Outdoor Cultivation Of Tropical Hibiscus

Basic requirements for healthy growth hibiscus are: sun and warmth, sharp drainage, ample moisture and nutrients.

Sun and warmth

Hibiscus should be planted in the warmest part of the garden where they receive day-long sun. The cooler the climate the more important it is to give them sheltered warmth in bright sunlight. Closer to the tropics these conditions aren't as crucial, and in fact growers in these lower latitudes have found that shrubs planted in part-shade, such as under the serrated umbrella of palm fronds, grow as well as or better than those in full sun.

Warmth means an absence of cold drafts, and again this is more of a challenge in cool climates where walls or fine-meshed trellises can give protection, and where raised beds with higher edging can cosset roots, especially when mulched, while at the same time ensuring drainage. Warmth also means an absence of very low temperatures. A short cold-snap in winter as low as 37°F (3°C) is about the limit for hibiscus.

If finding a suitably sunny site is so difficult that there appears to be nowhere to plant your yearned-for hibiscus -well, go ahead and plant it in semi-shade. The foliage will be sparser and the flowers fewer, but it will certainly grow there, even if it doesn't thrive.

Drainage

Good drainage is vital. If there is the remotest chance that the roots of your hibiscus might become waterlogged at times of prolonged rain, elevate the plant. Site it on higher ground, on the top of banks and slopes, or use raised beds. Or, if this is not possible, dig out the area and build a base of rocks and gravel before backfilling. Porous soils suit hibiscus. Fast-draining volcanic soil is good; sandy loam is ideal. Coastal sites which are almost pure sand may pose other challenges, such as how to provide and retain consistent moisture and nutrients, but drainage is not one of them.

The requirement is for aeration of the roots: hibiscus need a higher than average air-filled porosity. "Air-filled porosity" is a term used in the nursery industry to indicate the percentage, by volume, of air spaces in a container of potting mix that has been watered to saturation point and then allowed to drain off. A mix containing an abundance of coarse particles will have a high air-filled porosity. As an example, roses grow best where the air-filled porosity is about 10-12 percent. Epiphytic plants, which in nature grow in the forks of trees, need a growing medium of about 50-60 percent. While most bedding shrubs in cultivation prefer a lower percentage, the potting mix or soil best suited to hibiscus should have an air-filled porosity of about 25-30 percent.

Moisture

Hibiscus enjoy high humidity, but they'll grow happily in dry areas provided their roots have constant access to water, and provided their foliage is misted or regularly hosed. They need water most during the growing season. This means regular, thorough soaking. The faster the drainage, as in sandy beach gardens, the more watering will be needed. During the cold months of winter when the uptake of water is less, as it is with all garden plants, the need for watering will be less. In cooler climates with wet winters, watering will be unnecessary.

To test for moisture, examine the top few inches of surface soil. When it feels dry -really dry -your hibiscus needs water, and, as all experienced gardeners know, one infrequent but very thorough soaking is far better than lots of frequent but light sprinklings that may not penetrate.

Nutrients

Hibiscus grown in the ground do best when planted in well-prepared sites that have friable, loamy, good-quality soil with a pH reading of about 6 to 7, and plenty of organic matter that is routinely replenished. They really flourish if, during the warm months of active growth, they're given sustained nutrition. Potassium in particular should be in their diet as it promotes the best blooms, brightest color and most profuse flowering. Many growers claim success with citrus fertilizer. Others testify to results with commercial rose and vegetable fertilizers, and if you have these on hand you can try them. Most nurseries use chemical nutrients in soluble form.

Hibiscus thrive on compost. All organic matter, including well-decomposed manure, is beneficial because it supplies nutrients to the roots, promotes a healthy balance of organisms, and adds texture to the soil. The compost you give your hibiscus should be well aged. If you're in a position to prepare your planting site several months in advance, you can use fresh manure; otherwise make sure it's well-rotted. Dried manure, such as packaged sheep pellets, can be used at any time. You can also dig in decomposing leaf mold or straw, or purchased peat moss. Mix it in thoroughly.

