Prunus cerasus, or the sour cherry, is a species of Prunus in the subgenus Cerasus (cherries), native to much of Europe and southwest Asia. It is closely related to the wild cherry (P. avium), but has a fruit that is more acidic.
The tree is smaller than the wild cherry (growing to a height of 4–10 m), has twiggy branches, and its crimson-to-near-black cherries are borne upon shorter stalks.
There are two varieties of the sour cherry: the dark-red morello cherry and the lighter-red amarelle cherry.
Cultivated sour cherries were selected from wild specimens of Prunus cerasus and the doubtfully distinct P. acida from around the Caspian and Black Seas, and were known to the Greeks in 300 BC. They were also extremely popular with Persians and the Romans who introduced them into Britain long before the 1st century AD. The fruit remains popular in modern-day Iran.
In Britain, their cultivation was popularised in the 16th century in the time of Henry VIII. They became a popular crop amongst Kentish growers, and by 1640 over two dozen named cultivars were recorded. In the Americas, Massachusetts colonists planted the first sour cherry, 'Kentish Red', when they arrived.
Before the Second World War there were more than fifty cultivars of sour cherry in cultivation in England; today, however, few are grown commercially, and despite the continuation of named cultivars such as 'Kentish Red', 'Amarelles', 'Griottes' and 'Flemish', only the generic Morello is offered by most nurseries. This is a late-flowering variety, and thus misses more frosts than its sweet counterpart and is therefore a more reliable cropper. The Morello cherry ripens in mid to late summer, towards the end of August in southern England. It is self fertile, and would be a good pollenizer for other varieties if it did not flower so late in the season.
Sour cherries require similar cultivation conditions to pears, that is, they prefer a rich, well-drained, moist soil, although they demand more nitrogen and water than sweet cherries. Trees will do badly if waterlogged, but have greater tolerance of poor drainage than sweet varieties. As with sweet cherries, Morellos are traditionally cultivated by budding onto strong growing rootstocks, which produce trees too large for most gardens, although newer dwarfing rootstocks such as Colt and Gisella are now available. During spring, flowers should be protected, and trees weeded, mulched and sprayed with natural seaweed solution. This is also the time when any required pruning should be carried out (note that cherries should not be pruned during the dormant winter months). Morello cherry trees fruit on younger wood than sweet varieties, and thus can be pruned harder. They are usually grown as standards, but can be fan trained, cropping well even on cold walls, or grown as low bushes.
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