Citizen science projects, which engage non-professional scientists in one or more stages of scientific research, have been gaining popularity; yet maintaining participants’ activity level over time remains a challenge. The objective of this study was to evaluate the potential for a short-term, focused campaign to increase participant activity in a national-scale citizen science program. The campaign that we implemented was designed to answer a compelling scientific question. We invited participants in the phenology-observing program, Nature’s Notebook, to track trees throughout the spring of 2012, to ascertain whether the season arrived as early as the anomalous spring of 2010. Consisting of a series of six electronic newsletters and costing our office slightly more than 1 week of staff resources, our effort was successful; compared with previous years, the number of observations collected in the region where the campaign was run increased by 184%, the number of participants submitting observations increased by 116%, and the number of trees registered increased by 110%. In comparison, these respective metrics grew by 25, 55, and 44%, over previous years, in the southeastern quadrant of the United States, where no such campaign was carried out. The campaign approach we describe here is a model that could be adapted by a wide variety of programs to increase engagement and thereby positively influence participant retention.