One of the few tree fruits indigenous to the upper Midwest, northern Ontario and the prairie provinces of Canada and capable of surviving the harsh winters there, efforts were made to breed improved cultivars of these plums in the 19th century. Notable varieties still grown today include "Assiniboine" and "Cheney". "Prunus nigra" has the same number of chromosomes as "Prunus salicina", the cultivated Japanese plum, and so the two cross-pollinate readily. Breeding work in the 20th century resulted in improved "P. nigra x salicina" hybrid varieties that retain the high quality of Japanese plums and the hardiness of wild Canada plums, such as "Pembina", "Superior", and "Patterson Pride".