According to observations of naturalists and beekeepers.
Where are you?
Echium vulgare — known as viper's bugloss and blueweed — is a species of flowering plant in the borage family Boraginaceae. It is native to most of Europe, and western and central Asia and it occurs as an introduced species in North America.
This plant is present in at least 52 states/provinces in this country.
It is a biennial or monocarpic perennial plant growing to 30–80 cm tall, with rough, hairy, oblanceolate leaves. The flowers start pink and turn vivid blue and are 15–20 mm in a branched spike, with all the stamens protruding. The pollen is blue but the filaments of the stamens remain red, contrasting against the blue flowers.
It flowers between June and August.
Salix monticola (aka: Park willow, Cherry willow, Mountain willow, Serviceberry willow, White willow)
Salix geyeriana (aka: Geyer willow, Silver willow)
Rhus glabra (aka: Smooth sumac, Common sumac, Rocky Mountain sumac, Red sumac, Western sumac, White sumac)
Rubus laciniatus (aka: Cutleaf blackberry, Evergreen blackberry, Slashed blackberry)
Salix bebbiana (aka: Bebb willow, Beak willow, Beaked willow, Long-beaked willow, Diamond willow, Chaton, Petit Minou, Smooth Bebb willow)
Brassica napus (aka: Rapeseed)
Allium schoenoprasum (aka: Chives)
Sinapis arvensis (aka: Charlock mustard, California rape, Charlock, Corn mustard, Canola, Kaber mustard, Rapeseed mustard)
Asclepias tuberosa (aka: Butterflyweed, Butterfly Milkweed, Orange Milkweed, Pleurisy Root, Chigger Flower, Canada root, Fluxroot, Indian paintbrush, Indian posy, Orange root, Orange Swallow-wort, Tuber root, Yellow milkweed, White-root, Windroot, Butterfly love)
Salix brachycarpa (aka: Shortfruit willow, Barren-ground willow, Small-fruit sand dune willow, Small-fruit willow)
Rubus idaeus (aka: Raspberry, Black-haired red raspberry, Brilliant red raspberry, American red raspberry, Red raspberry, Smoothleaf red raspberry, Wild raspberry, Wild red raspberry, Grayleaf raspberry)
Rubus parviflorus (aka: Thimbleberry, Western thimbleberry)
Amorpha fruticosa (aka: Desert false indigo, False indigo-bush, Bastard indigobush, Indigo Bush)
Brassica rapa (aka: Field mustard, Common mustard, Wild mustard, Wild turnip, Forage turnip, Wild rutabaga, Birdsrape mustard, Rape mustard)
Cirsium arvense (aka: Creeping Thistle, Canada thistle, Field thistle, California thistle, Lettuce from hell thistle, Corn thistle, Cursed thistle, Green thistle, Hard thistle, Perennial thistle, Prickly thistle, Small-flowered thistle, Way thistle, Stinger-needles)
Echium vulgare (aka: Viper's bugloss, Blueweed, Blue thistle)
Rubus armeniacus (aka: Himalayan blackberry, Rubus discolor, Rubus procerus)
Chamaenerion angustifolium (aka: Fireweed, Great willowherb, Rosebay willowherb, Saint Anthony's Laurel, French-willow)
Agastache foeniculum (aka: Giant hyssop, Blue giant hyssop, Anise hyssop, Fragrant giant hyssop, Lavender giant hyssop)
Hyssopus officinalis (aka: Hyssop)