. Trailing varieties of the plant, such as ‘Blue Rain’ and ‘Huntington Carpet’, are especially good for indoor growing.
How to Grow Rosemary Indoors
Sunlight
Even when grown outdoors, rosemary is best suited to full sunlight (at least six hours per day) and it is only moderately tolerant of shade. This can cause issues when the plant is grown (or moved) indoors, where it is difficult to achieve the right sunlight conditions, especially during the short winter months. Ideally, potted rosemary should be kept on a bright windowsill that receives light all day, or in a well-lit room, like a sunroom. If you’re looking to move potted rosemary that has been outside all summer into your home for the fall and winter months, first put it on a sunlight “diet,” moving it to the shade for a few hours every day so it gradually gets used to reduced light and therefore is not shocked when relocated indoors.
Artificial Light
If you can’t provide your rosemary plants with six hours or more of direct sunlight each day, then you’ll need to supplement with artificial light. Almost no artificial lights provide the full-spectrum illumination that plants get from sunlight, but an affordable option that comes closest is full-spectrum LED lighting. LED bulbs can be hung very close over the plants since they do not get hot enough to burn them. When using artificial lighting as the principal illumination for rosemary, the plants will need a considerable amount of time under the lights—14 hours per day is not unreasonable.
Temperature and Humidity
Temperature-wise, rosemary is considered fairly hearty and will thrive in temperatures down to 50 degrees Fahrenheit, or up to 80 degrees. Even 40-degree temps will usually not harm these plants. Rosemary does not like the bone-dry air found in many winter homes, but excess amounts of humidity can actually pose a threat to rosemary, causing powdery mildew to cover the leaves of the plant, giving them a dusty, white appearance. In dry conditions, one standard method is to place the pots on trays filled with pebbles partly submerged in water. The gentle evaporation of the water provides just the right amount of humidity for the plants.
Watering
When it comes to watering a rosemary plant, it can be tricky to strike the right balance. When it doubt, err on the side of underwatering your plant, as it’s much more likely to die from too much water rather than too little. A good rule of thumb is to water when the top of the soil has dried out but never to let all the soil dry out completely. Additionally, rosemary is known as an “upside-down” plant, meaning it prefers to absorb its moisture through the air. Therefore, gently misting the plant every ten days or so can help fill the gap between waterings.
Air Circulation
Good air circulation will prevent rosemary from developing powdery mildew and other fungal diseases. This is especially useful in humid climates, but not necessary in colder climates where winter air is already very dry.
Fertilizer
For best results, feed your rosemary using a liquid fertilizer at the start of its growing season (spring) and continuing monthly through the fall. If you’re keeping organic in the hopes of cooking with your rosemary, be sure to use an organic fertilizer or fortify the soil with compost instead.
Pruning and Maintenance
Frequent harvesting and pruning will keep rosemary plants bushy and healthy. As stems become large and woody, you can cut them away and use the woody stems as skewers. Rotate the containers daily to ensure all sides of the plant get even sunlight.
Container and Size
In order to ensure proper drainage, choose a pot with adequate holes at its base—one made of moisture-wicking clay or terracotta can help ensure good drainage, too. Pots should be sufficiently deep to accommodate the plant’s roots—12 inches is a good depth.
Potting Soil and Drainage
Rosemary plants prefer dry, well-drained soil reminiscent of their Mediterranean upbringing. A sandy cactus soil blend works best for planting, but you can also try mixing sand into a traditional potting mix to make it lighter and airier.
Potting and Repotting
Unlike many other herbs, rosemary can grow into a substantial plant of up to 48 inches. Not many people have room for a massive rosemary plant in their homes, so the subject of repotting depends on your goals. If you’d like to keep your rosemary smaller and manageable, repot in the spring into a pot of the same size pot. During repotting, root prune the plant to stunt its growth by snipping off about one-third of the root material, then placing the plant back into the same size container with fresh soil. If you’d like a larger plant, step up the pot size and repot normally.
Moving Rosemary Outdoors for the Summer
Considerations
A potted rosemary plant can be moved out to a sunny spot on the patio or deck as soon as nighttime temperatures are reliably above 50 degrees. As with any indoor houseplant, it’s good to “harden-off” the plant by giving it increasingly long visits to its outdoor location over a period of a week or two. Bring the plant indoors at night during this hardening-off period.
When to Bring Rosemary Back Inside
Approaching frost is the signal that a rosemary plant should be brought back inside. If the plant has become too large and ungainly during the outdoor season, it can be repotted and root-pruned before bringing it indoors. Or you can take stem cuttings and root them to create new plants to grow over the winter.
