Native Plants of Hawai‘i - Solanum incompletum

Solanum incompletum

Solanaceae or Nightshade family [Five-minute families]

Synonyms: Solanum abollatum, Solanum forbesii, Solanum globosum, Solanum haleakalaense, Solanum incompletum var. glabratum, Solanum incompletum var. mauiense, Solanum tonsum

Pōpolo kū mai

Pōpolo

Thorny popolo

Natural Range & Environment

Solanum incompletum is a federally-listed endangered shrub endemic to the Hawaiian Islands with the only living wild plants growing on Hawai‘i Island. There it grows in dry to mesic forest, diverse mesic forest, and subalpine forest between 1,192 and 2,259 m (3,911 and 7,411 ft) [ECOS]. Reports of its past geographic distribution and elevational range differ somewhat depending upon the source. While Flora of the Hawaiian Islands and US Fish & Wildlife Service reports as late as 2015 [2015 5-Year Review] state that S. incompletum once grew on Kaua‘i, Moloka‘i, Lāna‘i, and Maui (but is now extinct on these Islands), later USFWS reports, starting in 2020 [2020 5-Year Review], state only that there are "historic occurrences" (i.e., recorded but now extinct) of the species on Lāna‘i, and Maui. Additionally, some sources report a lower (i.e., 600 m [1,969 ft]) elevational boundary [Wagner et al. 1990].

Solanum incompletum was listed as endangered in 1994. Around this time (conflicting reports of 1994 or 1996), two plants were (re-)discovered in the US Army's Pōhakuloa Training Area on Hawai‘i Island. Subsequent surveys increased this number to 83 in 2008 and 86 in 2015. Since 2008, State and Federal agencies have been engaged in numerous preservation and restoration activities, including: seed collection and storage (over 10,000 seeds in storage), propagation and outplanting (about 2,000 plantings with moderate survivorship), erecting fences to exclude ungulates, and reducing rodents, insect pests, and alien plants around wild and introduced plants [ECOS]. In their latest report [2025 5-Year Review], USFWS states that there are 106 wild S. incompletum on Hawai‘i Island with more than three times that number of planted (i.e., "reintroduced") individuals. Over the years, workers have also observed numerous (i.e.,100-plus) natural recruits.

Pōpolo kū mai produces clusters of small fuzzy white flowers. When pollinated, these mature into pea- to marble-sized berries that turn from green to orange when ripe. Within each fruit are a dozen or so flattened seeds. (Flower closeup, ripe fruits and seeds, and branch with leaves and unripe fruits photographs all courtesy of PTA Natural Resources Office staff via G.D. Carr ©.) While most pōpolo kū mai have prickles on their leaves, that's not always the case (Photograph courtesy of J.B. Friday ©.). A cultivated and flowering pōpolo kū mai. Unfortunately, this plant did not survive very long in the ground. Header photograph of leaf with prickles courtesy of Joel Lau ©.

Description

Solanum incompletum is an endangered small to medium-sized shrub potentially living longer than five years but probably less than ten. Both its stems and (usually) its leaves are covered with prominent red prickles. The leaves are slightly to deeply lobed and up to six inches (15 cm) long and typically covered with a yellow fuzz, particularly on their lower surface.

Pōpolo kū mai produces clusters of small (less than an inch (2.5 cm) across) perfect (bisexual) star-shaped flowers with fuzzy white petals. The flowers typically lack prickles. In the center of the flower is a ring of prominent yellow curved anthers and one two-lobed style. Cultivated plants flower sporadically, often with multiple periods of flowering throughout the year. Pollinated flowers develop into pea- to marble-sized berries that turn from green to yellowish orange when ripe. The ripe berries eventually harden and turn nearly black. Within each fruit are a dozen or so light-colored flattened seeds (see photograph above). Unfortunately, seeds often fail to germinate, either because of inviability or some type of prolonged dormancy.

Growth Requirements

General

Because of its rarity, I (Koebele) have only ever grown pōpolo kū mai in pots within two shadehouses. The few plants I've seen planted in the ground grew vigorously for several months, flowered profusely, and then quickly died with no apparent above-ground cause(s). I suspect (without proof) these plants were attacked and killed by some harmful soil pest (e.g., nematodes) or microbes. Therefore, unless you own an isolated hideaway in upland Maui or Hawai‘i Island, I recommend keeping your pōpolo kū mai in a container.

