Growing Hibiscus can be confusing to Iowa gardeners because the three most common species vary greatly in flower, habit and cold hardiness. But all three are worthy of consideration in the outdoor or indoor landscape.
The Rose-of-Sharon or Althea Shrub (Hibiscus syriacus) is a woody hibiscus that is blooming beautifully right now. This is the tallest of the 3 hibiscus species, often reaching 6 or 7 feet. Flowers are 3-4 inches in diameter and are borne by the hundreds starting in mid to late summer. Flowers can be single or double, and are usually pink, red, lavender/blue, or white, sometimes with a contrasting eye or center. Plants thrive in full sun and well drained soils. Since their cold hardiness is limited to zone 5, there are reliable bloomers in protected locations in central and southern Iowa.
Another hibiscus that tolerates Iowa winters is the Hardy Hibiscus or Rose Mallow (Hibiscus moscheutos). Unlike the Rose-of-Sharon, this hibiscus dies back to the ground every winter just like other herbaceous perennials. The plant habit can also be considerably smaller, with many of the new cultivars staying under 5 feet. But what it lacks in stature, it more than makes up for in flower size. Flowers are up to 12" wide and available colors include red, pink, plum, mauve, white, and lavender. Contrasting centers or eyes are also common with cultivars of this species. Like all hibiscus, Rose Mallow thrives in sun. But unlike the Rose-of-Sharon, it tolerates moist soils. In fact, it is native to marshy areas of the US. Cultivars vary greatly in leaf size, leaf dissection, leaf color, flower color, flower size, and mature height -so read the label carefully to find one that will match your preferences and landscape space. When in flower, rose mallow will give the landscape a tropical feel without the worry of winterkill. They do, however, take a while to emerge each spring. Don't worry if they are a bit tardy with their debut - simply be patient and they should appear by June.
Want something really tropical looking? Look no further than Tropical Hibiscus (Hibiscus rosa-sinensis). This is the species found throughout Hawaii and other warm winter locales. It has brilliant orange, pink, red, yellow, white, and lavender flowers. Flowers can be single or double; have frilled petals, and sometimes contrasting eyes or centers. Both the flowers and the leaves tend to be smaller than those of hardy hibiscus and are always lustrous and richly colored. Tropical hibiscus is often sold as a patio or container plant in the spring and will not survive an Iowa winter outdoors. Plants are usually brought indoors in September as the temperatures start to drop into the 40's and 50's. Carefully inspect plants for insects (especially white-fly) before bringing them indoors. Remove any insects that are present with a good wash with an insecticidal soap solution. Plants perform best outdoors in summer in full sun to part shade and require frequent moisture and fertilizer. Once brought indoors, they need bright, direct light to maintain a full complement of leaves. Long days (and short nights) are required to keep plants in flower throughout the winter months.