Chrysanthemums, commonly known as mums, are perennial plants celebrated for their vibrant fall display. Dividing mums is a horticultural technique used to rejuvenate older clumps, prevent overcrowding, and propagate the plant, leading to more vigorous growth and abundant flowering. Division becomes necessary when the center of the plant becomes woody and produces fewer blooms, typically every two to three years. The timing of this procedure is the most important factor determining the success of the new divisions.
Why Summer Division is Risky
While technically possible, dividing a chrysanthemum clump during the summer months is strongly discouraged due to significant physiological stress. Summer heat dramatically increases transpiration, the process where plants lose water vapor through their leaves. A newly divided plant has a severely reduced and damaged root system, making it unable to absorb water quickly enough to compensate for this rapid moisture loss.
This imbalance between water uptake and loss immediately leads to wilting. The plant must expend energy attempting to recover from heat stress rather than establishing new roots. Newly planted divisions require mild temperatures and consistent moisture to anchor themselves firmly in the soil. The intense heat of mid-summer can quickly bake the shallow root systems of the divisions, causing them to fail entirely.
Summer division also conflicts with the plant’s reproductive cycle, as many varieties are actively setting flower buds in July and August for their autumn bloom. Disturbing the root system during this stage diverts the plant’s stored energy away from flower production and toward survival. This results in a compromised or absent fall flower show. Performing a division in summer should only be considered an emergency measure if extreme aftercare, including temporary shading and heavy, consistent watering, can be provided.
The Optimal Time for Dividing Mums
The optimal time for dividing mums is in the early spring. The ideal window is just after the last hard frost, when new green growth first begins to emerge from the soil but before the plant has spent significant energy on vegetative growth. At this stage, the new shoots are typically only one to three inches tall, making the division process easier and less disruptive.
Dividing the plant in early spring gives the newly separated sections an entire season of mild weather to establish a robust root system. Cool soil temperatures encourage root growth before the plant shifts its focus to developing stems and leaves. This ensures the divisions are well-anchored and capable of handling the heat of the coming summer. This strong establishment allows the divisions to survive the summer and produce a healthy fall bloom.
Fall division is a secondary option, particularly in warmer climates, but it carries the risk of insufficient root development before winter frost. If dividing in the autumn, it must be done at least six to eight weeks before the ground typically freezes solid in the region. Divisions planted too late are susceptible to being pushed out of the soil by the freeze-thaw cycles of winter, a process known as frost-heaving. To mitigate this risk, fall divisions must be protected with a heavy layer of mulch immediately after planting to insulate the new root system.
Step-by-Step Guide to Dividing Chrysanthemums
Preparation for division begins the day before by thoroughly watering the parent plant to ensure the root ball is saturated and less prone to breaking apart. On the day of the procedure, which is ideally cool and overcast to minimize transplant shock, use a spade to dig a wide circle six to eight inches away from the center of the clump. Dig deep enough to lift the entire root mass intact, then carefully shake away the excess soil to expose the root structure.
The next step is to separate the viable, outer portions of the clump from the older, woody center, which should be discarded as it lacks vigor. Use a sharp, sterilized garden knife, a pair of secateurs, or the edge of a spade to divide the remaining healthy perimeter into smaller sections. Each new division must contain a healthy section of the root system and at least two to three new shoots or growth points to ensure successful establishment.
Replant the divisions immediately into prepared locations offering well-draining soil and full sun exposure. Place the new sections into the soil so that the crown, where the stems meet the roots, is positioned at or just slightly above the surrounding soil level. Space the new chrysanthemums approximately 18 to 24 inches apart to allow for proper air circulation and future growth. Immediate aftercare involves a deep watering to settle the soil around the roots, followed by applying a thin layer of mulch to maintain consistent soil moisture.