National Gardening Association A wealth of on-site information is provided by this nonprofit association including seed-swaps, zone maps, and an archived library. Their focus is on educating and helping people through gardening.
American Fern Society Includes fern basics, growing information, spore exchange, and links.
American Rose Society Lists local societies, has an "Ask the Expert" guide, offers an extensive photo collection, membership details, show schedules, and rose care tips.
Biodynamic Farming & Gardening Association Nonprofit organization open to all. Offer workshops, publications, and a corps of advisors for both farmers and gardeners. Based in San Francisco with groups in Canada and the US.
Tulsa Master Gardeners A certified volunteer group that provides information and technical support specific to northeast Oklahoma for home gardeners and landscapers. Offers articles, events, fact sheets, tips, MG Program information and links.