You gotta be kidding me
I went to a garden shop last night to pick up some stuff for repelling deer (they didn't have any) and while I was there, I heard a lady ask the sales staff for some tree fertilizer. The sales staff said that he didn't have any of that kind in stock and it was the wrong time of the year to fertilize trees anyway. He wanted them to wait until the tree leafed out.
That probably wasn't the right thing to say...
The best time to fertilize a tree is in the fall right after the leaves have fallen off. The roots are still active and they'll suck up the feed, storing it for the spring.
The second best time is early in the spring before the tree has leafed out. The roots are active with the warming soil and will suck up the nutrients then as well. They then send this energy up to the tops of the tree (sap folks - that's what sap is - tree energy)
Roots require these nutrients to feed the leaves. If they don't have enough in the early spring (hence the fall and early feeding) they can't send as much up to the leaves and the tree won't grow as much.
The leaf/root relationship is an interesting one. The leaf buds have enough carbohydrates stored in them to open up a small leaf and get photosynthesis started. Once this process starts, the rising sugars from the roots feed the leaves some more energy and the leaf starts to grow. The larger leaf is more efficient and starts sending energy back down the tree to feed the roots. It's one big cycle.
But it starts with feeding the roots in the fall and early spring. If you wait for the tree to leaf out before you feed it, you won't see as much new growth.
Hint: If you really want to see a tree grow - feed it in the fall. Don't bother when it has leaves on it.