The WUCOLS project was initiated and funded by the Water Use Efficiency Office of the California Department of Water Resources (DWR). Work was directed by the University of California Cooperative Extension, San Francisco and San Mateo County office. The first edition of the guide was completed in 1992. A second edition was published in 1994, and a third edition in 1999. In each new edition, additional species were evaluated and included.
The 4th edition (Costello, L.R. and K.S. Jones. 2014. WUCOLS IV: Water Use Classification of Landscape Species. California Center for Urban Horticulture, University of California, Davis. https://ccuh.ucdavis.edu/wucols-db) represents a substantial expansion in the number of plant evaluations. Over 1,500 entries have been added to the 3rd edition list, for a total of 3,546 entries. Essentially, the great majority of taxa available from wholesale nurseries in California are included.
Unlike the 3rd edition, the 4th edition includes evaluations of many cultivars and hybrids. For a number of species (e.g., Ceanothus spp.), water needs of cultivars were thought to be sufficiently different than water needs of the species and were evaluated separately. In addition, a number of cultivars do not have a specific epithet, such as Erysimum ‘Bowles Mauve’, and the species from which it is selected is not obvious. Hence, only the cultivar could be listed.