Pruning And Maintenance Of Hibiscus
Left unpruned, all hibiscus soon become shapeless and messy, and their flowers fewer, smaller and duller. Most hibiscus are evergreen. Apart from a cluster of species classified as herbaceous, which includes smaller annual or biennial sub-shrubs found growing in the wild, the only two fully deciduous species in cultivation are H. syriacus and H. mutabilis. These two hardy shrubs require heavy pruning to perform well, for like their tropical relations, they flower on new wood. Prune these two species only in winter, when the branches are bare. Other hibiscus should never be pruned in winter.
Early spring - at the start of the growing season - is the time to prune your tropical hibiscus. Occasionally, there may also be a need for a mid-season trim when the plant is in active growth, especially in warm climates. Most experienced growers prune just as cooler weather gives way to warm and when observation reveals the swelling of new growth buds. Don't wait for these buds to open out fully into leaves, but strike just before. Such timing will give you control over your plant by directing it where to put its energy. If you wait until after the new foliage has shot away, you've already lost some control, for the trimmings you discard are the wasteful product of the plant's energy reserves which might better have gone into leaves and blooms where you want them.
It is likely that along with the new foliage growth, you'll also be sacrificing flower buds. Don't think of this as a loss. Remember that you'll get far more flowers from pruning, because from each pruned branch or stem three new stems will grow in its place.
The purpose of pruning is to improve the shape of your hibiscus and to improve the vigor of its main branches (upright or spreading, depending on the hybrid) while also opening up the center to air circulation and sunlight, thereby encouraging new blooms. Pruning is more than just reducing the length of the branch and the number of its side shoots, for it also involves cleaning up any dead wood, removing weak or bug-eaten branches - and any diseased ones, of course - and cutting away any misshapen, spindly, odd-angled or aesthetically displeasing branches. Pruning means attaining desired shapes, and desired shapes can include espaliered and standard forms. If you prune wisely and your hibiscus are well-nourished, they will repay you with abundant and bountiful blooms.
By cutting back previous years' hardwood to reduce size, and by removing unwanted stems to induce vigor and shape, the result is likely to be a bush about two-thirds the pre-trimmed size. A healthy plant will tolerate even harder pruning to half size, or you may wish to take away only an eighth, or a quarter if the plant is young. With young shrubs, and sometimes even with larger ones, some experienced growers prefer to pinch out the first new growth rather than prune a longer branch, claiming that this method results in a thick-textured, better shape; they save the radical one-third-off prune for mature specimens.
Make sure your secateurs and lopping shears are sharp and scrupulously clean, so that your cuts are likewise sharp and clean. Cut at a slight slant as with all hardwood, about a 45° angle, with the highest part of the slant just above a bud or "eye". This will allow water to run off rather than collect on the wound. A jagged cut will invite bacterial infection.
To remove unwanted stems growing at angles from the framework, cut at a fraction beyond the join. This applies also to those very low branches that sweep the ground near the base of the trunk. These branches are often never allowed to grow. While some gardeners like the appearance of bushiness right to the ground, others argue for their removal for reasons of hygiene, and say that space at ground level makes mulching and maintenance easier.
Opinion is also divided about the use of pruning tar. Sealing over the cut immediately following surgery is believed to prevent infection and promote health, but many arborists now eschew this practice with forest trees in favor of allowing the tree to self-seal its wound. Self-sealing occurs, they say, when the cut is made further out on the limb from where it joins the trunk, so that when this amputated stump naturally rots away, the wound left on the trunk is smooth.
Neglected shrubs that have matured into quite hefty trees need a bold approach. First, feed and water them well over the growing season to promote health, then, after a winter rest period, arm yourself with a strong, well-sharpened pruning saw, and lop down the entire tree to about table height, retaining half a dozen, at most, strong branches that open outwards, vase shaped, while removing inner-angled branches that clutter the center. Clean up completely the ground below. Now, with more than half your bush or tree gone, together with by far the greater portion of the foliage, resist the urge to feed it. There is insufficient foliage left to take up and release moisture, and adding nutrients, especially in concentrated form, can harm the bush, so leave it to recover. Recovery may take another whole season, but in time it will flourish fully. The foliage and flowering will both be invigorated.