Best in the spring, after new growth starts naturally (May 1), through June.

If you do it in the fall (I don't recommend it), it would be after deciduous tree leaves fall. (October)

The limiting nutrient for trees is nitrogen, so a fertilizer high in nitrogen is most productive. A lawn fertilizer (without weed killer or insecticide) fits the bill nicely.

Rick

Has your soil been determined to be deficient?

Are they showing symptoms?...leaf sprays smaller than usual, light green or off-color foliage, ends of branches containing dead twigs, very short elongation of branches during the growing season, and a general lack vigor.

I've had some technys and globes for over 35 years that have never been fertilized and do beautifully anyway. I suppose a routine fertilizing program doesn't do much harm and it keeps the gazillion "must have" tree food products on the shelves.

There are always going to be two schools of thought on this: those who find the practice a plus even if not necessary, and those of us who think it's largely (when there's no obvious deficiency) an unnecessary bother and waste of money.