Prunus domestica ‘Mount Royal’ Considered the best blue plum. Extremely hardy, late-summer, freestone plum with blue skin and firm, yellow flesh. Spreading canopy. Self-pollinating but benefits from cross-pollination with European plum varieties. Good for eating, processing or drying. Prefers a well-drained soil. Height: 4m (13 FT.) Spread: 2.5m (8 FT.) Zone 2