The Lilac City and Spring Phenology

By Vincent Jansen, Graduate Research Assistant, University of Idaho

Well the month of May has come and gone. Here in the inland Northwest May kicks off with Bloomsday, a giant fun run in Spokane, Washington. Spokane is known as the Lilac City. I, Vincent Spokane Jansen, was born in the Lilac City and am interested in just how much these lilacs can tell us about climate in the inland Northwest.

Like humans, plants and animals are subject to the environmental factors where they live. By observing the life cycle events of organisms we can see how connected species are to climate [1,6,9,10]. The life cycle events for a species, particularly the annual cycle for plants, is called phenology. A good example of a phenology record is first leaf and first bloom dates, which often mark the arrival of spring and the payoff of enduring the long cold winter [2,4,6,7,8]. Since the 1950s, citizen scientists have been monitoring the first bloom date of lilacs across the US. Beginning in 1957, researchers led by Dr. Joseph Caprio at the Montana State University organized close to 1,000 observers across the 11 western states to monitor lilacs for agricultural purposes. They wanted to help inform decisions of when to plant crops [3] and how animals and plants interact with one another [6,8]. These data were collected by citizen scientists, and have recently been revitalized through efforts of  Project Budburst and the National Phenology Network.

Studies have shown that lilac bloom dates are strongly influenced by springtime temperature, with a cool spring (e.g., 2011) delaying the onset of the bloom, and a warm spring resulting in an earlier bloom [2,4,7,8]. To see if lilac bloom dates are an indicator of springtime temperatures for the state of Idaho, I selected a total of 13 monitoring locations statewide that had at least 20 years of bloom records from 1957-1993 (unfortunately, consistent records ended in 1993, and have only recently been re-initiated). I then acquired statewide spring (March-May) temperature data, from PRISM [5] using the Western Regional Climate Center’s Westmap climate tool.  These two time series, statewide spring temperature and statewide bloom anomaly dates, show a striking correspondence (Figure 1). The first bloom date anomalies significantly correlate (r2 =0.80, p <.001) with mean spring (March-May) temperature anomalies, suggesting that lilac bloom dates are a useful indicator of statewide spring temperature.

Figure 1. Average first bloom lilac anomalies (solid line) and mean spring (March, April, May) temperature (°C) anomalies (dotted line) for the state of Idaho from 1957 to 1993.

While a trend analysis may not be as robust for short-term records like the one above, the statewide average bloom date of lilacs arrived 2.8 (±1.9) days earlier per decade following overall warming trends from 1957-1993. A simple linear sensitivity analysis shows that for every 1˚ C rise in mean spring temperatures, the timing of bloom is 6.7 (±4.5) days earlier. These results are similar to lilac phenology studies across other regions of the United States [2,3,8].

Today, organizations like the USA National Phenology Network are continuing the phenology monitoring effort by enlisting citizen scientists across the nation to collect data, not only for lilacs, but all different kinds of plant and animal species. Data from these efforts are now being used to better understand how climate interacts with plants and animals over time and space. [2,4,7,8]. People around the Inland Northwest should participate to see if the saying “April showers bring May flowers” will continue to hold true in the coming decades. To participate go to


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  11. USA National Phenology Network (2010) Plant Phenology Data for the United States, 2009. Tucson, Arizona, USA: USA-NPN. Data set accessed YYYY-MM-DD at
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  13. White M A, Running SW, Thornton PE (1999) The impact of growing season length   variability on carbon assimilation and evapotranspiration over 88 years in the eastern   deciduous forest. International Journal of Biometerology, 42, 139-145.

2 thoughts on “The Lilac City and Spring Phenology

  1. Pingback: February 2013: short on days and interesting weather | Climate of the Inland Northwest US

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