Herbs are sun worshipers for the most part. As expatriates of the Mediterranean region, most flavorful herbs don't thrive in the un-Mediterranean environment and inadequate light our houses provide. Herbs don't tolerate north-facing windows, or any window that gets less than four hours of direct sunshine a day.
Even if your indoor herbs get their four hours of direct sunshine daily, installing supplementary lighting is a necessity. The light coming through a window may seem bright to your eyes, but its intensity in winter is often less than one-tenth of the outdoor light during a summer day. Grow lights will work if their light intensity is high enough and the spectral quality is right.
Acclimate Plants Gradually
Plants produce two kinds of leaves in response to strong or weak light. High-light leaves are thick, strong, and narrow. Low-light leaves are thinner, more delicate, and broader than high-light leaves. But narrow high-light leaves are less efficient in converting light energy into food than low-light leaves. High-light leaves are accustomed to an abundance of light, so they don't have be as efficient at food production. A plant that is adapted to abundant light often turns brown and drops leaves indoors. This is because it can't produce enough food to maintain itself. The plant tries to make food by shedding the inefficient leaves and producing efficient leaves higher up and closer to the light source. When you bring herbs indoors, this leaf drop and increased leggy growth can happen within weeks, or even days. Some herbs cannot make the transition fast enough to survive.
Rosemary is a case in point. This slow-growing evergreen doesn't have the chance to adjust to changes in light before the plant slowly starves itself. By January, February, or March, the leaves dry up, and the plant dies. This sudden death is by far the most common complaint about growing rosemary indoors. Here's what to do: Gradually adjust the plant to lower light. Place it in partial shade for two to three weeks, then in deeper shade for another two to three weeks before bringing it indoors. When plenty of new growth appears, the plant is ready to go into the house.
Soil, Fertilizer, and Water
After light, proper soil is the next most important factor in producing healthy herb plants. With few exceptions, herbs require excellent drainage, especially during the winter months, when transpiration rates are lowest (that's the rate at which plants release water from their leaves to the atmosphere). When roots are confined in a pot or planter, water and air cannot move easily. To improve drainage without sacrificing nutrients, add sharp sand or perlite to a good sterilized compost-based mix. Most herbs do well in soils of pH 6 to 7.
Many people incorrectly think that herbs grow better in poor soil. Flavors are stronger when culinary herbs grow outdoors in gardens. But in the confines of a pot, supplementary feedings with liquid fertilizer or organic fish emulsion are necessary. Feed herbs once a week when plants are actively growing, but not when dormant.
Watering is not a trivial matter with herbs. In general, water less often and more thoroughly, and only when the soil is actually dry. When the soil is dry to the touch, add water until it comes out the bottom of the pot. If the water doesn't come out, pots have a drainage problem. First, check that the holes aren't blocked; if not, you may have to repot with soil that has better drainage.
Pests and Diseases
Herbs are susceptible to common pests, including whiteflies, spider mites, aphids, mealybugs, scale insects, and thrips. Inspect herbs regularly.
If your herbs are in portable containers, control pests by dipping the whole aboveground part of the plant into a pail of insecticidal soap. Swish vigorously for a minute or two to wet all leaf surfaces (hold your hand over the pot to prevent soil loss). Dipping once or twice a week for three to four weeks will clear up most problems.
Photograph by Suzanne DeJohn/National Gardening Association