Elepidote rhododendrons are important landscape plants because of showy flowers and bold evergreen foliage. The most common flower colors are lavender, pink, and white, but red flower color is highly sought after. Only some elepidote rhododendrons possess sufficient cold hardiness to survive in the colder parts of the northern United States. These very cold-hardy rhododendrons that are able to withstand severe winter conditions are often referred to as “iron-clads.” The American Rhododendron Society categorizes rhododendron cold hardiness in groups from H1 to H6, with H1 being the hardiest and H6 being the least cold-hardy. Flower buds of H1 plants can tolerate cold temperatures down to around minus 25 °F (Kelley, 1991).
H1 elepidote rhododendrons with red flower color are not that numerous. Many of the red elepidote rhododendrons that have exhibited good cold hardiness over time have been faulted for having red flower color that is contaminated with blue (Brand and Mehlquist, 1992). Often the desire is to have ornamental plants with truer red color (Behe et al., 1997, 1999). One objective of this study was to evaluate flower color among H1 category, red-flowering rhododendron cultivars to identify those cultivars that produce the reddest flowers and those cultivars that are blue-red in color. Another issue with many red-flowered rhododendrons is that the flower color tends to fade and bleach, especially under full sun exposure. Studies documenting how red rhododendron flower color changes over time have not been reported. Therefore, another objective was to examine the rate and degree of flower color change over time from the start of flowering to the end of flowering.
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