We have two Japanese Maple trees in front of our house, about 15 feet apart. One started leafing out two weeks ago, and one just started opening leaves yesterday. There’s only a slight difference in the amount of sun, wind, and reflected warmth from the house between these two. One of them (the early-bird) is in soil that’s been in place for a while. The other guy is in some new soil that landscapers used to bring up the level of the yard a few inches. Could that be it? Or maybe it’s the mature Silver Maple near the slow-poke. That sends out a mat of fibrous roots at the surface, and so may be strong-arming the surface water away from the new guy.