Daniel Gill | 6/5/2015 12:15:04 AM
There is no denying that Louisiana gardeners are big fans of the tropical hibiscus. The tropical or Chinese hibiscus, Hibiscus rosa-sinensis, is a member of the Malvaceae or Mallow family along with such plants as cotton, okra, hollyhock and Turk's cap. Other commonly cultivated Hibiscus species include althea (H. syriacus), Confederate rose (H. mutabilis), Texas star (H. coccineus) and rose mallow or hardy hibiscus (hybrids involving H. moscheutos and other species).
As their common name implies, tropical hibiscus is native to frost-free climates and may be damaged or even killed by severe freezes. These shrubs are typically not hardy in north Louisiana when planted in the ground, and generally should be grown in containers.
Except for unusually cold winters, the tropical hibiscus is typically hardy in the landscape in the lower third of the state - especially with protection. Apply a thick mulch of pine straw about 8 inches deep around the base of the plant to save the crown and/or cover the plant with fabric or plastic sheets when temperatures lower than 30 degrees are predicted. Plants often survive quick freezes in the mid to upper 20s, but may die in extended freezes reaching the teens.
When planting in the ground, the beds should be prepared by digging in a 2 to 3-inch layer of compost, peat moss or manure, along with a light application of a general purpose fertilizer following label rates.
You can plant hibiscuses in the ground anytime during the summer. However, hibiscuses planted in late spring or early summer are better established and more cold resistant than hibiscuses planted into garden beds in the late summer or fall. So, planting in late spring or early summer is the best time.
Hibiscuses also make outstanding container plants. Growing them in containers sets apart and shows off these shrubs. And it is the most reliable way to grow them in north Louisiana. Repot the plants into progressively larger containers over time as they outgrow the pot they are in. Use good quality potting mixes and make sure the pots have drainage holes.
During the summer, fertilize your plants occasionally to keep them growing and blooming vigorously, especially those in pots (how often you apply fertilizer depends on the type you use). A slow release fertilizer is the easiest, as you only need to make one application to the plant in the spring and it will provide fertilizer all through the growing season.
Do not use high phosphorous fertilizers on hibiscuses. The percent of phosphate (phosphorous) in the fertilizer is represented by the middle number in the analysis (a three number analysis can be found on the label of all fertilizers, and they represent nitrogen, phosphorous and potassium levels in the fertilizer - always in that order). So, make sure the middle number is the smallest number in the analysis when you choose a fertilizer for your hibiscuses.
Hibiscuses prefer an even supply of water and should not be allowed to wilt severely before watering. Those in containers are especially vulnerable to drying out and may need daily watering in the summer. Water hibiscus plants growing in the ground regularly and thoroughly during hot, dry weather or whenever you notice they have wilted slightly.
Hibiscus should be given as much direct sunlight as possible for best flowering – at least six hours a day. Full sun – 8 hours or more of direct sun daily – is preferred.
Pruning may be done anytime you feel the need to control or shape the bushes. There’s really no special way to prune hibiscuses. It depends on why you are pruning and what you need to accomplish. Often, gardeners do major pruning in early spring, around early March, after the danger of hard freezes is past. Plants are generally not in bloom at that time due to unfavorable winter growing conditions. But, you may prune anytime during the summer if needed, even if the plant is in bloom.
How far back you cut depends on what you are trying to accomplish. If there was cold damage during the winter, prune to remove the damage by cutting back to living tissue. When pruning during the summer growing season, it’s best to prune lightly occasionally than to let the plant get way overgrown and have to cut it back a lot. After pruning flower production will stop until the plant has made sufficient new growth. Generally, the farther back you cut your plant the longer it will take to come back into flower.
So, if you want a plant about 5 feet tall, don’t let it get to be 10 feet tall and then cut it back to 5 feet. Cut it back to 4 feet when it reaches 6 feet. This helps keep the root system smaller and makes it easier to control the size of the plant.
Pruning is also done to reduce the size of bushes during the winter. This makes it more practical to cover them to protect them from cold weather. This should be done just before you need to cover them.
Yellow leaves often occur and are alarming but may not necessarily signal trouble. It is perfectly natural and healthy for a vigorously growing hibiscus to occasionally yellow and drop its older leaves (common in early spring).
Leaves may also yellow and drop due to sudden changes in environmental conditions, and may occur in a newly purchased hibiscus or those moved from one location to another (especially when moved to lower light conditions).
A plant that is allowed to wilt may recover when watered, but then yellow and drop leaves sometime later. On the other hand, a plant that is watered too often and kept too wet may also yellow and drop leaves.
An overall pale, yellowish look to the plant, with the lowest leaves especially pale, indicates a need to fertilize.
Yellow leaves may also mean the plant is infested with insects such as aphids or pink hibiscus mealybugs. Inspect a plant with yellow leaves very carefully for a possible insect infestation, and deal with the insects if necessary.
A dropped flower bud is like a broken promise. And lack of flowering is disappointing as the flowers are why we grow these beautiful plants.
Flower bud loss is most commonly due to stress from such factors as dry soil, low light, sudden environmental changes and high temperatures. Some cultivars of hibiscus seem to be more prone to bud drop than others, especially during the high temperatures of mid to late summer. There’s nothing that can be done when heat is the issue. When other stress factors are identified, improve the growing conditions.
