No matter how many or few characters you have available it will suggest family matches. Interactive Key of Common Iowa Trees from Iowa State University. Photographs assist each choice. Detailed illustrations and descriptions are quite useful. Ohio Historical Society Key provides leaf and fruit keys.
Users click on images that match their specimen.
European Garden Flora - this published set of encyclopedias provide the most complete set of keys to cultivated species ever offered. It is work of vast scope and was compiled by finest European taxonomists. In general the set replaces Rehder's Trees and Shrubs or Bailey's Manual due to the number of recently discovered taxa and updated nomenclature. Names are generally in compliance with Flora Europaea - which may or may not prove comforting. The keys to Lilium, Geranium, and Hosta alone are remarkable and worth the price.