Cherry trees (Prunus spp.) are cherished for their delicate spring blossoms and their sweet or tart summer fruit. Successful cultivation depends on matching the tree’s biological requirements to the local climate and geographic location. Nurseries and home gardeners rely on the standardized Plant Hardiness Zone Map (PHZM) to predict a tree’s likelihood of surviving the winter. This system is the initial assessment for determining where cherry trees can thrive.
Defining the Plant Hardiness Zone System
The United States Department of Agriculture (USDA) developed the Plant Hardiness Zone Map (PHZM) as a guide for growers across the country. This map divides North America into 13 distinct zones, numbered 1 through 13, that indicate the expected winter minimum temperature. The designation of a zone is based exclusively on the average annual extreme minimum winter temperature recorded over a 30-year period.
Each zone represents a 10-degree Fahrenheit difference, and is further refined by ‘a’ and ‘b’ subzones, which represent a 5-degree Fahrenheit increment for greater precision. The PHZM indicates whether a perennial plant, such as a cherry tree, can survive the coldest temperatures of an average winter. However, the PHZM is only a measure of cold tolerance and does not account for other factors affecting plant health, such as summer heat, soil type, rainfall, or the necessary period of winter dormancy.
Specific Zone Requirements for Cherry Varieties
Cherry varieties are separated into sweet and sour groups, each with distinct cold tolerance requirements defining their suitable growing zones. Sour or tart cherries (Prunus cerasus) are generally the hardier of the two and are better suited for colder climates. Varieties like ‘Montmorency’ and ‘North Star’ typically thrive across USDA Zones 3 through 8. Their robust nature allows them to tolerate colder winter temperatures, making them a popular choice in northern states.
Sweet cherries (Prunus avium), which are most often consumed fresh, are less forgiving of cold temperatures and more susceptible to damage from late spring frosts. Traditional sweet cherry varieties commonly grow in the milder climate of USDA Zones 5 through 9. While many sweet cherries are well-adapted to Zone 5, they struggle with the extreme cold that sour cherries can withstand.
Newer cultivars and rootstock developments have pushed the boundaries of these conventional zones. Certain low-chill varieties like ‘Royal Crimson’ and ‘Minnie Royal’ have been bred to thrive in warmer areas, extending the growing range into higher zones like 9 and 10. Conversely, hardier rootstocks can help sweet cherries survive in the colder reaches of Zone 4.
Why Chill Hours are Critical for Cherry Production
The ability of a cherry tree to survive the winter, as defined by the hardiness zone, is only one part of the successful cultivation equation; the tree must also receive a sufficient number of chill hours to produce fruit. Chill hours refer to the total accumulated time the tree spends in a specific temperature range during its winter dormancy period. This period is typically measured when temperatures fall between 32°F and 45°F (0°C and 7.2°C).
This cold exposure is a biological necessity that allows the tree to properly break dormancy and set flowers and fruit buds in the spring. Without enough accumulated chill, the tree may exhibit delayed bud break, uneven flowering, or a complete failure to produce fruit. Most standard sweet cherry varieties require a substantial chill period, often ranging from 700 to 1,200 hours annually, while many sour cherries require over 1,200 hours.
This requirement explains why a cherry tree rated for survival in USDA Zone 9 may still fail to fruit in a region with mild winters. A location in Zone 9 might never accumulate the necessary 700+ hours of cold to initiate fruiting, even if the tree survives. This has led to the development of low-chill varieties, such as ‘Stella’ or ‘Lapin’, which require 500 or fewer chill hours, making them suitable for regions with warmer winters.