Poor flowering may occur when newly purchased plants are repotted into a much larger container or planted in the ground. This will also occur when plants are cut back severely. Under good growing conditions the plants will eventually come back into flower.
Insufficient light is another cause of poor flowering. Remember to give your plants as much direct sun as possible.
Short days and cool to cold temperatures reduce flower production during the winter.
Plants that have been in the same container for a number of years may flower poorly or drop buds because they are pot bound. Transplant into a slightly larger container and the situation should improve.
Hibiscus plants drop their buds for a variety of reasons. Environmental factors often play a role. If hibiscus plants are allowed to wilt between waterings they will often drop flower buds (and yellow and drop lower leaves as well). However, keeping hibiscus plants too wet can cause root problems which can ultimately lead to a sick looking plant dropping buds.
I have seen many instances where hibiscus plants (particularly double flowered ones) begin to drop buds badly when daytime highs stay consistently in the 90s. Some cultivars are not bothered much by the heat and continue to bloom while others drop most or all of their buds. As the weather begins to cool down in late September/October, plants dropping buds due to the heat will begin to hold onto their buds and bloom.
Pests can also cause hibiscus bud drop. Aphids are small sucking insects that will cluster on the new growth and buds. They feed by sucking the sap from the leaves and flower buds, and this can cause the flower buds to abort. Aphids are easy to see and diagnose when they are the problem, and can be controlled with a general purpose insecticide.
Hibiscus bud midge or gall midge larvae feed inside the bud causing it to drop. Because they are inside the bud they are more difficult to see and control. You should check the buds that are falling off to see if there are any pinhead-size holes in them. Cut open several buds that have just fallen or are about to fall from the plants. The larvae of the gall midge are tiny and look like little maggots, so look carefully. Systemic insecticides should control these pests. Imidacloprid is a good insecticide for this problem. If you don’t see any midge larva, the problem is generally stress or environmental.
Aphids, pink hibiscus mealybugs, caterpillars and bud midge larva (see previous section) are the leading pests of hibiscuses.
Aphids are small, sucking insects that cluster on the flower buds, new growth and under leaves in the spring. Their feeding can cause the foliage to look shiny and feel sticky. Aphids are not fatal to hibiscuses, but as they feed on the sap, they weaken the plant and can cause stunted growth, deformed leaves, bud drop and yellow, dropping leaves. Control aphids with sprays of light horticultural oils (Year Round Spray Oil, All Seasons Oil), insecticidal soap or a pyrethroid, like permethrin, bifenthrin, cyfluthrin or others.
Pink hibiscus mealybugs are a relatively new pest in south Louisiana that showed up in the mid-2000s. The insects are covered with a white, fuzzy/powdery material. They cluster on the new growth and buds of the hibiscus plant and are very obvious when they are present. Like aphids, they feed by sucking out the sap of the bush. But, they also inject toxic compounds that can seriously damage or even kill a plant.
Pink hibiscus mealybugs can be controlled with several applications of a light horticultural oil. Or, you could use one of the pyrethroids (permethrin, bifenthrin, cyfluthrin or others). Imidacloprid, applied as a drench, can also be used to control pink hibiscus mealybugs, either alone or in conjunction with the insecticides already mentioned.
Where you have had serious or continuing problems with this pest, you can treat your plants with the insecticide imidacloprid in April or May to prevent infestations all summer. Some brands include Bayer Advanced Garden Tree and Shrub Insect Control, Bonide Annual Tree & Shrub Insect Control Concentrate, Bonide Guard & Grow, Ferti-lome Tree & Shrub Systemic Insect Drench, and Monterey Once a Year Insect Control II. This insecticide will also help control persistent aphid problems.
Do not use Malathion on hibiscuses as they are sensitive to this insecticide.
Caterpillars will occasionally feed on the foliage of hibiscuses. The damage is generally tolerable. But when it is severe enough to warrant control, the least toxic insecticides to use are Bt (Dipel, Thuricide and other brands) or spinosad (Monterey Organic Garden Insect Spray; Ferti-lome Borer, Bagworm, Tent Caterpillar and Leafminer Spray; Green Light Spinosad Lawn and Garden Spray; Dow Spinosad Home and Garden, Success Naturalyte; Bonide Captain Jack’s Deadbug Brew). Pyrethroids (permethrin, bifenthrin, cyfluthrin or others) and carbaryl (Sevin, Carbaryl) are also effective.
Always carefully read the entire label of a pesticide before purchasing it to make sure it is appropriate for your plants and pests.
Dwarf tropical hibiscuses pack all the punch of larger plants in a much smaller package. They are ideal for window boxes, containers sitting on tables and the front flowerbeds. Provide them the same care as for standard hibiscus, and enjoy the large colorful flowers produced on compact plants.
There is, however, a catch. Dwarf hibiscus plants are not smaller growing by nature – that is, they are not genetically dwarf. Instead, these are standard size hibiscus plants that have been treated with plant growth regulators. Eventually, the treatment will wear off (it generally lasts about one growing season) and the plants will begin to grow at their normal rate. Once that happens, you can continue to enjoy the plant as a standard sized hibiscus in a larger container or plant it into the ground.