I am going to refrain from giving any specific fertilization instructions because soil and climatic conditions vary so much.
I remember many years ago when I was starting in this business, Mr. Stoddard at the Experiment Station told me that he felt more plants were killed by over-fertilization than all other reasons combined. I feel the best advice for the homeowner is to fertilize lightly, and when in doubt — don’t. The ericaceous plants in particular are notoriously light feeders. Their tiny roots are close to the surface and great damage can be done by over fertilization. Junipers, pines and others can benefit from quite a bit of fertilizer. However, they can live without fertilization quite well and in some cases produce better trees with no fertilizer at all.
I think the basic rule to remember for any plant is to fertilize as early in the season as possible — just as soon as the ground has thawed. This gives the plant the vitality it needs early in the season, and by late summer the fertilizer should have been spent or leached, giving the plant an opportunity to harden itself and prepare for winter. Late fertilization often forces late growth which is susceptible to early frost and freezes.
In garden center situations where container grown plants are being held for sale throughout the summer or in situations where plants are put in planters or tubs, fertilization becomes a great problem because of leaching due to constant watering. For professional and amateur alike, I would recommend osmocote — used at half the recommended rate on all ericaceous plants, and the regular rate on all others.
Let me repeat, in my opinion, a bag of fertilizer in the hands of an unschooled amateur can do more damage to plants and to the reputation of the people that sold the plants than anything else I can imagine.