Chionoides seems to be fairly reliable in Southeast Michigan. According to Salley and Greer, Rhododendron Hybrids, 2nd ed., 1986, the parentage may be ponticum x maximum. This is likely since the plant appears to get its hardiness, narrow leaves, and the flower form from R. maximum. This plant was purchased at Kmart, Spring 1995.
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Parentage: R. ponticum x unknown (Unknown may be R. maximum)
Hybridizer: J. Waterer, before 1883