Plum is one of the most genetically complicated fruits in the Prunus group because it has a number of species, all which are commonly interbred. The resulting fruits fall into several broad categories, including European, Japanese, and prune plums. They differ in important ways:

• Japanese plums are round, usually require cross-pollination to set fruit, and are pruned to an open-centered shape. They’re generally hardy to Zone 6 and warmer parts of Zone 5, although some hybrids are hardier. Varieties include ‘Shiro’, ‘Early Golden’, ‘Burbank’, ‘Redheart’, ‘Santa Rosa’, ‘Methley,’ and ‘Beauty’. Hybrids ‘Elite’, ‘Superb’, ‘Tecumseh’, ‘Perfection’, and ‘Brookgold’ are hardy through Zone 4.
• European plums are oval, and most don’t need a second variety for pollination. Train these trees to a modified leader form. Most varieties are hardy through Zone 5, and some produce fruit even in Zone 4. Varieties include ‘Damson’, ‘Seneca’, ‘Verity’, and ‘Green Gage’.
• Prune plums are European plums with drier flesh and a high sugar content that makes them suitable for drying. Common varieties include ‘Stanley’, ‘French Prune’, ‘Fellenberg’ or ‘Italian Prune’, ‘Valor’, ‘Earliblue’, ‘Sugar’, and ‘Mount Royal’, which is hardy in Zone 4.

Japanese and European plums can’t pollinate each other, so if your trees require cross-pollination, be sure to choose compatible varieties. Thin the fruits to hang 4 to 6 inches apart, 5 to 8 weeks after bloom. Fruits ripen from July through September, starting with Japanese plums and ending with prune plums, depending on the variety.

Plant breeders have also crossed apricots with plums to create hybrids called apriums, plumcots, and pluots. These are hardy wherever European apricots grow, require cross-pollination, and have characteristics of both parents.