Answer: You really only need to feed your trees if they show symptoms of nutrient deficency. Symptoms of a nutrient-deficient tree include a slow rate and low amount of annual growth on twigs and trunk, smaller than normal foliage, off-color foliage, increased amounts of dead branches, tip die-back in branches, and increased rates of disease and insect problems. Trees that possess these symptoms generally would respond to a fertilization treatment. A good fertilizer to use include 10-5-5, 12-6-6, and 18-6-12. When you apply fertilizers, you want to broadcast the recommended amount (as listed on the fertilizer package) equally under the canopy of the tree. Tree roots will extend beyond the dripline, but fertilizer directly under the canopy will do the most good.
Your photinia has a common disease called black spot. This can be treated with a fungicide, or you can simply prune out the affected foliage each spring just as new growth begins. Since the beauty of these shrubs is in the reddish new growth, you'll be doing three things when pruning: getting rid of the damaged leaves, opening up the plant to better air circulation (to help avoid the problem in the future), and encouraging lots of beautiful new growth.
Best wishes with your landscape.
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