Like all plants, hibiscus need nitrogen, calcium, phosphorus, potassium, magnesium and sulfur as well as trace elements. Good-quality soil that is regularly enriched with organic matter should contain all of these -analyses of seaweed, for example, show it to contain every nutrient known to plants, and comfrey has similar properties - although occasionally there may be a need for supplementary additives. Dolomite is useful in providing magnesium and can modify a soil that is too acidic. Borax supplies the trace element boron; potash supplies potassium. If your soil is seriously deficient, the problem will show up not only in your hibiscus but in other plants as well, in which event it would be wise to consult a soil expert.

Maintenance feeding means lightly digging in beyond the outer limits of the shallow roots, taking care not to disturb them, as well as applying compost on the surface. Liquid feeding can also be used. During the growing season hibiscus have a vigorous uptake of nutrients and can be fed as often as every five or six weeks. In cooler climates  feeding can be lighter.

Sub-tropical climates often experience forceful downpours of rain that can leach the soil of soluble nutrients, which is one reason for heavier feeding and mulching. Hand-watering, as distinct from nature's deluge, should always be done after feeding, and is essential if you use chemical fertilizers. Hibiscus fed little and often will be healthier than those fed heavily but infrequently.

Mulching

Mulching preserves moisture and reduces the need to water by slowing down the rate of evaporation. At the same time the loose, coarse texture of mulch aids aeration. It can also add nutrients to the soil as it breaks down, and it inhibits the growth of weeds.

To some extent hibiscus are self-mulching in that they drop leaves and flowers almost year-round and these slowly decompose -another reason for never digging the soil at the base of the trunk. By all means dig a moat or shallow trench beyond the leaf drip-line and use this both for compost feeds and for placing mulch to spread over the center, but never dig down into the soil at the shrub's base. The area of the leaf drip-line is the area of the roots, i.e. the root area is defined by the diameter of the outermost foliage. It is within this circle that mulch should be applied -but don't pack it right up to the stem.

So, keeping clear of the trunk's base, apply your mulch as thickly as 6-8 in (15-20 cm), although it is best to apply it more thinly, say 2-4 in (5-10 cm) and top up regularly. The weather, too, can be an indicator of how thickly and how often to mulch: in droughts, apply more. Now to mulch materials. Use any, or a mix of, the following: pine needles, leaf mold, straw (barley and pea straw are both excellent), hay, corn husks, nut husks, chopped or shredded prunings (not infected or infested, of course), rinsed seaweed, peat products, shredded newspaper (not colored or gloss), tree-fern fiber, coconut fiber, dried pine cones, bark (fresh pine bark should be avoided because it leaches too much resin, but small quantities can be mixed with other materials), spent mushroom compost, by-products of the sugar cane industry, or anything similar. Fresh lawn clippings should not be used, other than in small quantities mixed in with other materials. A very effective and stable mulch can be had from discarded doormats made of open-weave sisal (or coir or jute) - they're organic and make excellent aerating covers that decompose slowly.

Selection and siting

Today's tropical hybrids give the gardener far greater range in shape, size, habit and flower than was known a decade or so ago. Although pruning can enable you to keep a bush at whatever size you want, it makes more sense to select appropriately at the start. Specialist growers and hibiscus society officials now recognize three sizes: tall (6-10 ft/2-3 m), medium (3-6 ft/1-2 m), and low (under 3 ft, or 1 m), in addition to the occasional very tall, or very low to almost prostrate. Note the use of the term, "low" rather than "miniature": unlike camellias, rhododendrons and other shrubs where "miniature" describes the overall size of the bush, in hibiscus culture the description refers to flower size, not shrub size.

There is also a three-fold classification of form: open, upright or bushy. Leaves should also be considered. All tropical hibiscus are evergreen, and leaf variation extends from the almost circular leaf to the very narrow, with many in between, while leaf margins can vary from smooth, or slightly serrated, to highly indented. As for flower classification, this is almost a specialist subject in itself, so great is the variety that modern breeding has produced. The formal registration of hybrids and the need for uniform standards in show judging have called for some rules of conformity. Hibiscus societies now recognize four sizes of blooms: miniature (under 4 in, or 10 cm), medium (4-6 in/10-15 cm); large (6-8 in/15-20 cm), and extra large (8 in, or 20 cm, and over).

