Herbs that are almost never troubled by insects and diseases in the garden are easy targets when grown indoors. As a bare-bones essential, you must offer indoor herbs artificial light. And even when you do, many of the plants will balk at confinement. Some herbs are too tall to grow under lights; some have deep tap roots; some require a period of chilling or complete dormancy during the winter months.
Many perennial herbs must come indoors during the cold months in most parts of North America. Leave them outside in December and January, and you're left with a few dead stems and a pot of soil to dump on the compost pile. Give these plenty of artificial light, and they'll struggle through winter. In their search for sunshine, many plants get leggy and impossibly tall, so prune stem tips frequently to keep them bushy. (Pinching off the ends tells the buds farther down on the branch to start growing.)
Gardeners have moderately good luck with growing these herbs indoors:
You can ease your herbs' transition from the outdoor to the indoor by moving them under a tree, an overhang, or a shelter of shade cloth (a mesh material available from garden shops or by mail order) for a few days. While the plants are in the transition area, check them carefully for any sign of insects.
You also need to reduce both fertilizer and water. Less fertilizer slows high-energy top growth (but the roots will keep growing). Drier soil "hardens off" the foliage, preparing it to cope with your home's drier environment. You can help plants adjust to the dry indoor air by enclosing them in a plastic bag for a few days — punch a few holes in the bag so that too much humidity doesn't build up.
In providing light for your indoor plants, the goal is to mimic as closely as possible the light spectrum of the sun. The special bulbs and tubes sold as grow lights are supposed to do this, but they don't use electricity as efficiently as fluorescent lights. Gardeners who've tried both swear by fluorescents, which are inexpensive and available at the local hardware store. Good air circulation is also essential to growing herbs indoors.
Just as outdoor containers tend to suffer from too little water, indoor plants often get too much. Plants that are growing rapidly use more water, but many perennial herbs go semidormant or completely dormant in winter. These plants need much less water than they did in summer.
Here are some watering tips:
Water most herbs only when the soil surface is dry. Observation is key. Some plants are simply thirstier than others. Some, such as basil, even seem to need more water as indoor plants than as outdoor plants.
Water less often if your home is cool, your herbs are growing in plastic pots, or if plants seem to be ailing. Rapid growth adds to their stress.
Don't let your plant sit in a saucer of water, and don't shock plants with cold tap water. Water should be tepid — about room temperature.
Use less fertilizer indoors than you would outdoors because most plants are growing more slowly. Give the plants a monthly shot at one-fourth the recommended dose.