Henry Fountain and Jeremy White with The New York Times report on the arrival of spring in their article Hang On, Northeast. In Some Parts, Spring Has Already Sprung. They offer a great breakdown on where, when, and why the first leaves bloomed- a.k.a. leafed out.
Based on a 30-year average, first leaf in the continental United States normally starts in the southern tip of Florida around the beginning of the second week of January, followed by South Texas about a week later. It usually reaches New York in early April. Parts of the high Rockies in Colorado, Wyoming, Utah and Montana are the last to show spring growth, at the end of May.