By Scott Aker
Throughout much of the country, now is the time to face the coming cold. Any plants you want to keep must be moved to warmer quarters. For many gardeners, that might be a sunny windowsill indoors. But is this the best place to overwinter your herbs?
Unless you have a room that you can keep below 60°F in your home, they may suffer. Many herbs hail from Mediterranean and subtropical climates and are programmed to thrive with distinct seasons of heat and chill. Bring them indoors, and they experience temperatures much higher than they ever would in winter in their native lands. The result is stress and conditions that favor pests, such as mites, powdery mildew, and scale insects, that can quickly overwhelm your plants.
I live in USDA Hardiness Zone 7B, and winters generally have long periods of mild weather with temperatures hovering around freezing punctuated with short periods of cold that may reach the single digits. I grow all my herbs in pots on the deck, because they are just steps away from the kitchen. Some stay there through the winter. Marjoram, parsley, oregano, chives, and mint have no issue with the cold, even in pots. Bay, sage, gardenia, and citrus go in the garage for winter. My garage is not heated, but it seldom freezes since the walls and door shared with the house are not terribly well insulated. Night temperatures dip into just above freezing and may rise to about 55°F during the day, on average.
While herbs would like more light than can be supplied even by a sunny windowsill indoors, they don’t really need light when held at temperatures in the 40°F to 50°F range. At these low temperatures, they are nearly dormant. They do need to be watered from time to time, but at low temperatures, they don’t need or appreciate frequent watering.
My garage herbs fall into two tiers. The hardy ones, such as the bay, gardenia, and sage go outside on the front steps whenever temperatures are forecast to remain above 25°F for a week or more. The citrus stay in the garage unless weather is above 30°F. Since watering is a chore that I dislike, I tend to move the plants outdoors anytime precipitation is forecast. My bay and gardenia have taken a heavy snow load with no damage, and they do appreciate sunny warm winter days.
If you are going to use your garage to overwinter herbs, pay close attention to weight. I grow all my large garage herbs in large plastic pots to keep the weight to a minimum and make them easier to move the short distance to the front steps. Terracotta and ceramic pots are more attractive, but I suggest using them in smaller sizes for smaller plants. Some pruning to keep things in bounds is helpful, too, since winter wind can easily topple top heavy plants.
On pleasant winter days, the site of my mandarin orange, with its full complement of ripening fruit, is a cheerful sight, and one that has attracted much attention from passersby. When cooking, it is only slightly less convenient to step in the garage to get what I need. When spring arrives, I find that the plants grow more vigorously because of the chill winter they experienced, and mites and other pests have been unable to prosper on my plants. Give some thought to your garage when it’s time to bring your herbs in for the winter.
Photo credits: 1) Herbs in containers (Creative Commons, freeformkatia); 2) Herb containers on deck (S. Aker); 3) Mandarin orange (Citrus reticulata) in garage (S. Aker).
Scott Aker is Head of Horticulture and Education at the U.S. National Arboretum in Washington, DC. He authored Digging In in The Washington Post and Garden Solutions in The American Gardener.