Asters are popular perennial flowers that provide vibrant color to the garden, especially as many other plants fade in the late season. Like many long-lived perennials, these plants benefit from routine maintenance known as division. Division involves separating a mature plant clump into smaller pieces for replanting, which rejuvenates the parent plant. This method of propagation and renewal ensures continued health and robust flowering for years to come.
Why Division is Necessary: Signs of Overcrowding
Asters that remain undivided for too long, typically every three to five years, begin to show visible signs of decline. The dense, extensive root systems start to become overcrowded, which negatively impacts the plant’s ability to take up water and nutrients efficiently. Gardeners will often notice a reduction in the number and quality of flowers produced on the clump.
This overcrowding also causes the stems to become weak and leggy, often leading to them flopping over instead of standing upright. The most definitive sign that division is needed is when the center of the clump begins to die out, becoming woody and unproductive. Splitting the plant revitalizes it, stimulates new root growth, and restores a dense, vigorous form with sturdy stems and full blooms.
Optimal Timing for Dividing Asters
The most favorable time to divide asters is in early spring, just as the new shoots are beginning to emerge from the ground. This timing is ideal because the plant is actively breaking winter dormancy and entering a period of vegetative growth, meaning its energy is focused on root and shoot development rather than flowering. Dividing at this stage gives the new sections the entire spring and summer season to establish a strong root system before the stress of blooming in the fall.
If a spring division is not possible, a secondary window exists in the late summer or early fall. However, this is a riskier period, particularly for fall-blooming asters, as the plant is using significant energy for its flowering cycle. Asters divided in the fall must be transplanted at least four to six weeks before the first expected hard frost date in your region. This allows the roots enough time to anchor themselves and begin healing before the ground freezes solid.
Step-by-Step Guide to Aster Division
Before beginning the process, thoroughly water the aster plant the day before to ensure the soil is moist and the root ball is hydrated, which minimizes transplant shock. To lift the clump, use a garden fork or shovel to dig a wide circle around the perimeter of the plant, staying several inches away from the stems. Gently work the tool underneath the root ball to loosen and lift the clump from the ground.
Once the clump is out, separate it into smaller, viable pieces. The old, woody, and unproductive center of the root ball should be identified and discarded, as it will not produce strong new growth. Use a sharp spade, a soil saw, or a sturdy knife to cut the healthier outer sections into divisions, or gently pull them apart by hand if the roots are looser. Each new section should be robust, containing strong roots and several shoots or buds.
The size of the divisions depends on the overall health of the parent plant, but sections about six to eight inches in diameter are generally suitable for replanting. Do not create overly small divisions, as they may lack the necessary stored energy to survive the initial establishment period. Immediately prepare the new planting sites, which should be twice as wide as the new root ball, to provide ample space for root spread.
Care for Newly Divided Plants
After separation, the new divisions should be replanted promptly at the same soil depth they were growing at previously, ensuring no part of the crown is buried. Following replanting, a deep and immediate watering is necessary to settle the soil around the roots and eliminate any air pockets. Consistent moisture is required for the first few weeks as the plant works to regenerate its damaged root system and establish itself in the new location.
Apply a two-inch layer of organic mulch, such as shredded leaves or compost, around the base of the newly planted asters, taking care to keep the mulch from directly touching the stems. This mulch layer helps to conserve soil moisture, regulate soil temperature, and prevent weed competition. New divisions should be placed in an area that receives at least six hours of direct sunlight daily for optimal recovery and future flowering. Avoid applying a high-nitrogen fertilizer immediately, as the plant’s focus needs to be on root development, not excessive foliage growth.