Figure 1. Crown buds in outdoor garden mum production. All photos by W. Garrett Owen, MSU Extenson.
Light and temperature influence plant growth and development, and for outdoor garden mum (Chrysanthemum × morifolium) production, growers usually rely on natural daylength (photoperiod) and temperature to control crop timing. Garden mums are short-day plants, meaning flower initiation and bud development occurs rapidly when the day length is short and nights are long. However, temperature interacts with photoperiod and can have a greater influence on flower initiation and bud development in mums.
In Michigan, outdoor production of fall garden mums can be challenging, especially when summer nighttime temperatures fall below 60 degrees Fahrenheit. For example, according to the Michigan State University Enviroweather station at Romeo, Michigan, the low temperatures at night averaged 55 F for five days in early July 2018. The several nights of cool temperatures can induce undesirable premature bud formation, which are known as crown buds (Figs. 1 and 2).
One cool night will likely not induce crown bud formation, but several consecutive cool nights can. During this time, flower bud initiation is triggered and the vegetative growth phase ceases, resulting in plants with fewer nodes and leaves, thus finishing small. In addition, stress such as inadequate irrigation and fertility can cause premature bud formation. Therefore, ensure the crop is irrigated as needed and supplied with adequate nutrition of 150 to 250 ppm nitrogen during the vegetative phase.
Figure 2. Crown buds in outdoor garden mum production.
- A spray application rate of 500 ppm ethephon is appropriate. Adjust spray water to a pH of 5.0 before adding the ethephon, and add a surfactant if the solution runs off the foliage.
- Apply the first ethephon treatment about 10 to 12 days after sticking of cuttings, once plants have sufficiently rooted.
- Repeat seven to 14 days after pinch. Earlier applications (at seven days after pinch) are more important when plants are exposed to cool night temperatures following pinching.