Plums are diverse stone fruits belonging to the genus Prunus, which also includes peaches, cherries, and almonds. While many varieties can be grown across the United States, commercial production is heavily reliant on a few specific geographic areas. The climate requirements for large-scale plum farming are quite demanding, resulting in a distinct concentration of the US supply chain.
California’s Central Role in Plum Production
California is the undisputed leader in US plum production, often accounting for around 90% of the entire fresh plum crop. This dominance is facilitated by the unique climate and fertile soils of the state’s Central Valley. The San Joaquin region, particularly Fresno and Tulare counties, forms the heart of the fresh plum industry.
The fresh market plums grown here are predominantly Japanese varieties (Prunus salicina), such as Santa Rosa and Blackamber. These varieties thrive in the mild winters and long, hot, dry summers of the Central Valley. The dry summer heat is advantageous for ripening and minimizes the risk of fungal diseases like brown rot.
California also leads the dried plum industry (prunes). European plums (Prunus domestica) are used for prunes, with the state producing an estimated 99% of the nation’s supply. Prune acreage is concentrated in the northern Central Valley, specifically the Sacramento Valley. The ‘Improved French’ cultivar dominates production, prized for its ability to ripen uniformly and tolerate the high summer temperatures required for mechanical harvesting and drying.
Secondary Growing Regions in the Pacific Northwest and Beyond
Outside of California, the Pacific Northwest states of Washington and Oregon represent the next most significant commercial growing area. Washington often ranks as the second-largest plum-producing state. The focus in this region shifts toward European plum varieties, which are better suited to the cooler climate.
These plums thrive in areas with true four-season climates, featuring longer, colder winters than California’s Central Valley. Popular varieties include the Italian plum and Stanley, which are often used for canning, preserves, or drying, contrasting with California’s fresh-market Japanese varieties. The Willamette Valley in Oregon is known for producing these European types, adapted to the region’s moderate summers and moisture levels.
Beyond the West Coast, plum production is much smaller and often caters to local or specialized markets. States like Michigan, Idaho, and New York have niche commercial production, primarily focusing on European varieties. These areas typically serve regional fresh markets or processing needs, such as jams and preserves. Their cooler climates and higher humidity levels limit the commercial viability of the fresh-market Japanese varieties.
Essential Climate Factors for Plum Growth
Commercial plum cultivation is heavily dependent on meeting specific climate requirements, which explains the concentration of production in a few regions. The most critical factor is the accumulation of “chill hours” during the winter dormancy period. Chill hours are the total number of hours when the temperature remains between 32°F and 45°F, necessary for the tree to properly break dormancy and flower in the spring.
Japanese plums, the main fresh market type, generally require less chilling, typically between 300 to 900 hours, making them suitable for warmer winter climates like California’s Central Valley. European varieties, including prunes, demand a higher chilling requirement, sometimes exceeding 1,000 hours, which is why they are successfully grown in the Pacific Northwest. If a plum tree does not receive enough chill, it can lead to delayed or uneven flowering, resulting in poor fruit set and reduced yields.
The second major climatic need is a consistently warm and dry summer, particularly during the ripening and harvest season. This warm, arid environment is crucial for maximizing the sugar content in the fruit and preventing fungal diseases. High humidity and excessive late-season rainfall can encourage pathogens that can quickly ruin a commercial crop. Furthermore, the timing of spring bloom must avoid late frosts, as a sudden drop in temperature can damage the delicate flowers and eliminate the entire year’s harvest.