Trees should be fertilized in early spring or mid-fall as long as the soil temperature is above 40º F two inches below the soil surface. Soil should also be moist. Avoid fertilizing in late summer and early fall as a nutrient application at this time could cause unwanted succulent growth that may fail to harden off before fall frosts hit.
Fertilizer Application Methods
Broadcast or topdress – fertilizer is added directly to the soil surface. This method is good for N, which moves readily through the soil, but poor for P and K that move slowly through the soil. Fertilizer should be applied to the drip line and at several foot intervals out from the drip line for mature trees.
Soil incorporated – dry or liquid fertilizer is added to holes in the soil beneath the canopy and extended beyond the drip line and provide a long lasting effect. Holes should be up to 12 inches deep and 1 to 2 inches in diameter and made in concentric circles 2 feet apart around the tree trunk with the first circle no closer than 3 feet from the trunk.
Foliar sprays – best for supply nutrients for plant use in only trace amounts, such as Zn, Mn and Fe.
Tree spikes are a dry soil injection method, with a hardened column or cylinder of fertilizer hammered into the soil.
Controlled release pellets are typically broadcast on the soil surface, but they can also be placed in holes augured into the soil.
Tree spikes and slow release pellets may delay the development of winter hardiness so it is best to use them in late fall or early spring.