Since no one is responding, let me venture to say that in Oklahoma, two
best flowering plants for the bees are 1)Buffalo Alfalfa and 2) Vitex,
both of which bloom nearly year-around although in a drought the latter
seems to suffer more than the former.  Please note that not all alfalfa
variety produce nectar and similarly, not all Vitex variety produce
nectar, either.  However, I heard an old friend of mine saying that an
acre of the right vitex variety will support hundreds of colonies.

FWIW,

Hum