University of Illinois Extension
University of Illinois Extension provides practical, research-based information and programs in Horticulture and Home Gardens, Urban Programs, Natural Resources and the Environment, and manyother areas to help individuals, families, farms, businesses and communities in Illinois.

Michigan State University Extension
Thousands of informational files on various topics, including Home Maintenance and Repair, Ornamental Plants Plus, Diversity and Pluralism, Preservation Food Safety, Water Quality, Tourism Education, and many others.

Ohio State University Extension Service Plant Facts
Guides for answering plant-related questions from 46 different universities and government institutions across the United States and Canada. Over 59,000 pages of Extension fact sheets and bulletins provide a concentrated source of plant-related information.

Purdue University Agronomy Extension
The Agronomy Extension group is diverse in its educational efforts, representing interest areas of turf management, soil fertility, soil management, environmental soil science, soil conservation, and others of interest to landscapers.

University of Wisconsin Horticulture Extension
Among the many, many useful sections of this umbrella site are the Wisconsin Urban Horticulture Program , UW Plant Disease Diagnostic Clinic, and Landscape Plants of the Upper Midwest.