A large rosemary plant in a 10” planter. If the top 1 to 2 inches (3-5 cm.) of soil feels dry, it's time to water. Water the plant deeply, then let the pot drain freely and never let the pot stand in water. Use care, as overwatering is the most common reason rosemary plants don't survive in containers. Rosemary likes full sun and does not tolerate any shade. This means it requires at least six hours of direct sunlight on most days. A south-facing window is ideal for indoor growth, and grow lights are often necessary to provide supplemental light.