March-April temperature balance gave no clear common link to US lilac bloom at fixed spring mean in 1965-2008

We tested whether a warmer April relative to March was linked to later lilac first bloom at a fixed March-April mean. We used 9,015 repeated plant-year records in four US region and period samples in 1965-2008. Models included plant and year effects and winter temperature. Contrast slopes ranged from -1.000 to +0.070 days per C. All four primary intervals included zero after correction for four tests. Plant trends, flag removal, and adjacent-year differences gave the same broad result. One nonlinear check gave a negative slope in late West. This is a model-dependent result and not evidence for a common delay. Monthly windows can extend past bloom. Coarse weather, missing visit schedules, different lilac types by region, selected long records, and few western state clusters limit inference. We do not test pollution, weather extremes, or causal climate-change effects. The package preserves code, the panel, raw source hashes, a raw rebuild check, numerical checks, all estimates, and a figure.

lilac · phenology · monthly temperature · observation bias · regional replication · null result