I've never heard of the individual parts of a plant having an elemental association, or I've never read anything about it, rather. Most plants are gernally associated with one element or another. For example, St.John's Wort is associated with the element of fire, dandelions are associated with air, ferns are associated with earth, and chamomile is associated with water.
"Herb Magic for Beginners: Down-to Earth Enchantments" by Ellen Dugan is a really good book on this topic. It has a lot of herbal correspondences throughout and various other herbal information in regards to magic.