Wisconsin's annual flowers never have to face the challenges of a Badger State winter. Blooming continuously over a single growing season to produce seeds for the next generation, they fill Wisconsin gardens with non-stop color. Blue flowers may be the most appropriate for a state bordered on three sides by water. Gardeners have several choices of blue annuals that thrive in Wisconsin.
Lobelia (Lobelia erinus), says Ohio State University, is a small, 6-inch high, spreading annual reaching up to a foot wide. Useful as a border edging or trailing container plant, it's a pest-and-disease-resistant annual at its best in spring and after the heat of summer subsides. "Crystal Palace" lobelia has tiny royal blue blooms against greenish-bronze to greenish-burgundy foliage. Light blue plants with medium to deep green foliage are also available.
Plant lobelia in part sun to shade, and cool, rich, moist well-drained soil for best results. Lightly shearing the plants when they rest from flowering in the hottest weather will encourage more flowers when temperatures drop. Except for the summer resting period, this annual normally blooms from May until first frost.
American bellflower (Campanulastrum americanum) grows wild in Wisconsin's moist woods and thickets and along shaded stream banks. This tall annual can reach 4 feet in height. Straight stems have dense, medium-green lance-like leaves. They bear showy light blue or purple flowers in clustered spikes from June to August. In spite of the plant's name, its blooms are shaped like stars, not bells. This annual, according to the Lady Bird Johnson Wildflower Center, would be a good choice for hummingbird gardens. Like lobelia, American bellflower prefers part shade and rich, moist soil. It can tolerate soil pH slightly higher or lower than 7.0.
Fringed gentian (Gentianopsis crinita), a native Wisconsin annual, brings stunning blue blooms to the autumn garden. Greater blue gentians are between 8 inches and 2 feet high, while lesser gentians stand only 6 to 18 inches tall and have narrower leaves, says the Lady Bird Johnson Wildflower Center. Both annuals have multiple straight stems bearing distinctive, clear-blue single blooms. Their four petals have fringed edges. The September and October flowers resemble iris buds when they close at night. These annuals grow in Wisconsin's bogs and limestone-rich areas. Plant them in part shade and moist soil.