The Stanley Plum tree (Prunus domestica 'Stanley') is a popular, self-fertile, European-type prune plum known for producing heavy annual crops of sweet, dark blue fruit with yellow-green flesh. It is ideal for fresh eating, drying, and canning, maturing at 12–20 feet in height depending on rootstock, and is hardy in Zones 5–8. 
Key Characteristics & Overview
Fruit: Medium to large, oval, dark blue skin with sweet, firm, freestone yellow-green flesh.
Uses: Excellent for drying into prunes, fresh eating, canning, and preserves.
Harvest Time: Late summer to early fall (September).
Tree Size: Typically 12-20 feet high and wide (can be smaller with dwarfing rootstock).
Pollination: Self-fertile; no other tree required, but a second tree may increase yields.
Hardiness: Zones 5–8 (some sources say 4-9).
Chill Hours: Requires approximately 700 hours. 
Growing Requirements
Light: Full sun is required for best fruit production.
Soil: Well-drained, fertile soil, with good moisture retention.
Maintenance: Low to moderate; requires pruning in late winter and is susceptible to early spring frost.
Spacing: 12-16 feet apart. 
The Stanley Plum is considered a reliable producer and an excellent, compact choice for home gardens.