You guys have stumbled into a very complex subject. It is simply not true that flower bloom is always caused by something as tangible as degree days. Some flowers may seem to some years, but it cannot be predicted using the numbers only. Further, the blooming periods of many plants are triggered by a variety of signals, including day length, soil moisture, frost, etc. 

* * * 

Janzen (1967) has argued that in the tropical dry forests of Central America, dry-season flowering of tree species is "a result of selection for sexual reproduction at the most opportune time in the year, rather than the result of immutable physiological processes which can only occur at that time of year." He regarded sexual selection as the "ultimate cause" of flowering time, environmental triggers as the "proximate cause". 

In the North American deserts, where germination, recruitment and other aspects of plant life cycles are adapted to take advantage of narrow climatic windows, one might expect that flowering phenology would also be climatically constrained. Most of the dominant species in the northern Sonoran Desert do indeed flower when seasonal soil moisture is greatest. 

It has been suggested, however, that the flowering time of certain Sonoran Desert dominants is determined by other considerations: pollinator availability for Fouquieria splendens, for instance, or seed germination for phreatophytes and succulents. Kochmer and Handel (1986) demonstrated that family membership strongly constrains flowering time in the South Carolina flora. 

In the northern Sonoran Desert, where floristic elements from cool-temperate and warm-subtropical climates mingle, it is not clear which should dominate - phylogenetic or climatic constraints - in determining phenological patterns. It is also not clear whether species that have evolved under similar climatic regimes will share the same phenological triggers; that is, flower in response to the same environmental cues. 

Flowering Phenology of Six Woody Plants in the Northern Sonoran Desert, Janice E. Bowers; Mark A. Dimmitt
Bulletin of the Torrey Botanical Club, Volume 121, Issue 3 (Jul. - Sep., 1994),215-229.

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