Native to both Asia and North America, the astilbe plant is a flowering perennial that can, like all perennials, be divided. Common species of astilbe include false spirea and false goat's beard plants, and they display vibrant foliage that attracts many gardeners. You can divide an astilbe plant to increase the size of your garden or to rejuvenate aging plants that are showing signs of approaching the end of their lives.
Check the weather forecast. Regardless of whether you choose to divide your astilbe plants in the spring or fall, it's best to do it ahead of expected rainfall. Newly divided plants need generous amounts of water in order to thrive, and it must be provided in the days immediately following planting.
Dig a deep hole, using a wide perimeter around the astilbe that will serve as the parent or donor plant. You must be able to remove the astilbe plant from the ground in its entirety, as you're going to use its roots when you divide it.
Extract the astilbe plant from the ground, taking extra care to make sure its roots are free (and not still attached to the ground) before you pull it out.
Inspect the roots of the astilbe plant you want to divide. Locate the crown (or "heart") of the stem. If the plant is truly ready to divide, the crown should appear dead and lifeless, though dividing an Astilbe before it's reached optimal maturity shouldn't prove to be too difficult in case you harvest a plant before it's ready.
Isolate the crown, cut it away and discard the remainder of the plant.
Replant the crown at a slightly deeper depth than the stem was at when you extracted it. Use enriched soil, and water the newly planted astilbes generously.
Astilbe plants are known to be rugged, and can find ways to survive even when divided incorrectly. They are one species of plant that definitely don't require an enormous degree of precision effort to divide. Unlike many perennials, you can divide an astilbe plant in either the spring or the autumn--your choice. Other perennials must be divided in the season opposite the one in which their flowers bloom. Plan to divide your astilbe plants every two to three years or so, depending on size. If a plant reaches its full adult size in two years, you can divide it then.