Rosemary (Rosmarinus officinalis) means "Dew of the Sea" and, true to its name, this aromatic herb grows wild along the coast of the Mediterranean. While it is hardy only in U.S. Department of Agriculture plant hardiness zones 7 to 10, rosemary thrives well outside in pots and indoors year round. The key to successfully growing rosemary in a pot is emulating the conditions it enjoys in the wild with well-drained soil for the roots, a high level of humidity for the leaves and plenty of sunshine.
Select a rosemary cultivar that meets your needs. Prostrate rosemary is a low-growing variety with a spreading growth habit that drapes nicely over the edge of a container. Upright cultivars can grow to a mature height of 2 to 6 feet, but can be pruned to the desired size and shape. Either type thrives successfully in a container.
Obtain a pot with good drainage. A 12-inch pot is large enough for most rosemary cultivars. Place a piece of fibreglass screen over the holes to keep the planting medium from plugging them and add a 1-inch layer of pebbles or gravel. Place the pot on a saucer to catch the overflow when you water.
Fill the pot two-thirds full with fast-draining potting mix that contains sand and a high proportion of perlite. Add commercial slow-release fertiliser to the potting soil, following the instructions on the label.
Plant the rosemary in the pot at the same level as in the nursery container. Add soil to fill the pot to within 1 to 2 inches of the top. Press down firmly to settle the plant in the soil and remove any air pockets. Add a 1-inch layer of peat moss on top of the soil as a mulch to keep the soil from drying out too quickly.
Place the container in a sunny location that has good air circulation. Both conditions can be a challenge when growing rosemary indoors. If you cannot provide enough natural light, place a grow light above the plant for at least six hours a day. Use a fan to boost circulation.
Water the rosemary plant every one to two weeks. It you are in doubt about whether it's time to water, feel the soil. If it's cool and slightly damp, wait one or two days and check again. The trick is to water rosemary as soon as the soil is dry -- not too soon and not too late. While letting the soil dry out completely will kill the plant, allowing the roots to become waterlogged makes it susceptible to diseases, so discard the water in the saucer after watering. If the air is dry, mist the leaves with distilled water at least once a week.
Use garden snips to harvest sprigs of rosemary regularly to promote a bushy, full shape. Use the fresh clippings in your kitchen or hang them to dry for future use. Do not remove more than one-third of the growth at a time.
Rosemary is more fragrant when grown in alkaline soil, so add a little lime to the potting soil.