How many species of Crataegus (or hawthorns) exist? The answer is, “It depends on what you mean by species.” Depending on the taxonomic interpretation, there may be a couple hundred species or there may be over a thousand. Charles Sargent, first director of the Arnold Arboretum, described 732 new species. Dr. Tim Dickinson of the University of Toronto cites W.H. Camp as pronouncing the determination of number of species as “The Crataegus Problem”.
Dr. Dickinson provides excellent resources about hawthorns on his lab and teaching site. A broad description of hawthorns is available on this page: Crataegus. A paper specifically about the black-fruited hawthorns (of which Crataegus douglasii is one) can be found here: North American Black-Fruited Hawthorns.
Landscape architecture / art / design resource link: Pruned, recently listed as one of the Best Blogs of 2006 That You (Maybe) Aren’t Reading. Hours and hours of readings and wanderings available here.