The perennial aster is a popular garden flower, known for its vibrant, late-season blooms. Asters are rhizomatous and grow in dense clumps, which naturally leads to overcrowding over time. Division is the process of splitting these mature root masses, a necessary gardening task that rejuvenates the plant and promotes vigorous new growth. This maintenance ensures the long-term health of the aster.
Recognizing When to Divide
Asters typically require division every three to five years to maintain their full potential. The most telling sign is a noticeable decline in flowering performance, resulting in significantly fewer or smaller blooms. Visually, the clump often shows a dense, woody, or hollow center, indicating the oldest growth has become unproductive. The stems may look spindly, causing the plant to flop over easily. If the aster displays poor overall health, reduced vigor, or a sparse appearance despite receiving adequate sunlight and water, it is time to divide the root ball.
Choosing the Right Time
Timing the division correctly minimizes stress and maximizes the plant’s ability to re-establish itself. The preferred time is early spring, just as new shoots begin to emerge from the soil. Dividing at this stage allows the roots to develop fully throughout the cool, moist spring and summer months before the autumn flowering period. Division can also be performed in early autumn after the plant has finished blooming. If dividing in the fall, do so at least four to six weeks before the first hard frost, giving the new divisions sufficient time to set roots. Avoid dividing during the heat of summer or when the plant is in full bloom, as the combination of high temperatures and active growth puts too much strain on the aster, risking plant loss.
Step-by-Step Division Technique
The division process begins by preparing the plant the day before, ensuring it is well-watered to reduce transplant shock and make the soil easier to work. To lift the clump, use a sharp garden fork or spade to dig a circle several inches outside the perimeter of the existing stems. Dig down deeply, aiming to get underneath the entire root ball before carefully lifting the mass out of the ground.
Separating the Clump
Once the root ball is exposed, a sharp, clean tool is necessary to separate the clump, which minimizes root damage and prevents the introduction of disease. For smaller or looser clumps, separation may be achieved by hand or by using two garden forks inserted back-to-back to gently pry the sections apart. For older, denser, or more woody root masses, a sharp spade, knife, or saw may be required to cleanly slice through the tangled roots.
Selecting Divisions
It is important to discard the dense, woody center of the old clump, as this is the exhausted portion that lacks vigor. Only the younger, more robust sections from the outer edges of the clump, which have healthy, white roots and active shoots, should be kept for replanting. Aim for divisions that are substantial, roughly the size of a dinner plate, ensuring each piece has a healthy amount of roots and at least a few growing points.
Replanting
Before replanting the divisions, enrich the soil in the new location with compost to improve drainage and nutrient availability. Plant the new sections at the same depth they were growing originally, taking care not to bury the crowns deeper than before. Space the divisions appropriately, usually 18 to 24 inches apart for most varieties, to allow for good air circulation and future growth.
Post-Planting Care
Immediate and consistent watering is the most important step after replanting the aster divisions. New divisions lack the established root system to draw moisture efficiently, so the soil around them must be kept consistently moist for the first few weeks. This helps the severed roots quickly form new feeder roots and establish contact with the surrounding soil particles.
To reduce the immediate demand for moisture, prune the foliage of the newly planted divisions back by about one-third. This action minimizes water loss through transpiration, allowing the plant to focus its energy on subterranean root development. Apply a light layer of organic mulch, such as shredded leaves or wood chips, around the base of the plant, avoiding direct contact with the stems. The mulch helps regulate soil temperature, reduces weed competition, and conserves moisture.