Plum trees comprise a number of species in the genus Prunus. Close relatives include apricots, peaches and cherries. Before the fruit is ready for harvest, it goes through several growth stages. The stages of the European plum (P. domesticus "Stanley") are typical, according to the Michigan State University Extension.
Dormancy to Green Clusters
Plum growth starts with the dormant stage when the branches are devoid of fruit or buds. The bud swell is when buds begin bulging from the branch. Bud burst is the stage when the white material inside the bud begins showing. In the green cluster stage, the multiple flowers within each bud are visible as green points, although the flowers are not yet open.
White Bud to Petal Fall
When the individual flowers' petals in bud begin showing, the white bud stage has been reached. First bloom follows when the first few flowers completely uncurl showing white petals and yellow centres. The time when all or nearly all of the tree's flowers are fully open is the full bloom stage. Eventually, the flowers fall to the ground in a stage called petal fall, leaving behind a bud remnant known as a shuck.
Shuck Split to Pit Hardening
When the green, immature fruits emerge from the former flowering site, the plum tree is in shuck split stage. From this stage, the fruits continue growing until reaching their full diameter of 1 to 2 inches. Inside the fully grown but still green fruit, the plum pits take shape in a stage known as pit hardening. Since the state of the pit is not detectable from outside the fruit, growers cut green plums open and check the pits.
Pale Green Fruit to Market
Once the plum pits harden, the fruits undergo a colour change from bright green to a more yellow hue. This is the pale green fruit stage. When the purple begins showing, it reached the colouring fruit stage. Finally, when the plums are fully purple, they are ready for harvest and sent to market. If kept in temperature and humidity-controlled conditions after picking, plums can be stored for two to three weeks.