Rosemary is an excellent indoor herb. Like other Mediterranean herbs, it's drought-tolerant and thrives in bright, sunny windowsills. Perhaps best of all, it's highly fragrant and delicious. Simply brush against the plant with your hand to release waves of delightful rosemary scent. Outside, of course, rosemary can grow into a rather large bush, even tolerating mild winters. Indoors, rosemary benefits from being kept slightly dry and trimmed to remain within its natural pot size.
Light: Rosemary thrives under bright light, so a bright window with morning sun is perfect. Alternatively, it can be very easily grown under standard or compact fluorescent lights, as well as halide lights. In general, the brighter, the better.
Water: Rosemary is much more likely to be killed by overwatering than under-watering. Use terracotta pots to increase evaporation in the root zone and let the soil dry between waterings.
Temperature: Rosemary is a hardy plant that will thrive down to 50˚F and up to 80˚F or higher. In hotter locations, be sure to provide plenty of air circulation.
Soil: Airy, light, fast-draining soil.
Fertilizer: Use liquid fertilizer, or supplement the soil with controlled-release pellets. For organic rosemary, use an organic fertilizer or fortify soil with compost. Contrary to common wisdom, indoor rosemary benefits from regular application of fertilizer.
Rosemary is a fairly slow-growing plant, so it's usually easiest to buy nursery-grown plants rather than start your own from seed. However, rosemary can be successfully started from seed if you don't mind waiting for the plant to fill in. Alternatively, you can take leaf-tip cuttings of established plants to start new ones. This is a particularly good idea if you have an outdoor plant that might not make it through the winter. Start a new cutting in the summer months in a container, then bring it inside when the weather begins to cool off. Not only will it provide rosemary all winter long, it will launch your herb garden next spring.
Unlike many other herbs, rosemary can grow into a substantial plant of up to 48". Clearly, not many people want four-foot tall rosemary plants in their homes, so the subject of repotting depends on your goals. If you'd like to keep your rosemary smaller and manageable, repot into spring into the same size pot. During repotting, root prune the plant to stunt its growth by snipping off about 1/3 of the root material, then placing the plant back into the same size container with fresh potting media. If you'd like a larger plant, step up the pot size and repot normally.
The common rosemary is known as Rosmarinus officinalis. It's an evergreen shrub native to the Mediterranean. This is the main species, but there are numerous cultivars that are typically bred for hardiness or their ability to withstand weather extremes. There is little taste difference among the various cultivars, but if you find yourself struggling with mildew or other common problems of rosemary, ask at your local garden center if they carry any locally appropriate rosemary cultivars.
Rosemary is generally a very easy plant to grow, yielding large quantities of piney scented leaves that are both medicinal and useful in cooking. The most common problem to beset rosemary is powdery mildew, which typically affects plants that are too wet or have insufficient light and/or circulation. Powdery mildew looks exactly like it sounds: white powder on the leaves. Because rosemary is an edible plant, you have to be careful what you use to treat powdery mildew with. When you first see signs of it, remove all affected parts of the plant (carefully, so as not to spread the mildew) and seal up the infected branches in airtight bags, then dispose of them. A variety of remedies can be used to treat the remaining plant, including neem oil and baking soda. Always test your remedy on a few leaves first before treating the whole plant.