Mountain ash trees, known botanically as the genus Sorbus, have two main cultivars, European mountain ash and American mountain ash. The European cultivar reaches 50 feet in height at maturity, while the American cultivar maxes out at 20 feet in height. They are both grown as large ornamental shrubs or small trees, producing white flowers in May and clusters of small orange to red berry-like fruits in the fall.
Fertilize your mountain ash trees in the spring after the last frost has passed and when the soil can easily be worked. Feed young mountain ash trees under four years of age once per year, and fertilize mature trees five years of age and older every other year. Mountain ash trees growing in nutrient-rich soil may need less frequent fertilizing or none at all if healthy annual branch growth is six inches or more without it.
Scatter a complete granular tree fertilizer with a guaranteed analysis of 20-15-15 around the drip line of the tree at least a foot to two feet out from the trunk and extending at least a foot beyond the outer edge of the canopy. Apply the recommended dose from the package label in keeping with the size and age of your tree.
Nestle the fertilizer granules into the top few inches of soil with a rake or cultivating fork. Water the soil and fertilizer deeply until the soil is drenched to a depth of at least 12 inches. This will begin the transfer of nutrients down towards the root zone where they can be put to use by the tree.