Plum trees, like other stone fruits, operate on an annual reproductive cycle that begins after a period of winter rest. This cycle moves sequentially from an inactive state to the development of mature fruit ready for picking. Understanding these specific growth stages is important for anyone wishing to successfully cultivate this popular fruit. The journey from a dormant bud to a fully ripe plum involves a precise biological timeline, governed by temperature, genetics, and successful interaction with the environment.
Bud Development and Pre-Bloom
The first stage involves the tree breaking its winter dormancy, a process controlled by the accumulation of chilling hours. Chilling hours are the total time the tree spends in cold temperatures, generally defined as between 32°F and 45°F, which is required for proper bud development and subsequent flowering. Plum varieties typically require 300 to 700 hours of chilling, though this varies significantly by cultivar. Without sufficient cold, the buds may open unevenly or fail to develop properly, leading to a reduced harvest.
As temperatures begin to warm, the dormant buds swell and differentiate into two distinct types: vegetative buds, which produce leaves and new shoots, and flower buds, which contain the reproductive structures. The visual progression of the flower buds is marked by several distinct phases, starting with the swollen bud stage. This progresses to the “silver tip” and then “green tip” stages as the protective scales part, revealing the developing flower inside. This pre-bloom development culminates just as the petals of the flower begin to separate, signifying the imminent opening of the bloom.
Flowering and Successful Pollination
Once the buds open, the tree enters the flowering stage, typically in early spring, which can last for one to two weeks. The timing of the bloom is dependent on the local climate and specific plum variety. Successful fruit development requires the transfer of pollen from the anther to the stigma of a flower, a process primarily carried out by insects like honeybees.
Many plum varieties are self-unfruitful, meaning they require cross-pollination from a different plum variety to set fruit reliably. This mandates that a second, compatible plum tree with a similar bloom time must be planted nearby, usually within 500 feet, to ensure adequate pollen transfer. Other cultivars are self-fertile and can produce fruit with their own pollen, but they often benefit from cross-pollination for a heavier crop and larger fruit size. The window of time during which the flower is receptive to fertilization is brief. Late spring frosts during this period can cause significant damage, turning the female organs brown or black and preventing fruit set.
Fruit Set and Pit Hardening
The moment a flower is successfully fertilized, the ovary at its base begins to swell and transform into a tiny fruitlet, marking the stage known as fruit set. A plum tree often produces far more flowers than it can sustain as mature fruit, leading to a natural thinning process. This phenomenon, commonly referred to as “June drop,” is the tree’s mechanism for shedding unviable or excess fruitlets, ensuring the remaining crop is adequately nourished.
Following fruit set, the plum enters a phase where size increase is minimal as the tree focuses its energy on developing the seed. This period is defined by the process of pit hardening, where the endocarp layer of the fruit, which surrounds the seed, becomes lignified and forms the hard stone. The construction of this protective stone takes precedence. This hardening phase must be complete before the fruit can begin its final expansion.
Final Swelling and Harvest Maturity
Once the pit has fully hardened, the plum enters the last stage of development, characterized by a rapid increase in size and the accumulation of sugars. This final swelling is dependent on warm temperatures and a consistent supply of water, which drives the expansion of the fruit’s fleshy mesocarp. The fruit also begins to change color, with the ground color shifting from green to yellow, and the variety’s final red, purple, or blue hues developing.
The reliable indicators for harvest maturity are a combination of color change, flesh firmness, and the sugar-to-acid ratio. For most varieties, the plum is ready for picking when the skin has fully colored and the flesh begins to soften. Plums picked for immediate consumption are allowed to soften to a flesh firmness of 2 to 3 pounds-force. Fruit intended for shipping or storage is harvested when slightly firmer to prevent bruising. This final stage represents the culmination of the tree’s annual reproductive labor.