I have a European Mountain Ash that has not bloomed in the 4 years I've had it. It now has about a 4" trunk and growing very well. It gets good sun and fertilized 2 times a year (spikes). I've been using super phosphate it in the spring thinking that might stimulate blooming.
Minnesota Master Gardeners say:
Mountain Ash do not bloom until they reach maturity, about 8-9 years of age and with a truck diameter of 2-2.5". Since your tree has a 4" diameter, it is probably mature or approaching maturity and may bloom next spring. Inadequate light can effect blooming, but Mountain Ashes grow in full sun or part shade so that does not seem to be a factor in your situation.
Another factor that can influence blooming in mature trees is over-fertilization. Fertilize your mountain ash only ONCE a year in spring before new growth emerges. Too much fertilizer encourages lush new leafy growth (susceptible to fire blight) and decreases the number of blooms. Fertilize with a balanced fertilizer such as 10-10-10 or organic fertilizers like blood meal, compost, milorganite, etc. Super phosphate is recommended only at the time of planting. The tree spikes are OK, but it is cheaper and easier to sprinkle fertilizer around the tree and water immediately to get the fertilizer into the root zone.
