In my first year of beekeeping, the locust tree was the only tree that clearly was favored by my bees. It blooms early to late spring and was loaded w/ bees. You could see them leave the hive, and immediately take flight to this tree. Another tree that had the soame reaction, later in the summer is the Catawba tree. As for late season flowers, Anise hyssop. Blooms from july to frost, and the bees sit on it and wont leave. This flower was available during my areas dearth, and helped bridge the gap until fall blooms