Winter minimum temperatures in southern Indiana drop as low as minus 10 degrees Fahrenheit annually, making it borderline U.S. Department of Agriculture hardiness zones 5b/6a. Plums need a prolonged winter chilling period to flower each spring, and planting multiple plum trees greatly increases the likelihood of a fruit-set thanks to better pollination. Standard trees grow much larger than dwarf plum tree selections.
European plum varieties (Prunus domestica) may be the hardiest choice for an orchard in Indiana since they tolerate winter cold, potentially cooler summer temperatures and flower slightly later in early spring to avoid damaging frosts. Prunes are typically made from European plums since they develop highly sugared fleshes. Varieties to consider include "Blufre," "President," "Reine Claude" ("Green Gage") and "Stanley." In colder microclimates in southern Indiana where frosts are more frequent or more winter wind occurs, also investigate "Dietz," "Yellow Egg" and "Damson" ("Shropshire"). Plant at least two plum trees near each other for better pollination rates although not required. Consider reducing the number of young developing fruits on variety "Stanley" to increase fruit quality and prevent branch breakage.
Japanese plums (Prunus salicina) develop excessive upright branches and require more pruning to yield the more heavily producing horizontal branches. The center of the tree canopy must also remain open. Thinning of fruits, spacing them about 4 inches apart on branches when they are young also develops better crops. These plums appreciate milder climates and warmer summers, and should be grown in southern Indiana in protected microclimates closest to the Ohio River. Varieties to try include "Methley" and "Ozark Premier." Plant multiple trees in a cluster as Japanese plums need cross-pollination to produce their abundant crops of tart, juicy fruits.
Hybrid plums resulting from crossing Indiana native wild plums (Prunus Americana) with Japanese types are also good choices for southern counties. Extremely cold hardy, varieties to grow include "Monitor," "Underwood" and "Superior." If growing selection "Superior," grow "South Dakota" as a pollinator companion tree, according to Purdue University recommendations.
Cherry plums (Prunus cerasifera), sometimes called myrobalan plums, develop into bushy plants that yield excellent small-sized fruit crops for fresh eating or preserves. Plant multiple plants for better fruit set. "Sapa," "Sapalta" and "Kaga" are good choices; use "Compass" as pollinator for "Kaga." Cherry plums demonstrate exceptional winter cold hardiness and may double as a windbreak on southern Indiana homesteads. For an inexperienced gardener, these may be the most rewarding orchard plants of all discussed here.