Update on results. Had to learn how to use R to the data tables. Somewhere on the order of a quarter million calculations were going into sorting through all the combinations of climate, years, species...I wasn't doing that in excel by hand.
After looking at ~50 species, species that flower earlier have lower minimum temperatures required for flowering. Not too surprising. It's colder in March than May.
More interesting is that species that flowered earlier were more sensitive to climate variability--required fewer days at threshold temperature to flower.
In short, it seems like climate variability (absent of changes in means) early in the season will have a greater consequence for phenology than later in the season.
In operational terms, a warm spell followed by a cold snap early in the season would disproportionately cause greater damage to plant reproduction than later in the season.