Several European and hybrid plum varieties can be successfully grown in Iowa and are self-fruitful. That is, a single tree will bear fruit. There are also several hybrid plum varieties with good cold hardiness that do well in Iowa. The hybrid varieties require two or more plants to achieve pollination and successful fruit set. More can be found at https://hortnews.extension.iastate.edu/faq/what-are-some-good-plum-varieties-iowa
As plums approach maturity, the fruit develop their characteristic color. Ther fruit of blue or purple varieties change from green to greenish-blue, then to dark blue or purple. The ripened fruit of other varieties vary from yellow to red. As they ripen, plums begin to soften, especially at the tip end. They also develop their characteristic flavor. Handle plums carefully.
Plums can be stored for approximately 2-4 weeks at temperatures near 32° F.