According to observations of naturalists and beekeepers.
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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.
Cercis canadensis (aka: Eastern redbud, Redbud, Cersis Reniformis)
Acer saccharinum (aka: Silver maple, Soft maple)
Malus angustifolia (aka: American crab apple, Buncombe crab apple, Crabtree, Narrowleaf crab, Narrowleaf crab apple, Southern crab, Southern crab apple)
Ulmus americana (aka: American elm, White elm, Water elm, Soft elm, Florida elm)
Prunus armeniaca (aka: Armenian plum, Ansu apricot, Siberian apricot, Tibetan apricot, Apricot tree)
Salix nigra (aka: Black willow, Swamp willow, Southwestern black willow, Gulf black willow, Scythe-leaved willow)
Celtis laevigata (aka: Hackberry, Sugarberry, Lowland hackberry, Sugar hackberry, Arizona sugarberry, Netleaf hackberry, Small's hackberry, Southern hackberry, Texas sugarberry)
Acer negundo (aka: Boxelder, Western boxelder, Arizona boxelder, California boxelder, Texas boxelder, Interior boxelder, Violet boxelder)
Pyrus communis (aka: European pear, Common pear)
Ilex opaca (aka: American holly, Dune holly, Hummock holly, Scrub holly)
Malus domestica (aka: Pyrus pumila, Malus pumila, Apple)
Amelanchier arborea (aka: Common serviceberry, Downy serviceberry, Juneberry, Shadbush, Shadblow, Sugarplum)
Malus coronaria (aka: Sweet crab apple, Garland crab, Alabama crab, Allegheny crab, American crab, Baltimore crab apple, Buncombe crab, Dawson crab, Dunbar crab, Fragrant crab, Garland tree, Lanceleaf crab apple, Missouri crab, Sweet-scented crab, Sweet wild crab, Wild crab, Wild sweet crab, Wild Crab Apple)
Prunus americana (aka: American plum, Goose plum, River plum, Wild plum)
Acer saccharum (aka: Sugar maple, Rock maple, Hard maple)
Prunus cerasus (aka: Sour cherry, Tart cherry, Dwarf cherry, Montmorency cherry)
Populus tremuloides (aka: Quaking aspen, Trembling aspen, Aspen, American aspen, Mountain aspen, Golden aspen, Trembling poplar, White poplar, Popple, Alamo Blanco)
Liriodendron tulipifera (aka: Tuliptree, Blue-poplar, Tulip-poplar, Yellow-poplar, Yellow wood)
Salix discolor (aka: Pussy willow, American pussy willow, Glaucous willow, Large pussy willow)
Acer rubrum (aka: Red maple, Scarlet maple)
Celtis occidentalis (aka: Common hackberry, Bastard elm, Nettle-tree, Northern hackberry)
Salix lutea (aka: Yellow willow)