Sideroxylon polynesicum
(formerly Nesoluma polynesicum)
Sapotaceae or Sapodilla family
Synonyms: Chrysophyllum polynesicum, Isonandra polynesica, Nesoluma polynesicum, Nesoluma polynesicum subf. gracilis, Nesoluma polynesicum subf. laurinum, Nesoluma polynesicum f. longipetiolatum, Nesoluma polynesicum f. macrophyllum, Nesoluma polynesicum f. microphyllum, Nesoluma polynesicum var. glabrum f. longipetiolatum
Keahi
Hawaiian nesoluma, Island nesoluma
Natural Range & Environment
Sideroxylon polynesicum is indigenous to the Hawaiian Islands, being found on all the main islands except for Ni‘ihau and Kaho‘olawe from 130 to 640 m (427 to 2,100 ft) [Wagner et al. 1990]. Outside Hawai‘i, the literature conflicts about where S. polynesicum is found (or, we just don't fully understand Pacific Islands geography). Wagner et al. state that the species is found "Occurring in the Austral Islands (Raivavae), Rapa, and Hawaiian Islands" [Flora of the Hawaiian Islands]. By Rapa, we assume Wagner et al. is referring to the island of Rapa Iti, one of the Austral Islands. In contrast, the World Conservation Monitoring Center (WCMC) [1998] states that S. polynesicum is native and extant to the "Cook Islands; French Polynesia (Tubuai Is.); United States (Hawaiian Is.)." Since Tubuai Island is part of the Austral Islands and directly adjacent to Raivavae Island, this is not a major discrepancy. However, the Cook Islands are about 800 miles (1,287 km) northwest of Raivavae, the northernmost island mentioned by Wagner et al. The Cook and Austral Islands are part of the same volcanic chain [www.MantlePlumes.org], so perhaps this is the reason for the differences between Wagner et al. and the WCMC descriptions. In any case, if anyone reading this has firsthand knowledge of the extra-Hawaiian distribution of S. polynesicum, please contact us (EickhoffandKoebele@gmail.com) so we can update this section.
In the past, Sideroxylon polynesicum was a much more common species in Hawai‘i's dry forests and lava fields. Today, it is considered uncommon (e.g., Lāna‘i and Maui) to rare (e.g., O‘ahu).
The milky sap of a ripe keahi fruit is super-sticky. While the upper leaf surface of a keahi leaf is nearly always shiny and hairless, the hairiness/fuzziness of the lower surface can vary tremendously; here are three examples (Photograph of the nearly hairless leaves of a Moloka‘i keahi courtesy of Spencer Len Pote CC BY-NC 4.0.). A young (but reproductive) shrub-like keahi at Maui Nui Botanical Gardens and a much larger and older wild keahi in Kanaio, Maui (Photographs of both Maui keahi, and header of flowers courtesy of Forest & Kim Starr CC BY 2.0.).
Description
Sideroxylon polynesicum is a long-lived (greater than five years) shrub to tree up to about 30 feet (9 m) tall. Nearly all the keahi we have seen were considerably shorter than 30 feet (with one exception in Nānākuli Valley) and had numerous branches emerging near its base, often making it look shrublike even when it was quite tall (see photographs above). Keahi has hard wood, gray bark, and a milky latex super-sticky sap; the bark on the trunk of old trees is often rough, cracked, and scaly.
The leaves of Sideroxylon polynesicum are elliptical to oblong, leathery, and typically about three inches (8 cm) long. While the stem tip and young leaves are usually covered top and bottom with a rust-colored fuzz, the upper surface of mature leaves is nearly always hairless and shiny. Some keahi retain the fuzz on the lower surface of their leaves while others do not (see photographs above).
Sideroxylon polynesicum flowers are small, greenish-white, usually in small clusters, and develop along the branches of the plant. Wagner et al. [1990] state that Sideroxylon polynesicum flowers are "unisexual (and the plants dioecious) or perfect (?)." However, this has not been our experience. Rather, all the keahi we have grown have produced fruit, whether they were alone or not, suggesting they all had perfect (bisexual) flowers or a mix of male and female (unisexual) flowers on the same plant. Keahi's blooming period may vary with location. However, in our experience, flowering is most common in the spring, with fruits ripening during the summer; cultivated keahi can flower and fruit over an extended period.
Sideroxylon polynesicum fruits look a lot like olives, turning from green to red, purple or black; however, you would not want to eat them because of the super-sticky white sap they contain (see photograph above). Within each fruit is a single large oval chocolate-colored seed.
Growth Requirements
General
Sideroxylon polynesicum is a truly underutilized plant in Hawaiian landscapes. In our experience, keahi is easy to grow and maintain, does well in both hot lowland and cooler upland environments, and (for a long time) remains relatively small, making it ideal for the often tiny residential front and backyards in Hawai‘i. Use it as an accent or, planted several feet apart, as an informal screen. Keahi can also be kept as a container plant, but will become stunted if not periodically transferred to a larger and larger pot.
