I am going to assume you are referring to the rockrose or Cistus shrub, a low water use plant with small gray leaves and pink, yellow or lavender blooms often with a darker eye but with fewer petals than a true rose. These are short lived shrubs so once they grow woody with old growth and become leggy it is often easier to replace them with young plants instead of watching them slowly decline. This shrub does best in hot, dry climates on a hillside with perfect drainage and it does not like heavy pruning. You can thin out the oldest branches by cutting them off near the center of the plant or tip prune the new growth in the spring or summer by shearing back the most outer tips by two to three inches. The yellow leaves are most likely from winter weather or too much rain and adding a fertilizer will not correct this. If you would like to send a photo of the plant I may be able to confirm what type of plant it is and why it is turning yellow. Also tell me how long you have had this plant. Anything older than 5 years and rock roses are on the decline. Keep growing, Marianne Binetti