Q: My rosemary has many yellow leaves.
A: The first thing I would check for is spider mites. My neighbor’s rosemary plant is “eat up with ’em”!
Clip off an eight-inch long branch and slap it onto a piece of white paper. If you see little yellow dots crawling across the white surface, my suspicions are confirmed.
Spider mites can partially be controlled by spraying the plant each day with a strong stream of water. You could also use insecticidal soap or horticultural oil on the plant but this won’t be very successful because there are so many leaves to cover. To be honest, I’d fertilize the plant lightly and hope that it can fight off the spider mites with the help of naturally-occurring beneficial insects.
If your plant is too far gone to save, remove it from your property and plant another rosemary in a different spot.