Cornell Cooperative Extension http://www.cce.cornell.edu/ Find information about the program. Under heading of Agriculture, find links on landscape horticulture, consumer horticulture, and urban horticulture.
OSU WebGarden http://webgarden.osu.edu/ Ohio State Univ. has a number of great resources here. Two are the Plant Dictionary, designed to compliment the program, and Factsheet Database, made up of "guides for answering pl ant-related questions... Over 20,000 pages of Extension fact sheets and bulletins provide a concentrated source of plant-related information."
USDA Economic Research Service http://www.ers.usda.gov/ Search under the terms "Floriculture and Environmental Horticulture" for federal information about industry growth rates, net income, and much more.
American Horticultural Society http://www.ahs.org/ Access events, membership info, a heat zone map (searchable by zip code), and partial text of American Gardener Magazine.
American Nursery and Landscape Association http://www.anla.org/ ANLA is the national trade association of the nursery and landscape industry. Find industry info, a legislative center, publications to purchase (very expensive), and more.
American Society for Horticultural Science http://www.ashs.org/ "The American Society for Horticultural Science promotes and encourages scientific research and education in horticulture within the United States and throughout the world." Over 4,500 members. Membership info, subcription info for their journals, conference info, and a career center.
American Society of Landscape Architects http://www.asla.org/ Based in Washington DC, this national organiations website contains member info, a job site, a list of publications, a sections for students, and much more.
Society of American Florists http://www.safnow.org/ Member info, news briefs, Capitol Hill Watch, and Floral Facts, among other topics.
Horticulture & Home Pest News http://www.ipm.iastate.edu/ipm/hortnews/ The electronic version of this newsletter is published by the Department of Entomology, Iowa State University. Well indexed, 1992-present.
Horticulture Magazine Online http://www.hortmag.com/ This online version cont ains limited text of current issue and full text of past issues. Subscribe for the full content.