High temperatures on mum production

The high temperatures this summer will likely delay flowering of early season potted chrysanthemums. Learn more about this phenomenon and how to deal with it.


Growers in Michigan produce the second largest crop of potted garden chrysanthemums, with wholesale sales of at least $15 million each year. Mums, especially early season varieties, are sensitive to high temperatures during flower initiation. This phenomenon is known as “heat delay.” When short days are provided to initiate flowers, heat delay occurs when temperatures exceed approximately 85 degrees Fahrenheit. In addition, high temperature can lead to poor branching, reduced growth, fewer flowers, flower color reversion and flower color fading.

According to Jim Faust of Clemson University, flowering responses of early season mums change with temperature. At cool temperatures (e.g., 65 F), early-season mums flower fastest under short days and eventually flower under long days. In contrast, at higher temperatures (more than 75 F), short days are required for flowering. This means that short days (long nights) are more critical when temperatures are high. Late season cultivars also require short days for flowering, but since flowers are initiated in late summer and early fall, high temperatures are usually not a problem.

About 15 years ago, Faust and Terri Starman at the University of Tennessee showed that some cultivars grown at a constant 75 F flowered earlier when the photoperiod was quite short (10 hours or less), compared to a photoperiod of 12 hours or longer. Therefore, a heat delay in early season cultivars can be partially overcome by providing a 10-hour day (14-hour night). In a small study we performed with three Cheryl mum varieties, which are late season varieties, plants grown at a day and night temperature of 95 and 77 F flowered at the same time whether the photoperiod was 10 or 12 hours long. Therefore, providing a day length shorter than 12 hours probably has no benefit to late season varieties.

What can a grower do to mitigate high temperature flowering delay in mums? For early season varieties, ensure the photoperiod is 12 hours or less. An even shorter photoperiod (10 hours of light, 14 hours of darkness) can at least partially reduce heat delay. Obviously, keep plants as cool as possible. For example, don’t pull blackcloth until very early in the morning (at least 40 minutes before sunrise), and keep it pulled until the dark period is at least 12 hours long. If blackcloth is, instead, pulled closed late in the day, hot air can be trapped under the blackcloth during the night.

Also, ensure mums receive enough water; in addition to high temperature, water stress can decrease growth, leading to smaller plants at flowering. When mums are produced in greenhouses, 65 to 70 F is recommended for the most rapid flower development. The use of lower temperatures can increase flower number and flower size and have relatively little effect on flower development time.

Dr. Runkle’s work is funded in part by MSU’s AgBioResearch.

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