Boxwoods are popular evergreen shrubs known for their dense foliage and versatility in landscape design. Their ability to provide year-to-year structure makes them a garden staple. Understanding their temperature limits is important for health and longevity, ensuring they thrive in various climates.
Boxwood Hardiness Zones
The USDA Plant Hardiness Zone Map is a tool that helps gardeners determine which plants are most likely to survive. This map divides North America into zones based on the average annual extreme minimum winter temperature. Each zone represents a 10-degree Fahrenheit range, with subzones further dividing these into 5-degree increments.
Most common boxwood varieties thrive in USDA Zones 5 through 9. However, some cultivars tolerate colder temperatures, extending viability into Zone 4. To find your hardiness zone, use online tools or consult local agricultural extension offices. Knowing your zone is an important first step in selecting boxwoods suitable for your climate, promoting successful growth and reducing cold damage.
Protecting Boxwoods from Cold
Boxwoods, while cold-hardy, can suffer damage from harsh winter conditions. Winter burn, appearing as bronzed or yellowed foliage, occurs when leaves lose moisture faster than roots can absorb it from frozen soil. More severe damage, like bark splitting, can result from extreme temperature fluctuations or very low temperatures, causing internal plant tissues to freeze and expand.
Adequate pre-winter watering is important for survival. Continue deep watering until the ground freezes, ensuring the plant has moisture reserves. Applying a 2-3 inch layer of organic mulch, such as wood chips or pine straw, around the base insulates the shallow root system and helps retain soil moisture. For protection against drying winds and sun, especially in exposed sites, consider wrapping boxwoods with burlap after the first hard frost.
Anti-desiccants can be applied to foliage to reduce moisture loss from winter winds, though they offer temporary protection. Heavy snow accumulation on branches should be gently removed to prevent breakage, especially for upright varieties. Healthy, well-hydrated boxwoods throughout the growing season also improves their resilience against winter stress.
Managing Boxwoods in Heat
Boxwoods require specific care to thrive in warmer climates and during hot, dry periods. Consistent and deep watering is important, especially for newly planted or shallow-rooted plants, as they are susceptible to drought stress. During extended heat waves, increased watering frequency may be necessary to prevent wilting and leaf scorch, which are signs of heat stress.
Providing partial shade, especially from intense afternoon sun, can benefit boxwoods in hot regions. Planting them where they receive morning sun and are shaded during the hottest parts of the day helps mitigate heat stress. Proper spacing between plants promotes good air circulation, preventing heat buildup within foliage and reducing fungal issues in humid conditions. Mulching also helps keep the soil around shallow roots cooler and conserves moisture during hot spells.
Choosing the Right Boxwood Cultivar
Not all boxwood cultivars have the same temperature tolerance, making selection important for long-term success. Some varieties are bred or adapted to withstand colder temperatures, while others excel in heat. For instance, ‘Wintergreen’ and ‘Chicagoland Green’ are known for their cold hardiness, performing well in Zone 4 and retaining green color in winter. ‘North Star’ is another globe-shaped, cold-tolerant option that maintains its green color through colder months.
Conversely, cultivars like Japanese Boxwood (Buxus microphylla japonica) are known for their heat tolerance, suitable for warmer climates, including Zones 9 and 10. ‘Green Velvet’ is a hybrid tolerant of both heat and humidity. When selecting boxwoods, always check plant tags or nursery information for the cultivar’s hardiness zone and temperature preferences to ensure it is suited to your climate.