buckwheat is a summer cover crop. that means the (good) beekeepers would allow the bees to keep some for their own food for fall/winter feeding. also...dont know a lot of people who summer cover crop anymore. i guess its definitely niche, but certainly available in the usa.
i cant control where my bees go foraging. so i just label it 'wild flower' honey. in early spring before they swarm, i make sure the brambles are flowering. at that time..even in california..there isnt a whole lot of forage for them when they come out hungry. they also gorge themselves before swarming and so usually there isnt a whole lot early spring honey. its a timing thing. so really..there is no good time window for collecting buckwheat honey. unless we steal it from them and replace it with something else for their feeding.