Azalea Society of America  
   Gives information about azaleas, "the royalty of the garden". There are azaleas for almost any growing situation you have, with almost any color, habit and height you desire
   http://www.azaleas.org/

American Hydrangea Society  
   Dedicated to the study of the genus, species and cultivars, as well as culture, habits, and hardiness. Event schedule, new introductions, photographs and resources.
   http://www.americanhydrangeasociety.org/

The American Boxwood Society  
   Not-for-profit membership organization devoted to the appreciation, scientific understanding, and propagation of the genus Buxus.
   http://www.boxwoodsociety.org

International Oleander Society  
   Dedicated to the promotion of the Oleander in culture, history and lore features articles and membership information.
   http://oleander.org