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.
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)
Acer negundo (aka: Boxelder, Western boxelder, Arizona boxelder, California boxelder, Texas boxelder, Interior boxelder, Violet boxelder)
Pyrus communis (aka: European pear, Common pear)
Acer glabrum (aka: Rocky Mountain maple, Douglas maple, Greene's maple, New Mexico maple, Torrey maple)
Salix lucida (aka: Shining willow, Greenleaf willow, Tail-leaf willow, Whiplash willow, Pacific willow, Lance-leaf willow, Longleaf willow, Red willow, Western shining willow)
Salix exigua (aka: Narrowleaf willow, Coyote willow)
Prunus americana (aka: American plum, Goose plum, River plum, Wild plum)
Acer grandidentatum (aka: Bigtooth maple, Canyon maple, Western sugar maple)
Prunus cerasus (aka: Sour cherry, Tart cherry, Dwarf cherry, Montmorency cherry)
Crataegus douglasii (aka: Black hawthorn, Douglas hawthorn, River hawthorn, Western thornapple)
Prunus emarginata (aka: Bitter cherry)
Prunus pumila (aka: Sandcherry, Western sandcherry, Eastern sandcherry, Great Lakes sandcherry)
Salix bebbiana (aka: Bebb willow, Beak willow, Beaked willow, Long-beaked willow, Diamond willow, Chaton, Petit Minou, Smooth Bebb willow)
Salix scouleriana (aka: Scouler's willow, Upland willow)
Salix discolor (aka: Pussy willow, American pussy willow, Glaucous willow, Large pussy willow)
Malus domestica (aka: Pyrus pumila, Malus pumila, Apple)
Acer platanoides (aka: Norway maple)
Rubus ursinus (aka: California blackberry, California dewberry, California grapeleaf dewberry, Douglasberry, Pacific blackberry)
