by Robin McCain, Woodland Park Zoo

Nitrogen and iron are the only nutrients in which woody plants are commonly deficient. Fertilize only as needed and only if other problems for poor growth have been eliminated. Avoid over fertilization, especially with high-nitrogen, fast-release fertilizers. Over-fertilization promotes excessive foliage growth that may result in an increase of pest populations such as mites, aphids, and psyllids. It also may cause bark to crack, allowing entry of disease, may damage roots, and cause burning or die back of foliage.