Rosemary, a fragrant, woody herb native to the Mediterranean, naturally thrives in warm, dry climates. This plant is typically hardy only in USDA Zones 8 through 10, meaning it struggles to survive sustained freezing temperatures. For gardeners in colder regions, ensuring the survival of this perennial requires specific preparation and attention to its environment. This guide provides the practical steps necessary to successfully overwinter rosemary, whether it remains outdoors or is moved inside.
Preparing Rosemary for Cold Weather
Preparation for winter begins in late summer or early autumn, well before the first expected frost. Lightly prune the plant to remove any dead or spindly growth and to shape the shrub, avoiding cuts into the thick, woody stems. This pruning helps the plant focus energy on existing foliage and roots rather than new growth.
If the rosemary is in the ground and will be potted to move indoors, the transfer should be done gradually. After potting, allow the plant a week or two outdoors in a sheltered spot to recover from the root disturbance.
Before moving the plant indoors, thoroughly inspect the foliage and soil for pests such as spider mites or scale insects. Treat any existing infestation with a mild insecticidal soap or horticultural oil to prevent an outbreak among houseplants. The warm, dry indoor environment is ideal for many common garden pests to flourish.
Overwintering Rosemary Outdoors
Gardeners in borderline climates, such as USDA Zone 7 or warmer parts of Zone 6, may attempt to keep established, in-ground rosemary outdoors with added protection. Select cold-hardy varieties like ‘Arp’ or ‘Madalene Hill,’ which better survive brief dips below freezing. Outdoor protection focuses on insulating the root ball and shielding the plant from harsh winter winds and sun scald.
Apply a substantial layer of organic mulch, such as straw, shredded leaves, or wood chips, three to six inches deep around the base of the plant. This mulch insulates the soil and stabilizes the root temperature. Keep the material away from the main stem to prevent rot.
Placing the plant near a south-facing wall can also help, as the wall absorbs solar heat during the day and radiates it back at night. For extreme cold snaps, use a temporary shelter, such as a framework covered with burlap or a frost blanket. Remove coverings once the severe cold passes to ensure adequate air circulation and light exposure.
Essential Care for Indoor Rosemary
Bringing rosemary indoors is the most reliable method for survival in regions with sustained freezing temperatures. The biggest challenge is replicating Mediterranean conditions by providing maximum light and a cool environment. Rosemary performs best in a cool room, ideally maintaining temperatures between 40°F and 60°F, which encourages a natural, semi-dormant state.
Locate the plant in the brightest possible spot, such as an unobstructed, south-facing window, ensuring it receives at least six hours of direct sunlight daily. If natural light is insufficient, use a supplemental LED or fluorescent grow light to prevent the plant from becoming leggy and weak. Avoid placing the pot near any heat source, including radiators or forced-air vents, as the resulting warm, dry air can quickly desiccate the foliage.
Watering is the most frequent cause of failure for indoor rosemary, as the plant is susceptible to root rot in cool, low-light conditions. Allow the soil to dry out significantly between waterings, applying water only when the top one to two inches of the potting mix feel dry. A terracotta pot is recommended because its porous nature helps wick excess moisture away from the roots.
Indoor conditions often trigger outbreaks of sap-sucking pests, especially spider mites, which thrive in low humidity. Regularly inspect the needle clusters for fine webbing or tiny moving dots, and check the stems for scale insects. Increasing humidity with a pebble tray or small humidifier can help deter mites. Pests can be managed by washing the plant with a mild solution of soapy water or horticultural oil.
Returning Rosemary Outside in Spring
The outdoor transition should only begin once the danger of the last hard frost has completely passed. Moving the plant directly from the dim, protected indoor environment to full sun and wind will cause severe shock, often resulting in sun scald on the needles. This process of “reverse hardening off” must be gradual to prevent damage.
Begin by placing the rosemary in a sheltered, shady location outdoors for just a few hours each day. Over seven to ten days, slowly increase the time the plant spends outside, incrementally exposing it to brighter sun and wind. This slow acclimatization allows the plant to adapt to the higher light intensity and fluctuating outdoor temperatures.
Once permanently settled outside, the rosemary can benefit from a light pruning to remove any growth that became weak during the winter. A diluted application of a balanced, water-soluble fertilizer encourages new growth as the plant exits dormancy.