Phacelia tanacetifolia is good for biomass and bees; a fast growing annual that can be sown through spring and summer. Often cited as a 'green manure' i.e. chop and drop to enrich the soil. Agriculturally the advice is to do that before the flowers set seed but self-seeding is fine by me.

 There's a bed on the allotment where I conducted an experiment to see if Wild Strawberry (Fragaria vesca) would form a ground cover that could suppress weeds and coarse grasses. Answer: no (though the Wild Strawberries held their own amongst them). 

 I dug the bed over in spring and sowed Phacelia rather than leaving it fallow. I hoped Phacelia would grow faster than weeds -thereby shading them out- which has proved to be reasonably successful. And very pretty with lots of bees.