Chrysanthemums, often called mums, are popular garden and potted plants valued for their vibrant, diverse blooms. Temperature significantly influences their health and prolific flowering. Understanding how these plants respond to varying temperatures is fundamental for successful cultivation and longevity.
Optimal Temperatures for Chrysanthemum Growth and Bloom
Chrysanthemums exhibit optimal vegetative growth around 25°C (77°F). Consistent temperatures within this range promote healthy plant structure and robust foliage. A relative humidity of 70-90% is also optimal.
Successful bud formation and flowering are highly sensitive to temperature, especially night temperatures. An optimal night temperature of 15-16°C (59-61°F) aids flower bud differentiation. Generally, the ideal temperature for flowering ranges between 17°C and 22°C (62.6°F and 71.6°F).
Chrysanthemums are short-day plants, meaning flowering is triggered by shorter daylight periods, typically requiring 11 to 12 hours of darkness. Temperatures below 12.8°C (55°F) can delay flower initiation in some varieties. Conversely, temperatures below 20°C (68°F) may promote earlier initiation in others.
Chrysanthemum Cold Tolerance and Winter Care
Chrysanthemums vary in cold tolerance, categorized as “garden mums” (hardy mums) and “florist mums.” Garden mums are typically perennial in USDA Hardiness Zones 5 through 9. Most potted hardy mums can survive down to -18°C (0°F) if well-rooted. Florist mums, often sold as potted decorative plants, are generally less cold-hardy and usually treated as annuals.
Light frost, with temperatures between -1°C and 0°C (30°F and 32°F), may cause minor damage such as wilting or slight discoloration. The plant often recovers from such mild exposure. Temperatures dropping below -2.2°C (28°F) pose a serious risk, potentially leading to significant damage. Signs of cold damage include blackened or discolored leaves and stems, wilting, or tissue that feels soft and mushy.
Winter protection is vital for chrysanthemums in colder regions. Planting hardy mums in spring allows ample time for root systems to establish before winter, significantly improving survival chances. Fall-planted mums have a lower survival rate because roots do not have sufficient time to develop. Well-drained soil is also important, as poor drainage can worsen frost damage.
Applying a protective layer of mulch in late autumn is a key overwintering strategy. A 3 to 4-inch layer of loose material, such as pine needles, straw, or evergreen branches, should be spread around the plant after stems have browned. This mulch insulates the soil, maintaining a more consistent temperature and preventing damaging freeze-thaw cycles that can heave plants out of the ground.
Leaving the plant’s foliage intact over winter provides additional insulation. For less hardy varieties or potted mums, bringing them indoors to a cool, dark, and frost-free location, such as an unheated basement or garage, is recommended. When overwintering indoors, reduce watering to keep roots only slightly damp.
Chrysanthemum Heat Tolerance and Summer Management
Chrysanthemums can experience stress when exposed to high temperatures, particularly during hot summer months. Their growth is inhibited when temperatures consistently rise above 25°C (77°F), and growth can essentially cease at 40°C (104°F).
Prolonged temperatures exceeding 32°C (89.6°F) can delay overall growth and development. Temperatures above 35°C (95°F) severely limit the plant’s photosynthetic capacity. Extreme heat, above 43°C (109.4°F), can significantly slow growth, cause leaves to lighten in color, and increase susceptibility to sunburn.
Heat stress manifests through several visible signs. Plants may exhibit wilting, leaf scorch, stunted growth, and a delay in flowering. High temperatures can also reduce the number of flowers produced, lead to poor branching, and cause the fading or reversion of flower color. This phenomenon, where flowering is delayed by temperatures of 29.4°C (85°F) or higher, is often referred to as “heat delay.”
Effective summer management helps chrysanthemums cope with warm conditions. Adequate and frequent watering is crucial, especially during periods of high heat, due to their shallow root systems. Soaking the soil deeply is recommended, while avoiding watering the leaves.
Providing partial shade during the hottest parts of the day can protect plants from intense sun, particularly in warmer climates or when daytime temperatures consistently exceed 24°C (75°F). Improving air circulation around plants is also beneficial. It is advisable to avoid excessive fertilization during extreme heat and to stop fertilizing after July to prevent late-season growth that could be vulnerable to early frosts. Fertilization should also cease once flower buds begin to emerge.