From our perspective as plants, our survival is dictated by our genetic programming, which is a product of our native habitat. We, rosemary (*Salvia rosmarinus*), originate from the dry, rocky hillsides of the Mediterranean basin. This means we are inherently adapted to mild winters, plenty of sunshine, and very well-draining soil. Our cold tolerance is not our strongest trait. While we can withstand a light frost, our vascular tissues, the intricate system that moves water and nutrients throughout our stems, are highly susceptible to damage from prolonged freezing temperatures. When the water within our cells freezes, it expands, rupturing the cell walls and causing the dreaded blackening and die-back that gardeners often see. Our fundamental answer to the question of winter survival is therefore heavily dependent on the specific climatic conditions of your location.
The USDA Plant Hardiness Zone Map is essentially a translation of average annual extreme minimum winter temperatures into a simple numbering system. This is the most critical factor for our survival outdoors. We are generally considered hardy in zones 8 through 10. In these zones, where winter lows typically dip no lower than 10°F to 20°F (-12°C to -7°C) in zone 8, and are much milder in zones 9 and 10, we can often survive winter in the ground with minimal protection. However, if you are gardening in zone 7, where temperatures can plummet to 0°F to 10°F (-18°C to -12°C), our chances of survival become precarious and are highly dependent on microclimates and winter protection. For any zone lower than 7, surviving an outdoor winter is highly unlikely for us without being dug up and moved indoors, as the sustained deep freezes will be fatal to our root systems and above-ground growth.
Your USDA zone provides the baseline, but several other factors from our point of view can mean the difference between life and death during the cold months. Drainage is paramount. We are far more likely to perish from cold, wet roots (which lead to rot) than from cold, dry roots. Planting us on a slope or in a raised bed with very gritty, sandy soil is essential. Our age and health also play a role; a mature, well-established plant that has been properly hardened off in the autumn has a much stronger root system and woodier stems, granting it significantly more resilience than a young, tender specimen planted late in the season. Furthermore, a sheltered location, such as against a south-facing wall that absorbs daytime heat and radiates it at night, can create a microclimate that is a full zone warmer than your surrounding area.
If you are in a zone 7 or a particularly harsh zone 8, you can take actions to significantly improve our odds. After the first hard frost has hardened our growth, applying a thick, loose mulch of straw, shredded leaves, or pine boughs around our base is crucial. This does not keep us warm, but rather keeps the soil consistently cold, preventing the damaging cycle of freeze-thaw-freeze that can heave our roots from the ground. For added protection, especially from drying winds and sun, wrapping our above-ground structure in burlap or placing a breathable fabric cover over us can be beneficial. It is vital that any covering does not touch our foliage directly and is removed during mild daytime periods to prevent overheating and disease.
