Plums are a popular stone fruit. They are primarily a summer fruit, with the harvest season beginning in mid-summer and extending through early autumn in the Northern Hemisphere. Understanding when this fruit is available requires knowing the progression of the plum tree’s annual cycle, which is influenced by genetics and environmental factors.
The General Plum Growing Season
The yearly cycle for a plum tree begins in late winter or early spring after the tree has satisfied its chilling requirement. Plum trees produce delicate blossoms, ranging from white to pink, typically in early to mid-spring before their leaves emerge. Pollination occurs during this flowering stage, marking the beginning of fruit development.
Following pollination, small plums begin to form in late spring, a phase known as fruit set. The fruit spends the next several months developing. The time from fruit set to harvest spans approximately 140 to 170 days, depending on the variety. This maturation period ensures the fruit develops its characteristic sweetness and size.
The general harvest window for most plums starts in July, continues throughout August, and can stretch into September. Plums are best picked when fully ripe, as they do not significantly increase in sweetness after being removed from the tree.
Key Plum Varieties and Their Ripening Windows
Plums are generally categorized into two main groups: Japanese plums (Prunus salicina) and European plums (Prunus domestica). These two types have genetically distinct ripening schedules. Japanese plums, such as the ‘Santa Rosa’ or ‘Beauty’ varieties, are typically the first to ripen.
These early-season plums are often ready for harvest from late June through July. They are characterized by their larger size, round shape, and juicier flesh. Japanese plums are primarily grown for fresh consumption and thrive in regions with milder winters.
European plums, including varieties like ‘President’ and the popular ‘Italian’ plum, mature later in the season. These varieties are often smaller, more oval, and contain more natural sugar, making them suitable for drying into prunes. Their harvest typically begins in late August and continues through September, with some late-maturing cultivars ripening as late as October.
Climate Factors Influencing Harvest Timing
The annual timing of the plum season is sensitive to local environmental conditions, which can accelerate or delay harvest dates. A significant factor is the requirement for “chill hours,” which refers to the number of hours spent between 0°C and 7°C (32°F and 45°F) during winter dormancy. Plum trees must accumulate a specific amount of chilling, ranging from 275 to over 1,200 hours depending on the variety, to properly break dormancy and flower normally.
Japanese plums generally have lower chilling requirements (118 to 685 hours), suiting them for warmer climates. European plums require a higher number of hours (579 to 1323 hours) and thrive in more temperate regions. If the winter is too warm, the chilling requirement may not be met, leading to delayed or uneven flowering and a smaller crop.
Late-season frosts pose a risk to early-blooming Japanese varieties, as cold temperatures during or immediately after flowering can damage blossoms and prevent fruit set. Additionally, summer heat is measured using metrics like Growing Degree Hours (GDH), which directly influences fruit maturation speed. Higher summer temperatures can lead to an earlier harvest but may result in smaller fruit size.