Rosemary is a highly fragrant, woody perennial herb that thrives in hot, sunny conditions. As a Mediterranean native, the plant is poorly adapted to freezing temperatures. Its survival becomes uncertain when cold weather arrives. The central concern for gardeners is how to protect the plant and ensure its longevity when temperatures drop below freezing and frost becomes a regular occurrence.
Determining the Need for Indoor Protection
Deciding the best winter location for a rosemary plant depends on the local climate and expected minimum temperatures. Gardeners use the USDA Plant Hardiness Zone Map as the primary tool for this decision, which correlates minimum winter temperatures with geographic regions. Rosemary is reliably hardy in USDA Zones 8 through 10, where winter lows typically do not fall below 10°F to 20°F. If a plant is located in Zone 7 or any lower zone, sustained outdoor survival is highly unlikely without extreme measures. Most varieties will not tolerate temperatures that remain below 20°F (-6°C) for an extended period, making indoor protection a necessity in colder regions.
Winterizing Rosemary That Stays Outdoors
For plants in mild climates, or those residing in Zones 8 and above, the focus shifts to minimizing stress and providing physical insulation. Even in these zones, strong, drying winds and winter sunlight can damage the foliage and cause desiccation. Planting the rosemary near a south-facing wall or another sheltered location can utilize passive heat to protect the plant from the harshest weather.
One important step is to dramatically reduce watering as the weather cools, since the plant enters a period of near-dormancy. Wet soil followed by a deep freeze is particularly damaging to the roots, often causing root rot or frost heaving. Applying a thick layer of protective mulch, such as straw or wood chips, around the base helps stabilize the soil temperature. This insulating layer prevents the damaging cycle of repeated freezing and thawing that can push the roots out of the ground. During brief, severe cold snaps, a breathable fabric cover or frost blanket can be draped over the plant to offer temporary protection against extreme cold and wind.
Transitioning and Maintaining Indoor Rosemary
Gardeners in colder regions must transition their rosemary plants indoors before the first hard frost arrives, typically when nighttime temperatures consistently drop below 40°F. Before moving the plant inside, it is important to inspect it thoroughly for pests like spider mites and whiteflies, which can quickly spread to other houseplants in the dry indoor environment. Lightly pruning any long, lanky growth will help the plant manage the change in light intensity and improve air circulation once it is confined indoors.
Light Requirements
Rosemary requires an enormous amount of light to survive indoors, which is the most challenging aspect of its winter care. A bright, unobstructed south-facing window is the best location, providing the maximum amount of direct sunlight available during the short winter days. If natural light is insufficient, the plant needs supplemental illumination from a full-spectrum grow light for at least six to eight hours daily to prevent etiolation, or stretching.
Watering and Soil
Overwatering is the single most common reason for indoor rosemary failure, as the plant’s growth slows considerably in winter. The soil should be allowed to dry out almost completely between waterings, and a well-draining potting mix is essential to prevent the root ball from becoming waterlogged. When watering, soak the plant deeply, but ensure all excess water drains away quickly, as rosemary is intolerant of “wet feet.”
Temperature and Air Circulation
The ideal indoor environment should be cool, replicating the mild winters of the Mediterranean. Rosemary thrives best when temperatures are maintained between 50°F and 65°F, which is often cooler than typical household temperatures. Placing the plant in an unheated sunroom or a cool basement with adequate lighting is often more successful than a warm living area. Furthermore, providing good air circulation is helpful to prevent the development of fungal diseases like powdery mildew, which can be an issue in a static indoor environment.