Global Warming and Daylilies

We have been measuring the date of first bloom, measured as days from the first of the year, for the past 25 years on Hemerocallis species. Four specific species are used; H minor, H middendorfii, H dumortieri, and H flava. The summary is below.

Spotty as it may be there is a consistent trend if we examine each species. We show all below.

Note that we have a consistent trend of shortening bloom dates by about 5-10 days per 25 years.

The question is; how dependent is this on micro conditions and how much is variability in general? The gross variability is significant. In addition given the severe winter of 13-14 we would have expected a later date but did not see it. However all trends are for earlier dates.