Fertilizing woody plants
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
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.