Development of inexpensive, automated ground methods is necessary to advance precision plant phenology monitoring across large spatial extents. We propose the use of free, publicly available, Internet-connected cameras, often associated with nonscientific monitoring, to monitor plant phenology at continental scales. We provide a methodology to detect changes in vegetation greenness and determine timing of spring and fall events from over 1100 public cameras across North America from February 2008 – 2009. Manual image segmentation facilitated spring detection for both deciduous and understory vegetation occurring within a single camera view. Deciduous spring green-up was highly correlated with visual ground truths, despite signal noise introduced by varying image exposure and automatic color correction. Compared to co-occurring satellite remote sensing products, public cameras had an equivalent or higher ability to detect spring with fewer days lost to cloud cover.