Populus nigra is a diploid with 2n=38 chromosomes. As one of 3 species included within the European Forest Genetic Resources Programme (EUFORGEN) a considerable amount of data has accrued as to the levels and distribution of genetic diversity within this species across Europe (Cottrell et al., 2005), this has recently been supplemented by local initiatives driven by LBAP’s which hopefully will provide a detailed picture across the British Isles (Adams, 2009). The species shows rather low genetic diversity across its wide range but the greatest levels are present south of the Pyrenees, Alps and Balkans indicating that these were refugial areas during the last glaciation and recolonisation of Northern Europe has been accomplished by relatively few clones, naturally and later with human assistance. Probably less than 100 clones exist in the British Isles, within Greater London 6 of the previously identified clones are known to exist in multiple locations, the most extensive being the Manchester Poplar clone. New clones are still being discovered as more intensive sampling is being made; recent work has shown another 9 clones to be present in Greater London, generally as single individuals, on the Thames bank in Kew, Syon Park and at Barnes, where they are threatened by planned renovation work.