Most garden chrysanthemums (Chrysanthemum morifolium) are short-day plants, meaning that they require short day length (or, more accurately, long nights) in order to bloom. Mums and many other plants contain a light-recognition substance called phytochrome that senses the day length. Flowering is one response that can be affected by phytochrome levels within the plant. Long, uninterrupted dark cycles trigger flowering in short-day plants such as chrysanthemum and poinsettia. Other plants require short nights (long days) to bloom. These are called long-day plants. For some plants, day length makes little difference in bloom time. As plant breeders have selected for earlier fall bloom in chrysanthemums, they inadvertently selected some that were no longer short-day plants but instead are day neutral. Your summer-blooming mum must be one of these day-neutral types.