If I had a farm somewhere in NC and I wanted to keep bees, I would probably plant some pollinator-friendly trees and crops. Trees especially, since the native ones should just do well on their own and they've got an extra dimension compared to cover crops. What plants would I collect? Here's a proposed list:

Trees

Tilia caroliniana
Nyssa sylvatica
Liriodendron tulipifera
Robinia pseudoacacia
Oxydendrum sp.

Cover crops

Clover
Buckwheat
Vicia caroliniana
Milkweed/Joe-Pye weed

Other

Rosa rugosa
Rubus sp.

Resources

NC Forestry Service Seedling Store - For those too lazy to hunt wild plants.
USDA Plant Guides - With state and county breakdown of plant locations