The transfer of genes from one species to another by hybridization then continued breeding (backcrossing) within one of the parent species. For instance, hybrids between Rhododendron ponticum and R. catawbiense proved to be tolerant to winter cold in the 19thC, and the rhododendron now naturalized in Britain (particularly in the Scottish Highlands) is believed to be R. ponticum with some genes from R. catawbiensis.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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