With big, showy flowers, hibiscus (Hibiscus spp.) are starring players in the midsummer garden. These tough plants put on an extravagant display when many other plants are struggling under high humidity and temperatures. It's a nice surprise to discover that several hibiscus species grow well in the moderate temperatures found in the Northeast.
About the Northeast
The northeastern United States, usually defined as the states from New York to Maine, enjoy moderate summer temperatures but cold winters with lots of snowfall. The area falls within U.S. Department of Agriculture plant zones 3b through 6, with average winter temperatures between 0 and minus 35 degrees Fahrenheit. Gardeners in most parts of the Northeast can choose between several different types of hibiscus.
Scarlet Swamp Hibiscus
The red or scarlet swamp hibiscus (H. coccineus) is hardy in plant zones 5 through 9, making it suitable for gardens in the more moderate areas of the Northeast. This native perennial wildflower, also called the Texas star, has 6- to 8-inch, bright red flowers that open into a star shape. The bold foliage is deeply divided. Grow scarlet swamp hibiscus in full sun in moist, even swampy, soil.
Often just called hardy hibiscus, rose mallow (H. moscheutos) is another perennial hibiscus native to the eastern United States. Rose mallow is slow to break dormancy in spring, so mark its location carefully so you don't accidentally disturb the roots. Once it starts to grow, rose mallow quickly becomes a 2- to 3-foot shrub covered in flowers up to 12 inches across. Many cultivars are available, mostly in shades of red, white and pink. Hardy hibiscus grows well in sun or partial shade in moist soil. It's hardy in plant zones 4 through 9, so it can be grown throughout most of the Northeast.
The shrubby rose-of-Sharon (H. syriacus), also called shrub Althea, is an 8- to 10-foot, deciduous shrub with a vaselike growth habit. This shrub blooms on new wood, so it tolerates severe pruning in spring to control its size or shape. Like hardy hibiscus, rose-of-Sharon is slow to leaf out in the spring. Once it starts blooming in midsummer, it continues until frost. The flowers are blue, white or pink and are often splotched in the middle with a deeper color. These shrubs are tolerant of a wide range of soils and grow well in the poor, compacted soils of urban areas. Rose-of-Sharon blooms well in sun or partial shade, and is hardy in zones 5 through 9.