From W. J. Bean (fourth edition) 1925
Rhododendron Altaclerense, Lindley (Bot Mag. , 3423)
Raised at Highclere, the seat of the Earl of Carnarvon in Berkshire, by crossing arboreum with ponticum ; it has beautiful, deep rosy red flowers in large trusses 6 ins through.
Very similar to it is:
Rhododendron Russellianum, Sweet, from catawbiense crossed with arboreum, but of a more crimson shade. It is as richly coloured as all but the richest forms of arboreum. Flowers in April.
For other arboreum crosses, see Nobleanum, pulcherrimum, and venustum. SMITHII with rich red flowers is arboreum+ ponticum ; and CUNNINGHAMI is arboreum+maximum.