Native Plants of Hawai‘i - Gossypium tomentosum

Gossypium tomentosum

Malvaceae or Mallow family

Synonyms: Gossypium hirsutum f. tomentosum, Gossypium sandvicense, Gossypium tomentosum var. parvifolium

Maʻo

Huluhulu

Hawaiian cotton

Natural Range & Environment

Gossypium tomentosum is endemic to all the main Hawaiian Islands except Hawai‘i Island [Wagner et al. 1990]. It was reported extinct in the wild on Kaua‘i in 1992 [NTBG], but with subsequent restoration efforts, it is unlikely this is still true. It is naturally found in dry coastal areas or sometimes slightly inland in dry lowlands, growing in sand, rocks (calcareous or basalt) or clay. Brushfires and development have eliminated ma‘o from much of its former habitat.

Ma‘o has bright yellow flowers that mature into dry capsules, each holding several fuzzy seeds. Wild ma‘o are often sparsely leafed because of the dry environments they inhabit (Photograph courtesy of G.D. Carr ©.). A line of ma‘o decorates this parking lot on Maui (Photograph courtesy of Forest & Kim Starr CC BY 2.0.). Two examples of cultivated ma‘o showing how differences in watering can affect the color of its leaves; more water typically equates with greener leaves (More silvery ma‘o photograph courtesy of Forest & Kim Starr CC BY 2.0.).

Description

Gossypium tomentosum is an at-risk long-lived (greater than five years) shrub 2 to 6 feet (0.6 to 1.8 m) tall and frequently wider than it is tall. However, there was(is) a treelike variety on Ni‘ihau that grew to at least ten feet (3 m) [David Orr, per. comm.]. Ma‘o leaves are 3- or 5-lobed, similar to those of kukui (Aleurites moluccanus) and maple, and appear grayish-green because of a dense covering of soft white hairs. (Over-watered plants will produce greener leaves with fewer hairs.) Its perfect (bisexual) 5-petaled flowers are light to bright yellow and about two inches (5 cm) across. Cultivated and most wild ma‘o flower and fruit repeatedly over the year. These cycles are initiated by heavy rains in natural locations. Fruits (capsules) ripen from green to tan, splitting open to reveal several seeds (¼ inch [0.6 cm]) which are densely covered in short reddish brown to tan hairs.

Growth Requirements

General

With its bright yellow flowers and silvery leaves, maʻo is a beautiful landscape shrub. It can be planted alone or in-line (3 to 6 ft apart) to create a hedge. Ma'o can also be kept as a container plant.

Gossypium tomentosum grows and looks best in a site with full sunlight for all or most of the day, and will languish in a continuously shady site. And, while it’s okay to water a newly-planted ma'o (once a week) for the first couple of months until it establishes a self-sufficient root system, after this, you should stop watering completely or water only during periods of prolonged drought. In fact, ma'o planted in a site with an automated sprinkler system are often overwatered and become covered with a black sooty mold on their leaves, stems, and trunk. Ma'o grows well in a variety of soil types or media mixes, including clay, coralline, cinder/lava, and organic-dominant, provided it drains well; avoid waterlogged soils. In continuous high rainfall sites, it's still possible to grow and showcase maʻo, provided it is given lots of sunshine and good soil drainage. Do this by planting your ma‘o on a slope, mound, or ridge. Maʻo is tolerant of extreme heat, drought, moderate winds, salt spray, and even plantings near brackish water.

When planting, keep in mind that maʻo can hybridize with other cotton species introduced to the Islands. Alex Lehman [University of Hawaiʻi at Mānoa] cautions that "this ability for hybridization (or gene flow) can severely impact the fragile genetic identity of maʻo. Couple with habitat degradation, the survival of this beautiful and culturally significant island endemic is uncertain in the wild." He continues, "to minimize the risk of gene flow, it is recommended that non-native species (any cotton species that produces white lint fiber) not be planted as ornamentals, especially if near native maʻo habitat (leeward, coastal arid environments) or other known maʻo plantings" [Lehman 2013]. With these facts in mind, exercise care before giving away any seeds from your ma‘o to friends. You can do this by sprouting and growing a sample of the collected seeds before giving any away. If the new plants do not display any hybrid characteristics (e.g., long-fiber seeds, atypical fiber, flower, or leaf color) then it's likely the remaining collected seeds are genetically pure ma‘o. If you want to be extra cautious, place paper bags around your ma‘o's flowers before they open, hand-pollinate each when they bloom, and then remove the bags about a week later.

For newly-planted maʻo, foliar feed for a few months with an organic or inorganic water-soluble fertilizer at ⅓ to ½ recommended strength to help it get its roots established quickly. However, don't overdo it. Too much fertilizer, especially those with a high nitrogen content, can result in large floppy leaves, fewer flowers, and more sap-sucking pests. Therefore, once established, stop fertilizing, or mulch with cinders, which will help improve drainage while retaining moisture (within the tiny cinder pores) and slowly add nutrients to the soil. For potted plants, infrequent but regular application of a balanced slow-release fertilizer with minor elements is recommended to maintain good health, color, and flowering.

Under optimum conditions, your ma‘o will be 2 to 3 feet in height and breath within a year; twice that in two to three years. Expect flowers and fruits within a year, two at the most. Once established, maʻo can be carefully hand-pruned to control its spread or height and keep it bushy. In fact, pruning away stems severely infested with pest insects should be part of its regular care. However, as with any plant, the cuts should be made with a clean sharp blade slightly above a node on the branch.

