International Lilac Society  
   Formed in 1971 to stimulate interest in the genus Syringa and to promote its use in public and private landscaping.
   http://lilacs.freeservers.com/

The Violet Society  
   Dedicated to Wild and Sweet Violets. Information on sources of plants, seeds, gifts, books, art and crafts.
   http://www.sweetviolets.com

National Chrysanthemum Society  
   Promotes the propagation and cultivation of the chrysanthemum and increase the bonds of fellowship among growers of the chrysanthemum.
   http://www.mums.org

American Hibiscus Society  
   Includes membership information, publications, chapter locations, events, and links.
   http://www.americanhibiscus.org/

American Brugmansia and Datura Society  
   Offers culture information, searchable plant database, registry and photographs.
   http://www.americanbrugmansia-daturasociety.org/

International Aroid Society  
   Dedicated to the dissemination of information on the plants in the family Araceae such as taxonomy, nomenclature, botany, pollination, horticulture, ethnobotany, literature, and art, as well as membership information.
   http://www.aroid.org/