Chrysanthemums are the quintessential flower of the autumn garden, providing color when most other plants have faded. Gardeners often wonder about their resilience as temperatures plummet, particularly concerning a dip to 32 degrees Fahrenheit—the temperature at which water freezes. Understanding how the chrysanthemum’s internal biology responds to cold is key to protecting its ability to thrive through chilly weather.
Survival at the Freezing Point
While 32 degrees Fahrenheit marks the freezing point of pure water, it does not automatically mean a chrysanthemum will freeze solid. Plant tissues contain dissolved substances, such as sugars and salts, within their cellular sap, which lowers the freezing point a few degrees below 32°F. This chemical property acts as a natural antifreeze, offering a small buffer against immediate freeze damage.
When the air temperature drops to 30°F or 32°F, the plant experiences a light frost, which primarily affects the most tender, exposed parts. The damage often remains cosmetic, resulting in the browning or wilting of the most delicate petals and new foliage. The deep-seated crown and established root system of a hardy chrysanthemum are typically insulated by the surrounding soil and remain viable, especially if the cold exposure is brief.
A more significant threat is a “hard freeze,” generally defined as 28°F or lower sustained for several hours. During a hard freeze, ice crystals begin to form outside the plant cells. This draws water out of the cells, causing them to dehydrate and potentially collapse, leading to irreversible tissue damage. The duration of the cold is therefore more damaging than the initial temperature dip, as a prolonged hard freeze can injure or kill the plant’s crown.
Identifying and Treating Cold Damage
After any cold event, visual inspection reveals the extent of damage to the chrysanthemum’s foliage and flowers. Tissues injured by freezing temperatures typically appear water-soaked, limp, or darkened. Severe damage can cause leaves and stems to turn black and mushy due to the rupture of cell walls.
Do not immediately trim or clean up damaged parts of the plant while they are still frozen or immediately after thawing. Manipulating frozen tissue can exacerbate the physical damage. The dead foliage actually provides a layer of insulation, helping to protect the plant’s crown from subsequent cold snaps.
Assessment and removal of damaged material should wait until the plant has completely thawed. Pruning too early can encourage vulnerable new growth that will be susceptible to the next frost. For perennial mums, it is often best to leave the dead stalks in place until new growth appears in the spring before cutting them back.
Preparation for Unexpected Cold Snaps
Proactive measures before a cold snap significantly increase a chrysanthemum’s chances of survival. Provide a deep watering approximately 24 hours before the temperature is expected to drop. Moist soil retains heat much more effectively than dry soil, releasing it slowly overnight and offering insulation to the roots.
Potted chrysanthemums are more vulnerable because their roots are exposed to cold air, so the best action is temporary relocation. Move containers to a sheltered spot, such as against a warm wall, under a porch, or into an unheated garage overnight. The surrounding structure provides a thermal barrier against wind and radiant heat loss.
In-ground plants can be protected by covering them entirely with a lightweight material like an old sheet, burlap, or a frost cloth. The covering should drape all the way to the ground to trap heat radiating from the soil. This creates a small, warm microclimate around the plant that can protect the tender stems and blooms from light frost.