Rosemary (Salvia rosmarinus) is a fragrant, woody herb native to the Mediterranean region, used widely in culinary and garden landscapes. Gardeners rely on the USDA Plant Hardiness Zone Map to determine which plants can survive the winter in their specific location. This system categorizes regions based on the average annual minimum winter temperature. Understanding the climate and the plant’s limits is necessary to determine if this sun-loving perennial can survive the winter in Zone 6.
Understanding Hardiness Zone 6
USDA Hardiness Zone 6 is defined by average annual minimum winter temperatures ranging from -10°F to 0°F (-23.3°C to -17.8°C). The zone is divided into two subzones: 6a (-10°F to -5°F) and the slightly warmer 6b (-5°F to 0°F).
For a plant to be reliably perennial here, it must withstand temperatures as low as -10°F during an average winter. This consistent threat of sub-zero temperatures presents a significant hurdle for plants originating from warmer climates.
Rosemary’s Cold Tolerance Threshold
Rosemary is generally categorized as a tender perennial, reliably surviving year-round only in Zones 8 through 10. Most common varieties have a cold tolerance threshold that does not extend much below 10°F (-12°C).
This limited tolerance means standard rosemary often suffers extensive damage or total dieback when exposed to Zone 6 minimum temperatures. Therefore, without specific intervention, rosemary is not a guaranteed perennial and is often treated as an annual herb that must be replanted each spring. The primary threat is prolonged or repeated exposure to temperatures significantly below the plant’s 10°F survival limit.
Selecting Hardy Cultivars and Ideal Planting Sites
Selecting Cold-Hardy Cultivars
Successfully overwintering rosemary in Zone 6 starts with selecting a cultivar specifically bred for cold-hardiness. Certain varieties demonstrate greater tolerance than the species standard, often surviving down to -10°F when properly protected.
The cultivar ‘Arp’ is widely recognized as one of the most cold-tolerant, capable of withstanding the lowest temperatures within the Zone 6 range. Other reliable cold-hardy options include ‘Madeline Hill’ and ‘Alcalde Cold Hardy.’
Optimizing the Planting Site
Selecting the right planting location is equally important for survival, as this creates a microclimate that can effectively warm the root zone. Planting rosemary near a south-facing wall or large structure provides thermal mass that absorbs solar heat during the day and slowly releases it at night, offering a few degrees of protection.
The planting medium must be extremely well-drained, as wet soil combined with freezing temperatures is a common cause of death for rosemary. Incorporating coarse sand or grit encourages rapid drainage and prevents the roots from sitting in icy, saturated conditions. A raised bed or mounded planting location can further ensure that water moves away from the plant’s crown during winter.
Active Overwintering Techniques for Zone 6
Outdoor Protection
For plants left in the ground, active protection measures are necessary. Apply a heavy, dry layer of mulch, such as pine needles or straw, around the base to insulate the root zone from deep freezing. Do not pile mulch directly against the woody stems, as this encourages rot.
Temporary structures, like a cold frame or a shelter covered with burlap or frost cloth, shield the plant from harsh winter winds and ice. Remove these covers during mild days to allow for air circulation and light exposure, preventing excessive moisture buildup. The goal is to keep the root ball consistently cold but dry, preventing it from freezing solid.
Indoor Overwintering
Overwintering rosemary indoors offers the most reliable method for survival in Zone 6. The plant should be potted up and moved to a cool, bright location, ideally a sunroom or unheated garage with a window.
The ideal indoor temperature range is between 40°F and 50°F, allowing the plant to enter a semi-dormant state. Watering must be drastically reduced during the indoor period, applied only when the soil is completely dry, to prevent root rot.