Pests and Diseases

More insecticide is used on commercial cotton (about 16%) than on any other single crop [EJF 2007] because it is a magnet to pest insects and diseases. While ma‘o is less attractive to pests, mature plants still get attacked by a host of insects and fungi. Most common are sucking insects like aphids, mealybugs, thrips, and whiteflies that tend to aggregate and distort the growing tips. Mites, too, can be a problem on the leaves. Chewing insects such as grasshoppers sometimes get a taste for ma‘o. Depending on the severity of the infestation, we have ignored the pests and let natural predators control them, pruned off the infested stem-tips or branches, or first pruned and then sprayed the ma‘o with horticultural oil and a systemic insecticide. Often, the sucking insects on your ma‘o are protected and spread by ants. Powdery mildew can appear on the leaves of ma‘o during rainy weather, on overwatered plants, or on those planted in a site with too much shade. The obvious solutions are to wait until the rains are gone, stop overwatering, and move your ma‘o into the sun, respectively. If the mildew persists, try spraying the plant with a non-systemic fungicide. A second fungus (Colletotrichum sp.) causes dark leaf spots. Treatment includes removing the infected leaves, stopping any irrigation that wets the leaves and can spread the fungus to healthy leaves, and spraying with a broad-spectrum fungicide. Others have reported nematode problems with ma‘o, but we have never encountered this, and, therefore, cannot offer any advice. Refer to the Pests & Diseases page for detailed ways to deal with each of the above-mentioned pests and diseases.

Uses

Ancient Hawaiian

Abbott [1992] and Krauss [1993], both legendary Hawaiian ethnobotanists, state that Hawaiians produced a yellow dye from ma‘o flowers and a green dye from its leaves, both used for decorating kapa. Further, Abbott notes that "any green kapa deserves close scrutiny, too, for the green obtained from maʻo leaves is fleeting, fading within a few days. The Bishop Museum collection contains no kapa that has retained its green coloration, but a few pieces may once have been green, judging by their overall design." Superficially, Abbott's and Krauss's statements seem logical; ma‘o flowers are bright yellow, so you would expect a yellow dye, and of course, ma‘o leaves are green. There’s only one problem: no one has been able to verify their claim. Dalani Tanahy is a friend and former coworker. She is also an exceptional kapa-maker (check out her website at: http://www.kapahawaii.com). Several years ago, Dalani told me [Koebele] she, too, had read the accounts of Hawaiians producing a yellow dye from ma‘o flowers and decided to check it out. Her investigation yielded a surprising contradiction: ma‘o flowers yield a bright green — not yellow — kapa dye. This green dye is photosensitive and fades over time, which may explain why there are no ancient examples.

Ma‘o flowers were used in lei [McDonald & Weissich 2003], while the seed fibers were used for medicinal swabs or coverings [Lincoln 2009]. For stomach cramps or aches, dried ma‘o flowers were eaten along with other ingredients while drinking a tea made of maʻo bark and other plants [Krauss 1981]. While ma‘o fibers were once used for stuffing pillows, they were never used as fabric [Lincoln 2009].

Modern

Breeding maʻo with other closely related species is possible, and has helped save the commercial cotton industry in modern times because the resulting hybrids are less attractive to insect pests that can destroy cotton crops. A commercial cotton industry was started in Hawai‘i in the 1800s and lasted for about a century, but never became an important trade except during the Civil War, when the Northern States, cut off from buying cotton from the Southern States, imported cotton from Hawai‘i [Lehman 2012].

Special Features and Information

General

The large Mallow family, Malvaceae, contains about 4,200 species, with notables such as okra, cacao, durian, baobab, kenaf, and cotton [WikipediA;Malvaceae]. Worldwide, there are about 40 species in the genus Gossypium. Maʻo is classified as a New World cotton and is the only member of this group not cultivated [Kimura & Nagata 1980]. Four Gossypium species are commercially used, with Upland cotton (G. hirsutum) composing about 97% of world production, while American Pima or Sea island cotton (G. barbadense) accounts for 1-2% — both are naturalized species in Hawaiʻi. The remaining 1% is produced from two other species, G. arboreum and G. herbaceum [WikipediA;Gossypium].

Though ma‘o has disappeared from many areas, there have been some successful restorations. Alex Lehman [University of Hawaiʻi at Mānoa] notes one of these on O‘ahu: "When off-roading near Makapuʻu lighthouse was halted, the maʻo population made a dramatic resurgence. Today, if you walk the trails between Sandy Beach and Makapuʻu in the springtime, you will be greeted by hundreds of flowering maʻo hidden amongst the koa haole and buffel grass" [Lehman 2013].

Hawaiian Name

Maʻo is derived from the Hawaiian word ʻōmaʻo for green. The endemic Hawaiian thrush, Myadestes obscurus, which has a greenish cast to its feathers, is called ʻōmaʻo. Huluhulu is a reference to its hairy leaves [Pukui & Elbert 1986].

Etymology

Gossypium is from the Greek word for cotton, gossypion [Wagner et al. 1990], while the species name, tomentosum, is a Latin word meaning "thickly matted with hairs" [Gledhill 2008].

BPK