Rosemary (Salvia rosmarinus) is a fragrant, popular herb known for its needle-like foliage and aromatic scent. This Mediterranean native is used widely in cooking and thrives under specific climatic conditions. Gardeners rely on the USDA Plant Hardiness Zone Map, which categorizes areas based on the average annual minimum winter temperature, to determine if rosemary can survive the winter in their region.
Defining Rosemary’s Core Hardiness Zones
Rosemary is a true evergreen perennial in the warmest regions of the United States, specifically USDA Hardiness Zones 8 through 10. In these zones, the plant grows year-round, developing woody stems and persisting for many years. The typical rosemary plant’s temperature limit is around 20°F, aligning with the minimum temperatures experienced in these warmer zones.
In Zone 7, rosemary is considered a semi-perennial, meaning it may survive the winter but often requires protection. Zone 7 minimum temperatures can drop to 0°F to 10°F, which is below the survival threshold for standard varieties. Consequently, many Zone 7 gardeners treat rosemary as a tender perennial that needs intervention to survive.
Specialized cultivars have been developed to increase cold tolerance for gardeners in slightly colder climates. ‘Arp’ and ‘Hill Hardy’ are the most cold-tolerant varieties available. ‘Arp’ rosemary, discovered in Texas, can survive temperatures as low as -10°F, allowing it to be grown in Zone 6.
‘Hill Hardy’ rosemary is also rated as hardy down to Zone 6, especially with proper winter care. These hardier varieties typically feature an upright growth habit, which is often more cold-tolerant than trailing types. However, survival outside of the core Zones 8-10 depends heavily on protecting the plant from sustained periods of deep cold.
Essential Environmental Requirements Beyond Zone
Successfully growing rosemary requires several environmental factors regardless of the hardiness zone. Full, direct sunlight is a requirement for this Mediterranean herb, needing a minimum of six to eight hours of sun exposure daily. Without adequate light, the plant will become weak and elongated, a condition known as etiolation.
The composition of the soil is equally important because rosemary is susceptible to root rot. The plant demands well-draining soil that mimics its native rocky, arid habitat; heavy clay soils that retain moisture are unsuitable. Amending the planting area with sand, gravel, or grit ensures the sharp drainage needed.
Rosemary thrives in soil that is slightly acidic to neutral, ideally maintaining a pH level between 6.0 and 7.0. Once established, the herb is drought-tolerant and requires very little watering. Overwatering is the leading cause of death for rosemary plants, so the soil must dry out completely between waterings.
Strategies for Growing Rosemary Outside the Ideal Zone
Gardeners in Zones 6 or colder must treat rosemary as an annual or use intervention to ensure survival. Container gardening is the most reliable strategy, allowing the plant to be moved indoors before the first hard frost. Planting rosemary in terracotta pots is beneficial because the porous material helps wick away excess moisture, promoting drainage.
To overwinter rosemary indoors, transition the plant gradually to the indoor environment to avoid shock and needle drop. The indoor location should be the coolest and sunniest spot available, ideally a south-facing window with temperatures between 55°F and 65°F. Supplemental full-spectrum grow lights are often necessary to provide the required light intensity during winter.
Potted rosemary requires less water during the winter months as its growth slows, and overwatering remains a risk. Gardeners should inspect the plant thoroughly for pests before bringing it inside to prevent spreading insects to other houseplants. For borderline Zone 7 regions, outdoor plants can be protected by applying a thick layer of mulch around the base. Covering the shrub with burlap or a frost cloth also provides insulation against winds and severe cold snaps.