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Overwintering rosemary

Q: I would like to overwinter my rosemary, but I often hear that it is susceptible to pest infestations and other problems when grown indoors. What kind of growing conditions should I provide to help it survive?

Scott Meyer, Pawling, NY

Overwintering rosemary requires well-drained soil and good air circulation. Overwintering rosemary requires well-drained soil and good air circulation. Photo/Illustration: Jennifer Blume

A: Debra Knapke, a designer of herb gardens and former curator of the herb garden at  Inniswood Metro Gardens  in Columbus, Ohio, responds:  Rosemary can be successfully overwintered indoors, but for healthy roots and shoots it needs good drainage and good air circulation.

When potting up a rosemary for the winter, use a soilless mix that has more bark and perlite than peat, then place the plant in a cool, well-lit room. Ideal locations include unheated enclosed porches or cool east-, south-, or west-facing rooms. Place your rosemary where there is air movement, or you may soon find sooty mold on the leaves. A fan will help to promote good circulation. Keep the potting mix moist but not soggy. Do not, I repeat, do not let the potting mix totally dry out, or you will have a dried-plant sculpture. 

Watch for whitefly, scale, and mealy bugs. If your plant becomes infested, eliminate the creeping visitors by using an insecticidal soap. To apply, place the ailing rosemary in a bathtub, cover the potting mix with plastic, and spray the leaves well with the soap spray.

When a rosemary is content in its indoor home, it will reward you with a beautiful show of blue flowers around the holidays. Winter is a good time to repot rosemaries that were grown as container plants in summer. If you remove some of the roots before repotting in the same pot, prune a proportionate number of the shoots. I also snip branches now and then to use in cooking, or for gentle shaping of a rosemary plant’s form.

Many rosemary cultivars will also survive winter outdoors if minimum temperatures don’t dip below 0˚F (USDA Hardiness Zone 7). If you’re willing to experiment, look for the hardier cultivars such as ‘Arp’ and ‘Madeline Hill’. Both have overwintered for me when temperatures have not dipped below –5˚F. The trick is to site your plant on a protected southeastern exposure, mulch around it after the soil temperature has cooled, and cross your fingers when the mercury dips below 0˚F.

From Fine Gardening 76, pp. 80