Cold Tolerance

• Mums have very shallow root systems, as noted by the Iowa State University Extension. Freezes, heavy frost and lack of snow cover could all cause severe plant damage. To increase the chances of survival through the cold winter months, plant mums in the spring to give them plenty of time to become established. When the tops of the plants die in the late fall months, cut the stems down to the ground and apply mulch to keep the ground frozen. Repeated freezing and thawing can damage plant root systems.
  
  

Heat Delay

• In the summer months, day length is very important for initiating the bloom cycle. Mums begin to bloom when the days become shorter, usually in late July. However, according to Purdue University Extension, temperature also plays a crucial role in flower development. Usually, hot temperatures will not kill mums, although they may cause abnormal growth. Nighttime temperatures that exceed 73 degrees Fahrenheit may cause mums to bloom much later in the season. Temperatures above 73 degrees may also result in abnormal bloom formations known as crown buds. This effect is also known as heat delay and is most common during the first two weeks of shorter days.
  
  

Planting Tips

• To prevent temperature-induced problems in your garden mums, plant them in the spring months, usually in April or May. If you plant in the fall, allow at least six weeks before the first frost in order to avoid damage. Plant mums in well-drained soil that receives full sunlight, and always allow at least 18 to 24 inches of space between plants. Avoid planting mums in areas that receive nighttime light, such as near street lights, in order to help them bloom in the late summer.
  
  

Cultural Practices

• Proper gardening practices may also reduce your chances of frost damage or heat delay. Indoor or greenhouse growers sometimes try to control heat delay by covering mums with an opaque black cloth from 8 p.m. to 8 a.m. each day. You can also prevent heat delay by purchasing heat-tolerant mum varieties. To avoid frost damage, the University of Iowa Extension recommends that gardeners avoid pruning mums in the fall months, and stop fertilizing them in late July in order to minimize growth in the late season.