A History of Lilac Phenological Observation in the United States

by Mark D. Schwartz Distinguished Professor of Geography University of Wisconsin-Milwaukee

The first extensive phenological observation networks in the United States began in the late 1950s with a series of regional projects, funded by the U.S. Department of Agriculture (USDA), and designed to use phenology to characterize seasonal weather patterns and improve predictions of crop yield.  J. M. Caprio at Montana State University began the first of these projects in 1956, and it eventually included around 2500 volunteer observers distributed throughout 12 Western states. Common purple lilac plants (Syringa vulgaris) were observed initially, with two cloned honeysuckle cultivars (Lonicera tatarica 'Arnold Red' and L. korolkowii 'Zabeli') added in 1968. Lilacs are not invasive, and thus they are acceptable for widespread distribution in a phenology network. However, honeysuckles are considered invasive, and no new plants have been distributed since the mid-1990s. Most observations from the original network in the West ended in 1994; however, a few observers started reporting data again in the later 1990s. Initially, only the dates of "first bloom", "full bloom", and "end of bloom" were recorded, with dates of "first leaf" and "95% or full leaf" added in 1967. These events were all precisely defined for the observers with verbal descriptions and photographs.

Encouraged by the success of Caprio's program in the West, similar projects were started in the central United States in 1961, and in the northeastern U.S. in 1965. All five events (phenophases) described above were recorded for plants in the Eastern United States networks from the start. Both of these networks observed cloned plants of the lilac cultivar Syringa chinensis 'Red Rothomagensis' and the same two honeysuckle cultivars used in the Western states project. The cloned lilacs are sterile (in that they do not produce seeds) and non-invasive.

In 1970, the Eastern networks were combined and expanded to about 300 observation sites. Between 1975 and 1986 observations continued under several additional projects, but the Eastern network lost funding and was terminated at the end of 1986. After the "decommissioning" of the Eastern network operations by the USDA, I corresponded with the most recent network supervisors, who granted me permission to contact the observers and to invite them to continue participating in an "interim" network, pending new funding. Approximately 75 observers responded to a renewed survey form sent out in March 1988, returning data for 1988 and in many cases 1987 as well. From that time to 2004, I continued to operate this interim "Eastern North American Phenology Network" with approximately 50 observers reporting lilac and/or honeysuckle event dates each year. Also, in the late 1990s the lilac observation protocols were added to the GLOBE program, and through that program and others, cloned lilac plants were distributed across Europe and in East Asia for observation.

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Starting in 2005, existing lilac observation stations were reorganized into an "Indicator Observation Program" and combined with a new "Native Species Observation Program" to form the prototype "Plant Phenology Programs" of the developing USA-National Phenology Network (USA-NPN) and are now part of the "Cloned Plants Project" in the USA-NPN. The historical lilac phenology data from these networks are publicly available on-line from the USA-NPN. Station density is highest in the western and northeastern portions of the country, allowing more detailed analyses of patterns in those regions. The USA-NPN has been working hard to expand distribution of cloned lilacs across the areas they grow in the United States, and is now regularly receiving annual phenology reports from over 70 stations observing cloned lilacs, and 110 more observing common lilacs since 2009. Lilac phenology stations are not present in the southeastern United States, as lilacs and honeysuckles do not receive sufficient chilling to grow successfully in that region. The USA-NPN began distribution of cloned dogwoods (Cornus florida 'Appalachian Spring') in the southeastern region this year for observation starting in 2012.