March-April extended past bloom in 11-41% of four US lilac samples in 1965-2008

We audited whether three fixed weather windows preceded reported first bloom in 9,015 repeated US lilac records. March-April contained days after bloom in 11.5-41.1% of records across two regions and two periods in 1965-2008. Keeping only bloom after April reduced warm-year retention by 8.9-16.2 percentage points relative to cool years within the same plants. This pattern remained after each state omission and flag exclusion. The restriction also changed latitude, bloom timing and weather composition. These are exact archive summaries, not causal effects or population estimates. Different plant types by region, selected long records, unknown visits, coarse weather and shared records limit inference. Delay scenarios give bounds under stated reporting assumptions. The package includes a raw-source rebuild, source hashes, a derived panel, code, record-level evidence, checks and a figure. It does not test pollution, weather extremes or species loss.

phenology · lilac · exposure timing · observation bias · sample selection · temperature