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Hibiscus: Hardy and Tropical Plants for the Gardenby Barbara Perry Lawton
Hardy hibiscuses will not thrive in tropical or subtropical climates such as those found in southern Florida. They require a cold season to flourish, much as apple trees do. Nevertheless, once established, these hibiscuses can endure weather and environmental extremes amazingly well. They thrive in full sun with moist, fertile soil that is of good texture, producing maximum numbers of flowers when provided with six or more hours of sun each day. They are adaptable, however, and can tolerate ordinary soil, dry conditions, and some shade once they have recovered from transplanting. Since many of these floriferous beauties descend from plants that grow in wetlands, they are tolerant of wet soils, although, like many bottomland natives, they are also tolerant of droughty conditions. Hardy hibiscuses are not particularly fussy with regard to pH, adjusting well to the usual garden soil, which will probably be anywhere from 6.5 to 7.5.
Photoperiodism defines a plant's physiological response to day or night length. Changes of season often determine such events as seed germination and flowering. Plants determine the time of year by means of the relative lengths of night and day. This can be especially important for plants that are native to areas that are far from the equal nights and days of equatorial regions. Some plants, such as poinsettias and chrysanthemums, are initiated into flowering by short days and long nights. Others, such as beans, corn, and cotton, are day-neutral plants, meaning they are unaffected by the photoperiod. Still others are triggered into flowering by long days and short nights.
Hardy hibiscuses are considered to be long-day plants, but in actuality they are short-night plants. It has long been believed that their flowering is initiated by day lengths exceeding twelve hours. However, recent studies suggest that it is the comparatively short period of darkness that spurs the development of flowers.
Knowing about the native environment of a plant and then trying to match those conditions at home is the key to success with any plant. Learn what kind of climate and soil it thrives in where it is indigenous. If you are growing plants bred from or descended from species that originated in marshy locations in the eastern United States, you can confidently assume that your plants will be a good choice for soggy, poorly draining locations. If, on the other hand, the plants derive from species native to the desert Southwest and Mexico, the last place you will want to plant them is in a site with poor drainage. Given a few simple environmental conditions, depending on the species or on the origin of the cultivar, hardy hibiscuses are easy to grow, and reward gardeners with both the large size of their blossoms and the profusion of bloom.
Most hardy hibiscuses found in the marketplace are hybrids developed from species native to damp or even wet places in eastern North America, often in the same locations where cattails are found. These handsome ornamentals are hardy to Zone 5 or even Zone 4 but need to be treated as annuals in colder climes. Of course, one never knows what winter will bring in the interior of the United States. The continental climate, not to mention the mountain chains that run north-south, means that
northern winters may be as mild as those in Memphis or as severe as those in the Yukon Territory. Most winters are a combination of the two extremes. When people ask what the climate is like in St. Louis, where I live, I can truthfully tell them that it's too hot, too cold, too wet, and too dry.
Hardy hibiscuses are late to emerge in spring. In fact, I have known gardeners who pulled up their plants thinking that they had died over the winter. Be patient with these plants. They will begin to show some budding foliage well after most other plants are in full leaf.
These perennials can become gangly, so it is wise to give them a thorough tip pruning once the new spring growth is 18 inches (45 cm) tall. If they are growing rapidly, you may want to give them a second tip pruning to further encourage branching and bushy growth. But do not tip prune after the first of June or you will delay flowering too much. If you do not tip prune hardy hibiscuses once or twice to encourage branching and bushiness, the larger, more vigorous varieties may fall over in the summer unless you stake them or cage them. In most cases, however, these strong-stemmed hibiscuses rarely need to be staked.
Varieties that ordinarily reach a height of 6 feet (1.8 m) often set buds and flower at half that height. These are plants that respond well to pruning. Once the first flush of blossoms is past, encourage more blooms by trimming off the old flowers, including any developing seedpods. You should see new flower buds developing below the old ones. Don't make the mistake of overpruning hardy hibiscuses once the first flush of bloom is past — this will delay the second round of flowers.
In fall, after a hard freeze, the foliage will die back, but it isn't necessary to prune the current year's growth until new growth begins in spring. The old growth, decorated with the last seed heads of the season, may provide winter interest. Similarly, some hibiscuses have red stems that keep their color well into winter. Most hardy hibiscuses are quite shapely if they have been tippruned during the growing season. If this is the case with your plant, leave it alone until new growth starts from the base in spring. At that time, prune back the old stems to 5 to 6 inches (12.5 to 15 cm) above the ground. If the plant becomes gawky or lopsided in late spring, go ahead and prune it back to 2 to 3 feet (0.6 to 0.9 m), shaping the plant as you do so; avoid, however, doing this after May. Dead, injured, and broken stems can be pruned out at any time of year.
Once there has been a hard freeze, put down 3 to 4 inches (7.5 to 10 cm) of mulch. There is no need to worry about root rot with these plants, since they originate from damp or wet places. The mulch will moderate temperature extremes, conserve soil moisture, and discourage weed growth.
Pruning hibiscus stems too close to the ground may open the roots up to damage from freezing. It also may lead to you forgetting the exact location of the plant. Never prune hardy hibiscuses to the ground midway through the growing season. The plants will not recover in time to set more blooms during that season.
Leave hardy hibiscuses in place for a decade or more. Don't bother trying to divide the crowns — a challenging task at best. It is far better to buy new plants or grow them from seed or cuttings. Just remember that if you do grow hibiscuses from seed, you probably won't get plants that are like the parents.
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