I seed my pasture with a lot of bee-friendly flowers each year. This year that included buckwheat, dutch white clover, yellow sweet clover, crimson clover, borage, white prairie clover, and more. With all of these blooming right now, the bees' preference is obvious--they love the yellow sweet clover. In fact, as long as the yellow sweet clover is in bloom, you're hard-pressed to find bees on the other flowers. This plant is a nitrogen fixer and was historically used extensively for hay, which is why it's a widespread "weed" now. If you've got a bit of land and you're looking to give your bees a source of nectar, consider putting yellow sweet clover down. I get mine from a local feed & seed store. It's surprisingly cheap, and pays you back in honey.