From a plant's perspective, a frost event is a severe environmental stressor. Chrysanthemums, as herbaceous perennials, have a cellular structure that is vulnerable to ice crystal formation. When temperatures drop below freezing, the water inside and between the plant's cells begins to freeze. The formation of sharp, intracellular ice crystals is particularly devastating; they puncture cell membranes and walls, causing irreversible damage. Once these cells rupture, they lose their structural integrity and their contents leak out. This is what you observe as the blackened, mushy, and wilted foliage characteristic of frost kill. The plant's vascular system, responsible for transporting water and nutrients, can also be damaged by freezing, leading to a systemic collapse even if some parts initially appear unharmed.
Not all chrysanthemum cultivars experience frost equally. Our genetic makeup dictates our level of cold hardiness. Garden mums (Chrysanthemum × morifolium) are often grouped by their ability to withstand cold. "Hardy mums" are typically cultivars that have been selected or bred to initiate crucial physiological changes as autumn days shorten and temperatures gradually decrease. This process, known as cold acclimation, involves the accumulation of sugars and other cryoprotectants (like proline) within our cells. These compounds act as a natural antifreeze, lowering the freezing point of cell contents and protecting cell membranes from freeze-induced dehydration. A plant that has experienced a gradual cooling period is far more prepared for a frost than one that is still in active bloom during a sudden, early cold snap.
A chrysanthemum's overall health directly influences its resilience. A plant that is well-hydrated, has received appropriate nutrients throughout the growing season (particularly potassium, which aids in cold hardiness), and is free from disease and pest stress has greater resources to allocate toward cold protection. Furthermore, our survival is tied to our growth cycle. A mum that has finished its flowering cycle and has begun to naturally senesce—shifting energy from tender top growth down to the crown and roots—is entering a state of dormancy. This crown, located at the soil surface, contains the meristematic cells needed for regrowth in spring. It is the survival of this crown, not the above-ground foliage, that is paramount for perennial life.
Human intervention can mimic natural protections that we might otherwise find in a more sheltered habitat. Applying a thick, loose layer of mulch (like straw, shredded leaves, or pine boughs) over our crown after the ground has frozen is the single most beneficial action. This blanket does not necessarily keep us warm but, crucially, it maintains a consistently cold temperature, preventing the damaging cycle of freeze-thaw-freeze that can heave our roots from the soil and destroy the crown. On a night when a light frost is forecast, covering us with a breathable fabric like burlap or a frost cloth can trap the residual heat radiating from the soil, creating a microclimate that may stay a few critical degrees warmer. It is essential that this covering does not touch our foliage and is removed during the day to allow for photosynthesis and prevent overheating.
