Gardening on Maui: Mite does not affect quality of flowers on hibscus shrubs

There are a number of hibiscus varieties that are found in landscapes on Maui. Many people know that hibiscus is Hawaii’s state flower. The actual species is hibiscus brackenridgei, also known as ma’o hau hele. It is a native and endemic, which means it’s naturally found in Hawaii, but it’s also only found in Hawaii. Its natural range includes Maui, Molokai, Lanai, Oahu and Kauai, although it is not commonly found in any of those areas. H. brackenridgei is a federally listed endangered species.

Specimens of ma’o hau hele can be found at Maui Nui Botanical Gardens. It can reach 30 feet tall with an 8-to-10-foot spread. The leaves feel like sandpaper, although a smooth form is found on Lanai. Ma’o hau hele is heat and drought tolerant shrub or small tree that prefers to be grown in dry areas with excellent drainage from 0 to 3,000 feet. It prefers full sun to partial shade.

Pruning should be done shortly after flowering, which occurs primarily in winter and spring. Pests include mealy bugs, whiteflies, spider mites, aphids and Chinese rose beetles. Overwatering can cause root rot and powdery mildew.

There are a number of Hawaiian native hibiscus, including hibiscus arnottianus (koki’o ke’oke’o) found on Molokai and hibiscus furcellatus (‘akiohala), found on Maui. A more commonly found hibiscus in Maui landscapes is the Chinese red hibiscus (hibiscus rosa-sinensis). Chinese red is a misnomer as flower colors also include a broad spectrum of colors including white, coral, yellow and pink. The shrub or small tree can reach 30 feet tall and 20 feet wide, which makes it an excellent candidate for a hedge. Each of the large flowers last just one day, but the shrub continues to flower year-round. The flowers also are edible with a citrus/cranberry flavor.

Chinese red hibiscus also requires excellent drainage but is only moderately drought tolerant. And while it can grow in a variety of soils, it does not do well in soils that contain salt. The shrub does best in full sun and should be well-watered and fertilized until it becomes established. Once established, it can get by with little care. Pruning also should take place after bloom and is best done with a hand pruner.

The shrub can be attacked by both fungal and bacterial leaf spots and bud drop can occur if plants are underwatered, overwatered or overfertilized. Pests include aphids, thrips, mealy bugs, Chinese rose beetles, whiteflies and hibiscus erineum mites. Hibiscus erineum mite is one of the most common pests found in Maui landscapes.

Hibiscus erineum mite was originally discovered on Oahu in 1989 and quickly spread to the other major islands. Erineum mites are not visible to the naked eye. As they feed, the plant reacts by producing leaf, stem and twig galls. These galls will only form on actively growing young leaves and buds, not on older growth. The mites do not feed on hibiscus flowers, so while the leaves and stems may be unsightly, the pest will not affect the number and quality of the flowers.

Hibiscus erineum mites are carried by birds, the wind and flying insects that also feed on hibiscus. To prevent spreading the mites, do not take cuttings from an area that has had an erineum mite infestation, even if the plants appear healthy. Predatory mites may help with managing hibiscus erineum mites by entering the galls and preying on the pest. The predatory mites are much larger than the erineum mite, can be seen with the naked eye and move very quickly. Miticides should not be applied if the predatory mites are observed. Miticides are pesticides that specifically control mites. Miticides are not likely to provide long-term control of hibiscus erineum mites because the mites are found inside the galls.

A two-year trial done in Kahului determined that cultivars such as Apricot, Empire, Pink Hibiscus, and Apple Blossom are less susceptible to hibiscus erineum mite infestation than Chinese red hibiscus. It is generally believed that native hibiscus plants are not susceptible to hibiscus erineum mite, however native red hibiscus plants also are prone to hibiscus erineum mite.

* Lorraine Brooks is an urban horticulture extension agent and the Maui Master Gardener coordinator with the University of Hawaii’s Cooperative Extension Service, part of the College of Tropical Agriculture and Human Resources. Her email is Gardening In Maui will run when there is a fifth Sunday of the month.