Like most herbs, rosemary is easy to care for and cultivate. It does, however, have its share of problems that can cause the leaves to begin turning yellow. Quick action can usually save the plant, since most of the problems that cause yellowing are caused by lack of proper care.
If the entire rosemary plant is turning yellow, including all of the leaves, it is likely that the plant has not received enough water. Check the moisture level of the soil by inserting a finger about 1 inch deep. If it is dry, give the plant some water. In some cases, it may be too late and the rosemary plant will die; however, many plants will bounce back to full health once they start receiving water on a regular basis.
If a rosemary plant is lacking essential nutrients, especially nitrogen, it may begin to turn yellow. Fertilise the plant with a general fertiliser, repeating the treatment every one to two months during the spring and summer, especially if the plant is being grown in a planter or pot.
Rosemary kept in a pot on a porch or indoors may not receive enough light to be healthy. A plant that does not receive light will begin to turn yellow and eventually will wilt and die. Move the rosemary to a place where it will receive eight to 10 hours of indirect sunlight daily, or move it outdoors.
Insect infestations, such as thrips, whiteflies and aphids can cause damage to the leaves of a rosemary plant. Eventually this damage can cause leaves to turn both yellow and brown. The damage is typically limited to a single section of the plant rather than all over, like in the case of lack of water, nutrients or sunlight. Spray the plant with a mixture of dish soap and water to kill the insects.