astilbes grow best when divided every 3 or so years. Dividing the roots of established plants eliminates the formation of overcrowded clumps, and promotes the healthy growth of divided plants.
Step 1 – Prepare the Planting Spot
Since astilbe plants thrive in partial to full shade, choose a suitable planting spot in your backyard. Astilbes require acidic soil, with a pH ideally below 6.0. Amend the soil in advance, as required. Addition of sulfur pellets is useful in increasing the acidity of the soil. Add some mature compost and peat moss to the soil. This will help improve drainage, which is essential to the healthy growth of astilbes. It also aids better moisture retention. Addition of compost will also increase the content of essential organic matter in the soil. Mix thoroughly and let the soil settle for a few weeks.
Step 2 – Digging out the Root Ball
You can divide astilbe plants in the spring, before they have started blooming, or in late summer, after they have finished flowering for the season. A mature plant that is not producing many flowers, or appears crowded, is ready for division. Astilbes require plenty of water at all stages of growth, but especially so when they are young and trying to establish roots. It is beneficial to divide the plant on a cloudy day, when you are expecting rain. Before digging, water the soil thoroughly, so that it will be soft, which will make it easier to remove the plant. Using a garden trowel, dig the area around the plant, trying to dislodge the plant. Force a spade under the plant to remove it. Gently remove the plant with the entire clump, making sure the roots are intact.
Step 3 – Divide the Roots
To divide the roots, gently pull clumps away from the central crown. For successful replanting, each division must look healthy. Make sure each division has plenty of healthy roots and shoots. Discard any divisions that show signs of rot or lack of vigor. The central crown can also be discarded.
Step 4 – Plant the Divisions
To plant each division, dig out the soil so that there is ample space for the roots. You can plant each division separately, or a few together to form a clump-like effect which will be more visually pleasing once the plant grows. Place the divisions slightly deeper than they were previously, and backfill with soil. Firm the plant in place and water thoroughly. Mulch the planting spot with a couple of inches of bark dust, wood chips, of leaf mold. As the plant establishes itself, provide it with ample water as required.