Winter Care for Rosemary Plants

If you have an herb garden or just want a few herbs in a mixed planting, save a spot for rosemary (Rosemarinus officinalis), one of the easiest-to-grow and most dependable herbs. It's a Mediterranean native that grows as a shrubby plant with pungently scented leaves. Rosemary grows outdoors year-round in warm-winter areas, where it might benefit from some protection, and can survive winter in colder regions when brought indoors to over-winter.

Rosemary grows as a hardy perennial in U.S. Department of Agriculture plant hardiness zones 8 through 10, where it's an evergreen that keeps its needle-like, grayish green leaves all year. It needs at least six hours of sun each day and continues growing slowly in cool winter weather in these regions, potentially becoming up to 7 feet tall.

In warm-winter areas, add 2 or 3 inches of organic mulch such as straw or shredded bark to the area under the plant in fall, to protect its roots from unexpected cold snaps and help conserve soil moisture during winter; keep mulch back several inches from the plant's base to discourage fungal growth.

Rosemary grows well in any type of garden soil, provided it's well-drained, and doesn't need supplemental water once established, tolerating dryness well. The herb doesn't need fertilizing but can benefit from an application of balanced, 20-20-20 fertilizer in spring if its growth is slow. Dilute 1/4 teaspoon per gallon of water, checking the product label for additional directions, and keep fertilizer off the plant's leaves and stems.

Rosemary also makes a good potted plant for a spot on a sunny patio or porch, growing well in commercial potting soil. A clay pot works best because it dries quickly, promoting good drainage. If your area has sub-freezing winter temperatures, bring a potted rosemary plant indoors as soon as temperatures drop to 30 degrees Fahrenheit. Dig up a garden-grown plant in late summer and plant it in a pot; add gravel to the bottom of the pot for extra drainage and keep it outdoors in partial shade sun for about six weeks to adjust to the new light level.

Before you bring a potted rosemary plant indoors in fall, prune it back hard so the plant's about half the size it was in the garden. Wipe pruning blade with rubbing alcohol between cuts to prevent spread of disease, and dry the cut stems by hanging bunches in a cool, dry place for several weeks.

The best indoor environment during winter is a south- or west-facing window that gets lots of light but is cool, with 50 to 60 degrees F being ideal. You can also grow the plant several inches from a fluorescent grow light. Indoors, water potted rosemary from the bottom when the soil feels dry, placing it in a water-filled tray or sauce until the soil's surface is moist. After watering, let the plant drain well; never leave it in a water-filled saucer.

When over-wintering rosemary indoors, watch for powdery mildew, a white fluffy growth on leaves that's promoted by dampness. This is best prevented by keeping multiple pots well-spaced and using a small electric fan nearby to increase air circulation.

Rosemary is usually pest-free when grown outdoors, but dry indoor air can promote an infestation of aphids, small greenish crawling insects, or spider mites, which aren't visible but produce web-like coverings on leaves. These are best controlled by spraying the plant with insecticidal soap, diluted at a rate of 5 tablespoons per gallon of water; repeat the spray every week or two as needed.