This is a great question. Yes, plant names are usually written including both the genus and the specific epithet, for example Rhododendron maximum.

However, if a certain plant is used in the creation of many cultivars, the specific epithet is often dropped, leaving just the genus and cultivar name. Usually this is customary for a common or well-known plant with many different cultivars. In addition, sometimes people don't write out the entire name of a well-known hybrid.

For example, Artemisia ‘Powis Castle’, commonly called wormwood, is a popular perennial. It is a cross between two plants, A. arborescens and A. absinthium. But it's not customary to write out the entire name, Artemisia arborescens x absinthium ‘Powis Castle’. Instead, the name is shortened to Artemisia ‘Powis Castle’.

In the case of azaleas, many of those you see are known as PJM hybrids. These hybrids, started in the 1930s and 1940s, have been crossed so many times that they are known simply as the PJM Group. So the name for these azaleas is usually Rhododendron ‘Cultivar Name Here’ with no attempt to include the specific epithet or the parent plants. See the Missouri Botanical Garden's information page for more information about PJM rhododendrons.

Some cultivars are not developed from hybrids but rather from breeding the same plant over and over for a particular trait such as flower color. If one species is used to create many cultivars, the specific epithet will often be dropped in favor of just the genus and cultivar name.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service