Protecting Plants from Frost: Winter Container Gardening

How to keep your herbs healthy when temperatures drop


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Question: This spring, I bought a little rosemary plant. I planted it in a clay pot and put it on our sunny patio. It grew well all summer and looks wonderful now. How can I keep it over the winter?

Answer: It depends on where you live. Rosemary that’s been growing in the ground for several months can normally survive winter temperatures in the teens, but potted plants are more sensitive to cold. Their roots are more likely to freeze because the soil in a pot gets as cold as the air, while the ground stays warmer.

Unless you live in a mild region where frosts are infrequent, you must protect a potted rosemary. In areas where temperatures drop no lower than 20°F, you can overwinter it outdoors. Choose a site close to the house where the plant will be sheltered from cold winds and bury the pot in the ground up to the rim. Do this now, before hard frost strikes, and dig it up in midspring. For extra protection, mulch the soil on and around the pot with about an inch of gritty sand or pea gravel. Water the rosemary every week or two during the winter, or whenever the soil in the area around the pot gets dry.

In most of the United States, you must bring potted rosemary indoors for the winter. Stop fertilizing the plant now and begin watering less frequently to slow its growth. Think of its indoor stay as a time of rest: all it has to do is stay alive, not grow larger or make new leaves. It may start flowering in late winter, but it won’t increase much in size until spring.

Acclimatize the plant to lower light levels by moving it from your sunny patio to a shady location outdoors, under a tree or on the north side of the house. Leave it there for a few weeks. If frost threatens during this period, bring the plant indoors at night and set it back outside in the morning.

Providing enough light indoors is a challenge. Most rooms are too dark, especially during the winter when days are short and skies are often cloudy. With insufficient light, rosemary can weaken and die by midwinter. If you can’t set the pot next to a large, unshaded east or south window that gets at least 4 hours of sun daily, use artificial light. Fluorescent fixtures are best because they give off less heat than incandescent bulbs. An inexpensive shop light with two standard 48-inch tubes will provide enough light for several potted herbs. Hang or mount it over a table or shelf with the fluorescent tubes about 6 inches above the top of the tallest plant. Raise shorter plants on bricks or other supports so that their tops are also 6 inches from the tubes. Set an automatic timer to keep the light on for 14 to 16 hours a day.

The ideal temperature for indoor rosemary is cooler than normal room temperature. Try for 50° to 70°F during the day and about 10° cooler at night. Rosemary will do better under artificial light in a cool room than with natural daylight in a room that’s too warm.

Rosemary’s generic name, Rosmarinus, means “dew of the sea”. The plant grows wild on windswept cliffs along the Mediterranean Sea, where the winter weather is often cool and foggy. It doesn’t like dry, stuffy air. Although you can’t simulate a sea breeze, do the best you can by misting the foliage with fresh water every few days, and run a small fan constantly to circulate the air in the room. If you have a forced-air heating system, don’t put rosemary near a register.

Judge when to water rosemary by sticking a finger into the soil—when the top inch of soil is dry, water. Your plant will need much less watering in winter than it did in summer, but never let it dry out so much that it wilts.

Watch for spider mites, whiteflies, mealy bugs, aphids, and other pests, and spray with insecticidal soap if needed. These pests seem to appear out of nowhere, and they multiply fast.

Come spring, wait until forsythias and tulips are blooming before taking the rosemary outdoors. Set it in a shady, sheltered location for a week or two before putting it back on your sunny patio.

You can repeat this cycle for many years. From time to time, you’ll have to move the rosemary into a larger pot.

Other herbs that can be overwintered in a cool place with bright natu­ral or artificial light include sweet bay, lemon verbena, eucalyptus, jasmine, myrtle, curry plant, lemongrass, pine­apple sage, and all kinds of scented pelargoniums. Saving your own herbs will give you a head start next spring, and you’ll relish their fresh aromas on a cold winter day.

Rita Buchanan grows many herbs in her garden in Winsted, Connecticut.