Knight's Early Black is a traditional English early-season black cherry. The flavour is well-regarded, and the tree is reasonably problem-free and productive.
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Knight's Early Black is self-sterile and needs to be pollinated by another variety which blooms at the same time and is in the same incompatibility group - the simple answer is to plant a modern self-fertile variety nearby, such as Stella, but Van and Amber Heart are also options.
It is likely that Knight's Early Black was raised by T.A. Knight at his orchard near Elton, Herefordshire, in the early 19th century. It achieved some commercial success particularly in Kent, and there are probably several different forms of this variety.