What flowers to plant/buy for our area of Zone 5? These flowers have done well for us…
Bees love California Lilac which provides early nourishment for the bees
Bumble bee on the Comfrey, which can be considered invasive, but provides food for the bees.
Echium plant grew from 2 ft (.6 m) high last year to about 10 ft (3m) high this year
Bumble bees go for the Echium in a big way
SIX BEES ON A SINGLE BLOSSOM
Bees go crazy on these blossoms (which I was under the impression were Island Bush Poppies) as can be seen in this fuzzy photo of six bees on a Hypericum. It’s covered with bees mid June to late July.
Bee diving into Penstemon blossom. These start blooming June.
Dahlias, blackberries (main Oregon crop for bees), sunflowers, wall flowers, rosemary are also good bets for bees, as well as cotoneaster which grows wild and provides food late in the season.
These are plants we’ve had in our garden. I’m sure there are many more. Please fill free to add to the list in the comments.
Many of these flowers serve as butterfly attractors also. See Butterflies.
Adventures in Natural Beekeeping 
A footnote about bees and flowers. I read this in one of the beekeeping trade magazines.
A study revealed that flowers have Velcro petals so the bees won’t
slip when they land if there is a breeze. The plants evolved this accommodation by spreading the cells out in the petal surface so they
resemble a Velcro matt.
( Nobody is going to believe this. I should hunt for that article in Bee Culture or The American Bee Journal.)
Bees and flowers have co-evolved over millions of years. I’m ever amazed at how this relationship has helped both species survive and develop.