Herbarium specimens provide a critical source of phenological data that can be used to identify the direct

and indirect drivers of variation in flowering date within and among species. Specimen-based phenological

research in California has been accelerated by digitization efforts such as the California Phenology Network,

which has scored and archived the phenological status of over 1.4 million specimens to date. Using this new

data source in the Consortium of California Herbaria’s CCH2 data portal, we obtained data from 993

specimens of the iconic California Poppy, Eschscholzia californica Cham., along with climate data

representing all collection sites. Our goal was to determine how long-term and interannual climate

variation affect flowering dates, and whether the magnitude of phenological sensitivity to climate varies across

the species’ range. We found that specimens collected from chronically warm or dry sites flowered relatively

early, and flowering date was more sensitive to long-term mean temperature than to long-term mean

precipitation. Independent of these effects of long-term conditions, flowering date in E. californica was

sensitive to interannual variation in seasonal precipitation, but the direction of this effect depended on the

season in which the precipitation occurred. Specimens sampled from sites experiencing warmer-than-average

springs in the year of collection flowered 2.7–3.3 days earlier for every 18C increase in spring temperature

relative to long-term mean spring temperature. The magnitude of these effects, however, varied across the

range of E. californica, with greater sensitivity to temperature in relatively cooler regions and no discernible

sensitivity in relatively warm regions. Consistently, California Poppies exhibited significant phenological

advancement over the last 120 years, but this advancement was restricted to the cooler portions of its range.

Our results provide one of the first accounts of intraspecific variation in both phenological sensitivity to

climate and the magnitude of phenological shifts over time, and demonstrate that, for a single species,

location- or population-specific estimates of phenological sensitivity or of temporal trends in phenology might

not accurately predict phenological responses to climate change in other locations throughout its range. In

this study, we highlight the utility and promise of herbarium specimens for addressing novel questions about

the phenological responses of plants to climate trends.