Any spot in your garden that never has standing water, but retains some moisture even when summer is at its hottest, qualifies as having moist soil, according to Purdue University's Dr. Michael N. Dana, Ph.D. and Consumer Horticulture Extension Specialist B. Rosie Lerner. These conditions mimic those in woods, where humus prevents soil moisture from evaporating. Many flowering plants--including groundcovers, shrubs, vines and trees--require consistently moist to wet garden soil.
Sweet pepperbush (Clethra alnifolia) is a deciduous shrub native to the eastern United States' seashores, swamps, bogs and streambanks. Growing up to 8 feet tall and 6 feet wide, it's hardy to minus 40 degrees F. Its glossy, deep green leaves become bright yellow in fall. In July and August, according to the Missouri Botanical Garden, plants bears 3- to 6-inch spikes of fragrant white flowers. Brown seedpods follow the flowers and persist into winter. This largely disease- and insect-resistant shrub is a good choice for water gardens or pond edges. It tolerates sun but prefers partial shade and well-drained, consistently moist or wet acidic (pH below 7.0) soils.
Butterfly lily (Hedychium coronarium) is a tender perennial hardy to 10 degrees F. Belonging to the ginger family, it's native to India, where it can grow up to 10 feet high. In other settings, plants normally stand between 3 and 6 feet tall and up to 3 feet wide. The lily has large--2 feet long by 5 inches wide--green leaves. Heavy, elliptical clusters of fragrant, white butterfly-shaped blooms appear between late summer and fall.
Use butterfly lily, recommends the Missouri Botanical Garden, in borders and containers. It thrives in full sun to partial shade, moist, rich well-drained soil, and high heat and humidity. Where not winter hardy, lift its rhizomes (thick roots) for winter storage in vermiculite and set them out again after the last spring frost. Plants are susceptible to root rot, scale, aphids and spider mites.
Rose mallow (Hibiscus laevis) is a late-blooming perennial shrub found in the moist soils along streams, rivers, ponds and swampy areas throughout the central and eastern United States. From 4 to 6 feet high and up to 3 feet wide, it has 6-inch, three-lobed green leaves. In August and September, its 4- to 6-foot-high stems have maroon-centered, white or pink blooms. Showy, 6-inch flowers resemble hollyhocks.
Hardy to minus 30 degrees F, rose mallow is a good choice for water edges, native plant gardens and the backs of borders. Plant it in full sun--for best results--to light shade. Provide consistently moist or wet, fertile, well-drained soil. The tallest plants may need staking.