Besides these four flower sizes, and besides the three categories of single, double and semi-double, there are many forms and textures. Forms include cartwheel and cartwheel overlap (twisted formation of the base of the petals -wheel-like); windmill (narrower petals separated by gaps between each); fringed petals (outer edges are split and fringed); crested (both single and double forms may have this petaloidal or small petal-like growth on the style tip); cup-and-saucer (a central tuft of petaloids arises from the center and is distinctly separated from the outside petals); and recurved (the outer edges of the petals curve backwards). Petal textures include fluted (waves or soft ribs), ruffled, veined (contrasting color visible as thread-like veins) and picot or frilled edges.

From the grower's point of view, the main consideration - apart from, obviously, choosing the varieties that most appeal- is the space and location for the intended plant. Consider the ultimate size of each hibiscus you buy. You are the best judge of the best plants for the best places in your garden. And when you buy, make sure you select plants that show vitality: strong stems, good healthy foliage, and signs of plentiful new growth.

Planting and transplanting

Always plant out in spring, at the start of the growing season. In mild climates it may be possible to plant from late-winter through to fall, but caution is best where weather is doubtful. The shock of a too-early planting in spring can be fatal in cool areas, as can a delayed fall attempt when winter is fast approaching. A young, small plant can go straight into the ground untrimmed. A larger plant may need to have any long or heavy bud-bearing stems trimmed back before planting. Handle the roots gently. Invert the container or growing bag, then cup the base of the plant in the palm of your hand to remove the container, leaving the roots and clinging soil uppermost. Observe the spread of the roots: they spread out at the sides more than down at the center. To accommodate this tendency, your planting hole should be wide and shallow, but well-dug and prepared deeply below, and mounded gently in the middle.

Plant your hibiscus so that the join of the base of the stem is at the same level of soil as before, i.e. the bole/collar area must be clear of the surface. It's especially important that grafted plants have the graft area kept clear of soil and mulch, so don't plant deeply. Remember that the shallow roots grow close to the surface. Cover lightly and press gently, then water well.

If you're transplanting a long-established in-ground hibiscus to a new location, it will be necessary to prune it first by cutting back to about two-thirds, perhaps more. It should be watered very thoroughly a day or two before transplanting. Have the new site well prepared before you attempt the move. Dig around the plant's base from the outer edge of the leaf drip-line, taking care not to cut the roots; if they do extend beyond where you dig, sever them cleanly. Dig from the sides down in stages, then work your fork under the shrub and lift it carefully. A fully-grown, mature hibiscus will have a correspondingly fully-grown root system, so its removal and relocation will need more than one person to handle it. You'll need a sack or old rug under the roots for portability. Lower the shrub into its prepared new home, keeping the base of the trunk at the same level. Cover the roots, and water well. Never feed a hibiscus immediately after planting -especially if you use chemical fertilizers -but give it a settling-in period first.

Staking

The shallow roots of hibiscus are not deeply anchored. To stake or not to stake? The safest advice is to stake. And if in doubt, still stake. It's not just anchorage that's the concern. A hibiscus blown over by the wind may have its roots injured and, if damage is severe, root rot can set in. Hardwood stakes are best. Bang them into place before planting and use soft ties to hold the stem. Never use hard twine or wire or any material that can wound.

If you've found a spot up against a wall, or well-sheltered by overhang and surrounded by dense foliage, or in the niche of a recessed pocket, or other such special place, you may not need to stake. Or you may have trellising, or banks of boulders -it all depends on the site. Or you may be blessed with a climate that is always calm -a real rarity -so that your hibiscus need no buttressing. Obviously, because low-growing hibiscus are a lot less prone to damage, selecting plants that don't grow very tall is a wise preventive measure in windy areas.