Rosemary (Salvia rosmarinus) is a fragrant, needle-leaved herb that grows as a woody, evergreen shrub in its native Mediterranean habitat. Gardeners often wonder if this perennial can withstand freezing temperatures and return after a cold season. The plant’s survival depends entirely on the local climate and the specific cultivar planted. Rosemary often survives year-round where winters are mild, but it must be protected or brought indoors where the cold is more intense.
Understanding Rosemary’s Hardiness
Rosemary is classified as a tender perennial or woody shrub, meaning it does not reliably shed its leaves or go completely dormant like deciduous plants. Outdoor survival is primarily mapped using the United States Department of Agriculture (USDA) Plant Hardiness Zone system. Most varieties thrive where winter temperatures rarely drop below 15°F to 20°F, correlating generally to USDA Zone 8 and warmer.
Below this temperature threshold, the plant’s cellular structure sustains damage from ice crystal formation, leading to tissue death. Roots are susceptible to freezing when the ground temperature drops significantly, especially in poorly drained soil. Cold, desiccating winter winds also cause moisture loss from the evergreen leaves. This moisture loss, which the roots cannot replenish from frozen soil, is known as winter burn.
The specific variety influences cold tolerance; for instance, cultivars like ‘Arp’ or ‘Madeline Hill’ are slightly more cold-tolerant than others. Upright, traditional forms tend to be less resilient to extreme cold than lower-growing or creeping varieties. Prolonged exposure to temperatures below 10°F will likely result in significant damage or complete plant death, regardless of the type.
Winterizing Techniques for Survival
Gardeners in marginal climates, such as USDA Zones 7 and 8, can take preventative measures to encourage outdoor survival. Successful outdoor winterizing requires excellent soil drainage, as cold, waterlogged roots are significantly more prone to rot and freezing damage. Rosemary planted in raised beds or sandy soil fares better than those in heavy clay.
Apply a thick layer of organic mulch, such as straw or shredded leaves, around the base of the plant to insulate the root crown. The mulch should be 4 to 6 inches deep but kept a few inches away from the main stem to prevent moisture buildup and fungal issues. Erecting a temporary windbreak or placing the plant on the south side of a structure mitigates the drying effects of winter wind.
In colder regions where reliable outdoor survival is impossible, containerized rosemary must be moved indoors before the first hard frost. Before bringing the plant inside, inspect the foliage and soil for common pests like spider mites or scale insects. Treat pests immediately to prevent an indoor infestation.
Once relocated, the plant requires a period of semi-dormancy, needing high light but reduced water and fertilizer. A south-facing window offers the best natural light, but supplemental light from a simple fluorescent or LED grow light may be necessary. Watering should be infrequent—only when the soil is completely dry to the touch—to mimic the plant’s winter rest cycle.
How to Check for Life in Spring
When spring arrives, a rosemary plant that looks brown and brittle may not necessarily be dead; it could simply be suffering from winter dieback. The definitive way to determine viability is by performing the “scratch test” on the woody stems after the threat of the last hard frost has passed. Gently scrape a tiny section of bark off a lower stem using a fingernail or small knife.
If the tissue immediately beneath the bark is bright green, the stem is alive, meaning the plant survived the winter. If the tissue is brown, dry, and brittle, that section is dead and should be pruned back until living green wood is reached. Often, only the tips of the branches are damaged, requiring light corrective pruning.
If the scratch test reveals brown tissue all the way down to the base of the plant, it is unlikely to recover and should be replaced. Rosemary is slow to break dormancy in the spring compared to many other garden plants, so patience is required. Wait until consistent warm weather has arrived before making a final determination or discarding a seemingly lifeless plant.