Fertilizing Woody Plants

Nitrogen and iron are the only nutrients in which woody plants are commonly deficient. Fertilize only as needed and only if other problems have been eliminated as the cause of poor growth. Avoid overfertilization, especially with high nitrogen fertilizers. Overfertilization promotes excessive foliage that may increase populations of pests such as mites, aphids, and psyllids. Excessive growth may cause bark to crack, allowing entry of fungi. Excessive fertilizer may also kill roots and burn or kill foliage.

With the possible exception of young plants and fruit and nut trees, woody species in landscapes should not be routinely fertilized. As long as woody plants exhibit normal leaf size and color and desired growth, nutrients are probably adequate. Learn to recognize symptoms of nutrient deficiencies in established plants. Deficiencies cause foliage to discolor, fade, distort, or become spotted. Fewer leaves, flowers, and fruit may be produced and these can develop later than normal and remain undersized if plants are suffering from deficiencies.

Common deficiency symptoms

Nitrogen: Older leaves, needles are yellowish; new growth sparse, undersized; plants grow slowly and may drop foliage prematurely.

Iron: New foliage small and yellowish, except green along veins; dead spots may develop between veins; leaves drop and dry prematurely.

Zinc: Leaves yellowish; new growth may be delayed; new leaves may be small, narrow; foliage may be purplish.

Phosphorus: Foliage dark green, bluish, or may develop spots; shoots short and spindly; slow growth.

Potassium: Foliage growth is sparse; older foliage is yellowish and may have brown tips and margins near leaf edge or between veins.