> Buckwheat is similar with some varieties producing a lot more nectar than others.

I was told a story by a friend who kept bees in the 1950s in the San Diego County. He used to move bees several hundred miles from home to the San Joaquin Valley to get honey from cotton. For years it was a pretty reliable plant but yields began to fall off, leading to a fair amount of head scratching among beekeepers. 

Turned out that the University of California had developed varieties of cotton that didn't produce much nectar, since the growers didn't want their mechanized equipment getting all sticky. The thing is, nobody told the beekeepers this was happening because they figured they'd make a big stink about it. The change happened and they finally figured it out, too late. 

Alfalfa and clover yield very differently in different soils, different climates, etc. Same with goldenrod. Some regions have vast acreages of goldenrod that yield pollen but not nectar, dependent on soil pH. Finally, populations of wild plants transition over time, like when pastures fill in with shrubs, and finally convert to forests.

PLB

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