Toka plum
Toka is an incredibly tasty plum that is sometimes called the bubblegum plum – and I can attest that this fruit really does taste like candy. The fruit is purple-red on the outside with yellow, aromatic flesh.
Cold hardiness: Toka is cold hardy to at least zone 4 (for reference, see this map of Canada’s Plant Hardiness Zones – Guelph is zone 5b). Japanese/Asian plums generally flower earlier than European plums, and so there is a slight risk of a late hard frost damaging their flowers, but I believe Toka's fruits are worth the risk.
Pollination: Self-fertile. Toka is a hybrid of a Japanese plum (Prunus salicina) and an American plum (Prunus americana) and is a great pollinator for both Japanese plums and native plums (but not European plums, which are only cross-pollinated with other European plums).
Rootstock: Myrobalan 29C.
References:
Tree fruit culture and varieties in North Dakota.
Japanese-American hybrid plums.
Image 1: Glority.
Image 2: Daniel Wachenheim.