|Growing Herbs Indoors
Herbs are hot or cool, depending on your choice of slang adjectives. Everyone seems to want to grow herbs these days. And why not? Herbs pay triple dividends in good looks, good flavors, and good scents. The magic of freshly chopped chives sprinkled over an omelet or soup; the Mediterranean charms of fresh rosemary, oregano, and thyme; the intoxicating aroma of lemon verbena all make it difficult not to get passionate about herbs. And these rewards aren’t limited to the summer garden. Even just a few pots indoors can supply you with wonderful flavors and herbal gifts through the rest of the year.
Herbs that Grow Well Indoors
Not every herb likes indoor life. Coriander (cilantro), garden cress, and dill are short-lived annuals that, when cut for harvest, do not regrow. You have to resow these herbs to produce a continuous crop. Three pots of each plant, each at a different stage (seeded, intermediate growth, and ready to cut), are usually enough. Forget trying to grow coriander, dill, or other spice herbs indoors for their seeds: They won’t set enough to warrant the effort.
You can grow parsley in pots, but I prefer to bring in established plants from the garden at the end of the season. The older leaves will fall off, but the thick taproot will drive new growth from the center. However, parsley grown indoors from seed never reaches the size and productivity of plants dug from the garden. That’s why I dig outdoor plants in fall and bring them inside. Keep the soil around the taproot intact, and be sure to use a pot that’s deep enough to accommodate the root.
Unless light is plentiful, growth of most indoor herbs will slow or even stop during the winter, even with enough warmth. When growth slows, reduce harvests and hold back a little on the water. Reducing the indoor temperature to 60º to 65ºF, if possible, also helps.
French tarragon and chives in particular benefit from a cool period. When growth flags in winter, place them in an unheated shed or garage (or in the refrigerator) for a month or two; freezing temperatures are fine. When returned to room temperature and good light, they’ll put out succulent new growth.
My mother, co-founder of Richters Herbs, grows herbs indoors in window boxes. She "plants" herbs in their pots in a window box filled with soil up to the rim of the pots. This system may seem odd, because the roots can only get at the soil outside through the holes in the pots. But herbs do precisely that, with faster and more lush growth than in stand-alone pots. The extra soil prevents the plants from becoming potbound, humidity and soil moisture remain more even, and the herbs seem to grow better. Also, the roots don’t become so intertwined that it’s difficult to rearrange or replace plants. A firm yank dislodges them.
How to Grow Herbs Indoors
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.
Provide light. 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. Several types of supplemental lights are described below.
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 to 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.
Supplemental Lighting for Herbs
Even with a bright sunroom, sun-loving herbs will need 12 to 16 hours a day of supplemental light. You have two choices: fluorescent or high-intensity discharge (HID).
The brightness of a light is measured in either lumens or foot candles. Lumens refers to the amount of light at the source, foot candles to the amount of light falling on a given area. As you move farther away from a light, the lumens stay the same, but the foot candles decrease. For perspective, a typical bright but overcast day is about 1,000 foot candles, and a bright summer day at noon is about 10,000 foot candles.
FLUORESCENT LIGHTS. Home gardeners have used fluorescent lights for starting seeds and growing plants for many years. Because their light intensity is low, they are best for seedlings and low-growing herbs so that even the lowest leaves are no more than 8 inches from the tube. A standard 4-foot unit with two 40-watt tubes will light an area about 8 inches wide. A variety of special tubes for growing plants are available, but a combination of standard cool- and warm-white tubes is also effective. Verilux tubes approximate the color of natural light and cost about $10 each. Vita-Lite "power twist" tubes produce somewhat more light per watt, and the quality of light is balanced for optimum plant growth, but they cost about $18 each.
Fluorescent lights at 6 inches provide 700 foot candles, and at 12 inches the light drops to 450 foot candles. Fluorescent efficiency, 60 to 80 lumens per watt, though superior to that of standard incandescent, is far below that of HID lamps. Cost is $30 to $60 for the fixture and tubes, and about $2 to $4 per month for electricity.
HIGH-INTENSITY DISCHARGE LIGHTS. Serious herb growers, including commercial producers of fresh-cut herbs, rely on high-intensity lamps. Our stock plants grew fabulously through the winter under these lights. They produce a much greater intensity of light, meaning the bulb can be several feet above the plants and still deliver adequate intensity to lower leaves. There are two types: metal halide and high-pressure sodium. Both utilize large, long-lived, and relatively expensive bulbs filled with various combinations of rare gases and a remote ballast.
A 400-watt HID lamp delivers about 1,000 foot candles of light 3 feet below the bulb and can sufficiently illuminate 25 square feet. Cost of electricity is $4 to $8 per month, depending upon lamp size and your electricity rates. (Operate HIDs only during off-peak hours to reduce energy cost.)
Most gardeners prefer metal halide lamps because the light they produce is good for healthy growth and flowering and pleasant to work around. Efficiency is 120 to 130 lumens per watt. Bulbs are available in 150-watt ($200) to 1,000-watt ($350) versions.
High-pressure sodium lamps are the most efficient type of HID lamp, producing about 140 lumens per watt. The light they produce is reddish, so it’s less pleasing to work around, but is preferred for some flowering plants. Bulbs are available in 150-watt ($160) to 1,000-watt ($400) versions.
For more information, check with manufacturers such as Diamond Lights, (800) 331-3994; Hydrofarm, (800) 634-9999; Mr. Greentrees, (800) 772-1997; and Worm’s Way, (800) 274-9676.