Cause (or causes): The most apparent cause of umbrella bloom appears to be a lack of exposure to temperatures low enough to fulfill the chilling requirement of the plant during its dormant--or resting--period.  When furnished with natural lighting, cranberry plants in the northeastern United States need a minimum of about 1700 hours of exposure to temperatures below 45ºF (older studies found the number to be 2500 hours, but that was in the absence of natural lighting). Studies in Oregon suggest only 500 hours are needed there. But regardless of the number, it is believed that if the number falls short of what is needed, umbrella bloom will occur. It may also be true that chilling hours could be lost during the characteristic ‘ups and downs’ of temperatures experienced in December and January in the northeast, in effect resetting the cranberry plant's internal 'cold counter' to zero and forcing the number of hours below 45ºF to begin accumulating all over again.

Some additional causes of umbrella bloom that have been suggested in the literature (and suspected) are frost injury or other types of damage to the terminal bud of an upright, such as insect or mite injury.