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U.S. Forest Service

Plant of the Week

Lobelia siphilitica range map. Lobelia siphilitica range map. USDA PLANTS Database.

Lobelia siphilitica. Plant in flower. Photo by David D. Taylor.

Lobelia siphilitica. Close up of flower. Photo by David D. Taylor.

Lobelia siphilitica. Close up of flower from side. Photo by David D. Taylor.

Great Blue Lobelia (Lobelia siphilitica)

By David Taylor

Great blue lobelia, is a member of the Campanulaceae (Bellflower) family. The family includes around 2,400 mostly herbaceous species, most of which occur in North America, Europe and Asia. A number of species are from South Africa. Well known species include bellflowers (Campanula species) and balloon flower (Platycodon grandiflorus). About 415 species of Lobelia are known. About 43 species are known from the U.S., including Hawai’i, and Canada. A particularly well known species is cardinal flower (Lobelia cardinalis) which has intensely red flowers.

This lobelia generally produces a single upright stem 5 to 15 decimeters (20 to 59 inches) or more tall. The stem is smooth or scarcely hairy. The alternate, toothed leaves are narrowly oval shaped. Leaves are 8 to 12 centimeters (3 to 4.75 inches) long by 1.5 to 4 centimeters (0.6 to 1.6 inches) wide. Flowers are arranged in an elongated cluster (raceme) 10 to 30 centimeters (4 to 12 inches) long at the end of the stem. Flowers are blue-violet or sometimes white, and about 2.5 centimeters (1 inches) long. The backsides of the flowers have short hairs.

The lower three petals of the flower provide a landing pad for a bee. A bee of the correct weight, a bumblebee most of the time, will depress the petals while it works to crawl inside the flower to get nectar. The style (arched yellow part in photo C) is bent downward and the stigma (the pinkish blob at the end of the style), pollen from a previous flower is wiped on the bee’s back. If the bee continues into the flower pollen is wiped on its back. Some bumblebees just chew a hole in the base of the flower and take the nectar that way.

This species is found in damp to wet ground, often in areas with some shade. Habitat includes roadside ditches, floodplains, lake margins, swamp forest, and wet prairies. It is known from Maine to Georgia west to Texas, Colorado, Wyoming and the Dakotas. It is also known from Ontario and Manitoba.

Bumblebees, other bees, hummingbirds, and butterflies will also visit the flowers. This species is frequently cultivated and is available from many commercial sources. This species was used to treat respiratory and muscle disorders by Native Americans and once was considered a cure for syphilis by European settlers.

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