Various scenarios can take place when growing mums during these very hot days. Here are a few tips to think about.
If plants are drying out quickly during these very high temperatures, and plants are being watered and fertilized more frequently, growers are advised to reduce fertilizer to 200 ppm (constant feed) and use plain water once a week. This will help to lower risk of high soluble salts. High soluble salts can lead to root injury and Pythium root rot.
If irrigating more than once a day, reduce feed to 100-125 ppm for each feed if using constant feed. For example a grower may apply two applications of 100 ppm per day (each time they water) instead of one application of 200 ppm and one clear water each day.
If night temperatures are warm, some growers may find that flowering is delayed by 6-8 days. This might be more likely to occur with mid-season varieties. Chrysanthemums are short-day plants. Both flower initiation and development of the flower buds occur more rapidly under short days than under long days. However, temperature has a greater influence than day length on flowering of garden mums. Delay of flowering and in more extreme cases, failure of buds to develop properly (crown buds) are often associated with heat delay. Heat delay occurs when night temperatures are greater than 73F. Garden mum cultivars vary in their sensitivity to heat delay. Also research has shown that plants are more sensitive to heat delay when plants are subjected to high night temperatures during the first 2 weeks of short days, during flower bud initiation. Plants subjected to high night temperatures after 3 weeks of short days showed no permanent damage and only exhibited delayed flowering.
Tina Smith, UMass Extension
Garden Mum Tips by Ed Higgins, Ball Chrysanthemums, Aug. 2016
Runkle E. and T. Vaid. 2013. Temperature on Chrysanthemum, GPN Magazine, July issue.