|XID Services interactive electronic keys to weeds of North America
1,200 Weeds of the 48 States and Adjacent Canada on DVD (or even more species on flash drive). That’s a lot of weed species and covers most things one is likely to run into in North America.
Flora of North America
An important but technical reference intended to cover all the plants in North America. Seventeen of the planned 30 volumes have been completed. Print and electronic publication is available for most volumes. This is a hard reference to work with from scratch. Descriptions of families (and other taxa) are useful. In addition it lists many references for various groups of plants. Illustrations (except in the 2 volumes on grasses) are far from a complete coverage.
An online flora (and in print). Mostly useful in eastern Canada. Somewhat technical, but with excellent keys and comments.
The Families of Flowering Plants
L. Watson and M. J. Dallwitz
An information retrieval system (DELTA) with an interactive key which allows free choice of characters, is “easy” to use, and can lead to correct identifications in spite of occasional errors. It can display illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, genera included in each family, and classifications. The program and data are also available on DVD or thumb drive (see http://delta-intkey.com/www/datadvd.htm).
Department of Environmental Horticulture, College of Agricultural and Life Sciences, University of Florida.
A multiple-entry key to the families of flowering plants as defined by the Angiosperm Phylogeny Group in 2009.
Key: Plant Families
This Android app is for both amateur plant enthusiasts and professional botanists. It is an interactive key to identify a plant to its respective family. You select characters and enter their values, such as flower color; the app will present you with possible identifications including family details and images.
[I have no idea how well this works.]
Dichotomous Key to Families (GoBotany)
An online key to plant families by the New England Wildflower Society (regionally useful in eastern Canada).
Key to California Plant Families
David J. Keil et al., The Jepson Herbarium.
An online key to plant families (regionally useful in British Columbia).
Plant Family Web
An online guide to the major plant families. A limited website with poor descriptions and a few photos, but with useful links to further information (some links are broken).
What Plant Family Is It?
A useful site from Clermont College (University of California) with descriptions and illustrations of common plant families.
The Plant List. A working list of all plant species.
This is a pretty comprehensive list of the world’s plants. Not much help if you are trying to identify a plant, but it is a list of families with the genera (and species) they contain.
Angiosperm Phylogeny Website
Stevens, P. F. (2001 onwards), Missouri Botanical Garden
This site has extensive descriptions of plant families (and orders), distribution maps and some images. It is mainly concerned with plant evolution and is not very helpful for identification, except to confirm family descriptions. It also has a long list of references which may be useful at times.
Common weeds of the northern United States and Canada
Covers only common weeds, but is constantly being added to. No keys, but many excellent photos for comparison IDs of common weeds.