Chrysanthemums, often called mums, are a ubiquitous symbol of autumn, transforming gardens with a late-season burst of color. Their success as garden perennials or seasonal annuals is inextricably linked to temperature, which acts as the primary regulator of their growth, survival, and flowering cycle. Temperature dictates the speed of vegetative development, governs winter survival, and precisely times the display of blooms. Understanding the specific thermal boundaries of this plant is the most important factor for successful cultivation. The plant’s life cycle is a delicate balance where deviations from the preferred range initiate responses ranging from rapid growth to dormancy and flower initiation.
Optimal Temperatures for Growth and Health
Chrysanthemums engage in peak vegetative growth within a specific thermal range, which maximizes leaf production and root development. Productive daytime temperatures for this stage fall between 20 and 28 degrees Celsius (68 to 82.4 degrees Fahrenheit), facilitating efficient photosynthesis and metabolism. These warmer days should be paired with cooler night temperatures, ideally between 15 and 20 degrees Celsius (59 to 68 degrees Fahrenheit).
Maintaining this differential between day and night temperatures is important for the plant’s overall health and structure. When temperatures remain consistently within this range, the plant directs energy toward establishing a robust root system and dense, bushy foliage. Deviation to either extreme can slow growth, but this range is where the plant operates most efficiently. A strong foundation developed during the summer is necessary to support the spectacular fall flower display.
Resilience to Cold and Overwintering
A chrysanthemum’s ability to survive winter is highly dependent on its specific cultivar, with a distinction existing between garden and florist types. Hardy garden mums, classified as perennial varieties, are capable of surviving winters in USDA Hardiness Zones 5 through 9, and some specialized types can tolerate zones as cold as 3. These resilient varieties typically endure temperatures dropping to at least -5 degrees Celsius (23 degrees Fahrenheit), and some may survive lows of -15 degrees Celsius (5 degrees Fahrenheit).
Conversely, the large florist mums sold in fall are often less cold-tolerant and are best treated as annuals, especially in colder climates. For hardy garden mums planted in the ground, a common overwintering strategy involves cutting the stems back to a few inches and applying a heavy layer of mulch once the ground has frozen. This mulch, composed of materials like straw or chopped leaves, does not keep the ground from freezing but helps maintain a consistent, cold temperature. This consistency prevents the damaging freeze-thaw cycles that can heave plants out of the soil. In regions with extremely cold winters, an alternative is to lift the entire plant after the first frost and store it dormant in a cool, dark, and frost-free location for the winter.
Strategies for Managing Summer Heat
The upper limits of a chrysanthemum’s thermal tolerance are reached when temperatures consistently exceed 29 degrees Celsius (85 degrees Fahrenheit), inducing heat stress. Sustained high temperatures, especially warm nights above 20 degrees Celsius (68 degrees Fahrenheit), can delay the onset of blooming and cause a decline in flower quality. Signs of this stress include wilting foliage, yellowing of leaves, leaf curl, and fading flower color.
To mitigate these effects, gardeners can provide afternoon shade, particularly in the hottest summer months, to lower the plant’s temperature and reduce water loss. Increasing the frequency of watering is also important to maintain turgidity, as the plant transpires more rapidly in the heat. Avoid applying fertilizer during periods of extreme heat, as the salts in the fertilizer can intensify the plant’s stress response and potentially lead to leaf burn.
How Temperature Triggers Flowering
Chrysanthemums are fundamentally short-day plants, meaning the primary trigger for flowering is the lengthening of the dark period, which must be sustained for at least 12 hours to initiate the process. While this change in light duration is the initial cue, temperature plays a powerful supporting role that refines and enhances the final bloom. The most significant thermal requirement for successful flower development is the presence of cool night temperatures.
Night temperatures consistently dropping below 15 degrees Celsius (60 degrees Fahrenheit) are highly beneficial during the bloom-setting phase. This cooling effect helps to slow the plant’s respiration rate, which conserves energy that can then be directed toward flower formation and development. Cooler nights also serve to enhance the saturation and depth of the flower’s color pigments, resulting in richer, more vibrant blooms. Conversely, if night temperatures remain too high after the short-day cue is received, the flowering process can be delayed, and the resulting flowers may be abnormally formed.