Keahi does best in a site with full sunlight for all or most of the day. When initially planted, water your young keahi weekly until its roots get established and you see significant new growth. Then, cut back or cease watering and observe your plant's response. If it continues to do well, you're all set. If it declines, resume weekly watering and try again in a couple of months. Eventually, your keahi should require little to no watering on your part, surviving on natural rainfall and groundwater. If you keep your keahi in a pot, of course, you'll need to water it; do so once or twice a week and make certain your container is filled with an easy-draining media mix (i.e, lots of black cinder or perlite).
Sideroxylon polynesicum does not grow very fast, perhaps one to two feet (30 to 61 cm) each year. Shrublike forms tend to branch early and get bushy rather than grow tall (except for when they are grown in a constantly shaded space). If possible, ask the person you got your keahi from what the parent plant looked like — shrublike or a tree — and where it was growing. This information will help you a lot with how you care for your keahi. After three or four years, expect your keahi to begin flowering and fruiting. Keahi can tolerate extreme heat, drought, and strong winds. We have grown keahi in clay and organic-dominant soils or (in a container) cinder- or perlite-rich media. (Based on its habitat on Maui, lava/cinder landscapes should also be suitable.)
In the ground, Sideroxylon polynesicum does not appear to need any or very little fertilizer. In a container, apply a balanced controlled-release fertilizer every six months, or spray your plant with an organic or inorganic water-soluble fertilizer every couple of months at ⅓ to ½ the recommended strength. Because keahi grows slowly, avoid pruning except to remove dead branches.
Pests and Diseases
Few pests seem to attack Sideroxylon polynesicum, perhaps because of its milky sap. However, sap-sucking insects like scale insects, aphids, and mealybugs may occasionally get established on its stem-tips and leaves. Black twig borers, too, can become a problem in wet windless sites. Refer to the Pests & Diseases page for ways to deal with these pests.
Uses
Ancient Hawaiian
While we could not find any documented ancient uses of keahi, it may be that Hawaiians of old made good use of its sticky fruits, perhaps to help them catch birds.
Kia Manu (Hawaiian bird catchers) used the sap of ‘ulu (breadfruit; Artocarpus altilis) or the sticky fruits of pāpala kēpau, smeared onto branches of non-pāpala kēpau trees, to catch native birds for their feathers, which were then fashioned into 'ahu'ula (cloaks), mahiole (helmets), akua hulu manu (images of gods), and kāhili (standards) [Abbott 1992]. Birds such as 'ō'ō and mamo were plucked of their few yellow feathers and set free to grow more for the next season. However, not all native birds were as fortunate. ʻiʻiwi, ʻamakihi and ʻapapane were often plucked of all their red or green feathers and then eaten [Mitchell 1992].
Modern
Special Features and Information
General
Depending on the literature source, the number of species (and genera) in the Sapodilla family (Sapotaceae) varies dramatically from a few hundred to over a thousand. Given this uncertainty, the following information is taken primarily from WikipediA. According to WikipediA, the Sapodilla family includes approximately 800 species of shrubs and trees within about 65 genera and has a pantropical distribution. Many species have a white blood-like sap and produce edible fruits or fruit products. Examples include: one of the original chewing gum chicle (Manilkara chicle); Star apple (Chrysophyllum cainito), a dessert fruit; several species of Pouteria; shea (Vitellaria paradoxa), used to make shea butter; and the incredible miracle fruit (Synsepalum dulcificum), which when eaten alters the tongue's taste receptors turning bitter or sour foods, such as lemons and limes, sweet!
One truly notable non-native relative of keahi is the dodo tree or tambalacoque (Sideroxylon grandiflorum), endemic to Mauritius. There was a decades-long controversy regarding the claim that the extinction of the dodo bird (Raphus cucullatus) contributed to the decline of S. grandiflorum in the wild because their seeds needed to be eaten and pass through the digestive tract of a dodo bird to germinate. However, recent research has suggested this claim is false or exaggerated, and that the decline of the dodo tree is more likely the result of competition with alien weeds and the unripe fruits being eaten by the invasive introduced crab-eating macaque (Macaca fascicularis) [WikipediA;Sideroxlon grandiforum].
In addition to Sideroxylon polynesicum, other native members of Sapotaceae in Hawai‘i include: two endemic species of ʻālaʻa, Planchonella sandwicensis and P. spathulata [Flora of the Hawaiian Islands].
Hawaiian Name
Etymology
The genus name, Sideroxylon, comes from the Greek words, sideros, meaning "iron," and xylon, meaning "wood," [Gledhill 2008] referencing its very hard wood. The species name, polynesicum, is a direct reference to Polynesian, where this species is found. The suffix -um was added to Polynesia in order to Latinize the word [Gledhill 2008].
BPK