Interesting, never heard of bullace before, maybe I've been calling bullaces damsons all these years !
Do they also go dark red when stewed ? The ones we used to find did...
I guess they're all plum family at the end of the day..
Bullace, damson, common plum, sloe and mirabelle are all types of small plum (Prunus genus, along with cherries, peaches etc). There are several species in there - spinosa, cerasifera etc. and a lot of hybridisation between them which creates a lot of intermediate characteristics, usually classified as domestica. There are however some general groupings of physical characteristics which pretty much describe the 'type' of each in general convention, although what they are known as locally/colloquially is a lot more variable.
Mirabelles (P.cerasifera) grow on bigger trees which don't sucker. They can be red, yellow or purple, ripen in July and are sweet enough to eat raw.
The damson and wild plum types (domestica) ripen next, through August and September. Wild plums are sweet enough to eat raw, just small, and may be anything from green to yellow to purple, whereas damsons are a distinct group and have a much stronger, more intense flavour (although Merryweather is marginal for being classed as a damson as it is sweeter and less intense). This means they are usually cooked. It is the intensity of flavour in damsons which means they work well in flavouring spirits and has them classified as a distinct group - subspecies institia. Most damsons are elongated rather than spherical (again, Merryweather is an exception).
Sloes are next (P.spinosa) in September/October. They are distinguished by smaller trees with much more spines and exceedingly astringent fruit making them inedible raw. The tannins are broken down by alcohol, hence sloe gin is not astringent and neither is the flesh afterwards.
Bullace are the last to ripen - October to December. These are also varied in colour from pale green (known as white) to yellow and purple and spherical shaped. There are some distinct named varieties. Bullace are like a hybrid between sloes and damsons - small and very acid until they are properly ripe (Shepherd's may not actually ripen enough to eat before it is destroyed by frost).
However, the above groupings are pretty fluid as they are all constantly hybridising anyway!
Alec