Cold Hardy Rosemary Plants

Hardiness

• Rosemary is a perennial shrub, hardy in U.S. Department of Agriculture planting zones 7 to 8 and able to withstand winter temperatures down to minus 3 degrees F. There are varieties of rosemary, differing in growth habit, size and leaf shape. Varieties that have an upright growth form, thinner leaves and lighter flowers tend to be hardier than those that grow close to the ground and have broad leaves.

Research

• The U.S. National Arboretum, in Washington D.C., has test plots that are devoted to finding the best, most cold-tolerant varieties of rosemary. Located in hardiness zones 7 and 8, winters in the District of Columbia can reach temperatures near zero degrees F, exposing rosemary plants to the lowest temperatures they can stand.

Cold Tolerant Varieties

• Rosemary varieties that have proved to be cold tolerant at the U.S. National Arboretum include: 'Albus,' 'Bendenen Blue,' 'Goodwin Creek,' 'Herb Cottage,' 'Logee's Light Blue,' 'Miss Jessup's Upright,' 'Russian River' and 'Salem.'

Winter Protection

• Protecting rosemary plants in the winter can improve chances of the plants survival. Rosemary plants should be planted in early spring to allow the roots to establish over a long, warm period. Following the first frost, prune rosemary plants to just above ground level and mound soil over the plant stubs. Adding a 4- to 5-inch layer of mulch over the soil will help to further insulate the plants roots. An alternative method is to enclose the plant in a cage made of chicken wire and then fill the cage with mulch.

Finding the Right Rosemary

• To find a variety of rosemary for your garden, know your area's plant hardiness climate zone. Garden centers and greenhouses stock a variety of herbs in the spring, but it may be necessary to special order uncommon types of rosemary. In climates that are colder than hardiness zones 7 and 8, situate rosemary plants in sheltered areas, such as near the house, to provide extra protection. In areas where rosemary cannot be successfully overwintered, consider growing it in a container and moving the pot indoors to a sunny location for the winter.