Chrysanthemums (mums) provide a burst of color in the late summer and autumn garden. The success of growing these vibrant flowers depends heavily on environmental conditions, with temperature being the primary factor. Mums require distinct temperature zones to successfully transition from robust vegetative growth to the formation of abundant blooms. Temperature requirements shift dramatically across the growing season, dictating when the plant develops its structure, initiates flowering, and prepares for winter dormancy.
Temperature Range for Optimal Vegetative Growth
The initial phase of a chrysanthemum’s life, spanning spring and early summer, focuses on establishing a strong root system and dense foliage. During this vegetative stage, the plant grows most efficiently when temperatures are consistently warm but not excessively hot. The optimal temperature range for robust growth is generally between \(64^\circ\text{F}\) and \(77^\circ\text{F}\) (\(18^\circ\text{C}\) to \(25^\circ\text{C}\)).
This period requires a balance between daytime and nighttime temperatures to support rapid cell division. A constant temperature of around \(77^\circ\text{F}\) (\(25^\circ\text{C}\)) is often cited as the ideal thermal point for development. Growers frequently aim for a day-to-night difference, such as \(82^\circ\text{F}\) during the day and \(64^\circ\text{F}\) at night, to maximize plant vigor.
Sustained high temperatures quickly become detrimental once they exceed the optimal range. Growth begins to slow above \(77^\circ\text{F}\) and is inhibited past \(90^\circ\text{F}\) (\(32^\circ\text{C}\)). When the temperature reaches or exceeds \(95^\circ\text{F}\) (\(35^\circ\text{C}\)), heat stress severely impairs the plant’s ability to photosynthesize, leading to wilting and stunted development. Prolonged exposure to extreme heat, such as \(104^\circ\text{F}\) (\(40^\circ\text{C}\)), can cause growth to stop entirely.
How Temperature Dictates Budding and Flowering
Chrysanthemums are classified as short-day plants, relying on the shortening daylight hours of late summer to trigger flowering. Temperature plays an important, limiting role, particularly during the first few weeks of this transition. The process requires a specific, narrow thermal window to ensure proper bud formation and timely blooming.
The most rapid flower production occurs when temperatures are maintained between \(65^\circ\text{F}\) and \(70^\circ\text{F}\) (\(18^\circ\text{C}\) and \(21^\circ\text{C}\)). While slightly warmer days are tolerated, the nighttime temperature is especially important for signaling the change from vegetative growth to reproductive development. High temperatures during this sensitive period can cause “heat delay.”
Exposure to temperatures of \(85^\circ\text{F}\) (\(29^\circ\text{C}\)) or higher, especially during the first two weeks of short days, can delay flowering by one to three weeks. This heat stress can also lead to uneven flowering, malformed buds, and the reversion of flower color. Managing heat is necessary for a successful autumn display.
Conversely, night temperatures that are too cool can also disrupt the process. If temperatures drop below approximately \(60^\circ\text{F}\) (\(15.5^\circ\text{C}\)) for several consecutive nights, it can prematurely trigger the formation of “crown buds.” These early buds often develop deep within the foliage, resulting in small, poorly positioned flowers. For some varieties, temperatures consistently falling below \(55^\circ\text{F}\) (\(13^\circ\text{C}\)) can delay the overall flower initiation process.
Cold Hardiness and Winter Survival
Once blooming is complete, the focus shifts to ensuring the plant survives the cold months and enters dormancy. The cold hardiness of chrysanthemums varies by cultivar, but garden mums are generally hardy in zones 5 through 9. This resilience allows them to withstand a certain degree of freezing temperatures.
Most hardy varieties can tolerate temperatures down to at least \(23^\circ\text{F}\) (\(-5^\circ\text{C}\)). A light frost, where temperatures hover between \(30^\circ\text{F}\) and \(32^\circ\text{F}\) (\(-1^\circ\text{C}\) to \(0^\circ\text{C}\)), will typically damage the foliage and flowers, but the main plant structure and roots usually remain unharmed.
The danger level increases significantly when temperatures drop below \(28^\circ\text{F}\) (\(-2^\circ\text{C}\)), especially if the hard frost is prolonged. In these conditions, the upper parts of the plant can blacken and die back, and the roots become vulnerable to damage. For plants established in the ground, a layer of insulating mulch is a protective measure once the tops have died back, helping to shield the roots from freeze-thaw cycles.
Well-rooted, established plants in protected garden beds can sometimes tolerate temperatures down to around \(20^\circ\text{F}\) (\(-6.6^\circ\text{C}\)). In colder climates, where temperatures frequently drop below \(0^\circ\text{F}\) (\(-18^\circ\text{C}\)), many gardeners move potted mums to a protected location, such as an unheated basement or insulated garage, to prevent root kill. Consistent, cold temperatures are required for true dormancy, which ensures vigorous regrowth in spring.