My pōpolo kū mai grew just fine under moderate (50%) shade, but the previously-mentioned in-the-ground plants did even better in a site that received full sunlight for all or most of the day. The media in my pots was nearly 100% black cinder with a little sphagnum peat moss mixed in. The in-the-ground plants were planted in clay-dominant soil that drained water reasonably well.

Wild pōpolo kū mai grow in fairly dry places. Therefore, do not overwater your plant. I found that my plants did best with infrequent watering such that the surface media remained dry to the touch most of the time.

In contrast to the above-mentioned in-the-ground plants, my containerized pōpolo kū mai grew rather slowly. In particular, they struggled (i.e., lost leaves and/or failed to grow) during the very hot summer months; both shadehouses were located in leeward O‘ahu at less than 100 ft (30 m) elevation. While my plants grew slowly, this didn't prevent them from flowering, even when they were quite small (i.e., less than a foot [30 cm] tall). Usually, they flowered once or twice during the cooler winter months. Since my pōpolo kū mai were in pots, I regularly fertilized them once every six months with a balanced controlled-release fertilizer with minor elements. However, as I mentioned, they still grew slowly despite the fertilization.

There is still a lot we don't know about the cultivation of this striking, unique, and beautiful shrub. And, unfortunately, because pōpolo kū mai is so rare, State agencies have yet to invite the public to help them discover its cultivation secrets. However, if you are one of the fortunate few to grow this species, and discover one of its secrets, we hope you will contact us so that we can improve upon the information shared here. You can contact us at: EickhoffandKoebele@gmail.com.

Pests and Diseases

Pōpolo kū mai is prone to infestations by several sap-sucking pests, particularly mealybugs (both above and below the media) and spider mites. Aphids, scale insects, thrips, and whiteflies can also become a problem. Kerin Lilleeng-Rosenberger [2005] has reported seeing pōpolo attacked by leaf miners, black twig borers, and root-knot nematodes. She has also seen mice and rats eat pōpolo seeds. Refer to the Pests & Diseases page for ways to deal with each of these pests.

Uses

Ancient Hawaiian

Solanum incompletum shares the Hawaiian name pōpolo with five other plant species in Hawai‘i: Cyanea solanacea, Phytolacca sandwicensis, Solanum americanum, S. nelsonii, and S. sandwicense [Flora of the Hawaiian Islands]. This makes it difficult to pair specific species to recorded Hawaiian uses. However, in our search, we were unable to find any Hawaiian uses directly attributed to Solanum incompletum. Rather, all the noted Hawaiian ethnobotanical experts and historians seem to attribute most Hawaiian uses of pōpolo to one species, Solanum americanum. These include: fruits being eaten [Handy & Handy 1972]; fruits used to make a blackish purple dye [Krauss 1993]; and several medical uses [Abbott 1992; Krauss 2001; Ka‘aiakamanu 2003].

Modern

Special Features and Information

General

Solanum incompletum is in the large Nightshade family (Solanaceae), which has about 103 genera and 2,700 species, with species on every continent except Antarctica. Many members of the family have a high alkaloid content in their tissues, which makes them toxic. Examples include: Apple of Sodom (Calotropis procera), mandrake (Mandragora officinarum), tobacco (Nicotiana spp.), and the extremely deadly belladonna (Atropa belladonna). However, others, such as tomatoes, potatoes, eggplants, and peppers, are common food plants. [WikipediA;Solanaceae].

Other native Hawaiian Solanaceae include: the indigenous ʻōhelo kai (Lycium carolinianum var. sandwicense); four species of ʻaiea (in the endemic genus Nothocestrum); and three or four species of Solanum [Flora of the Hawaiian Islands].

One hypothesis to explain the prickles on pōpolo kū mai is that they protected the plant from ancient, now extinct, herbivorous native Hawaiian forest birds. A few other native Hawaiian plants such as the endemic lobeliad, Cyanea horrida, have similar prickles on their leaves believed to serve the same purpose. For more on this hypothesis, see Chapter 15 of Ziegler [2002]. Unfortunately, these prickles offer little protection today from the more aggressive introduced mammalian herbivores like goats and cattle.

Hawaiian Name

Pōpolo kū mai can be translated to mean "the pōpolo that stands up" [Pukui & Elbert 1986], likely referring to the species growing to as much as ten feet (3 m) tall [Wagner et al. 1990]. Solanum incompletum shares the Hawaiian name pōpolo with five other plant species in Hawai‘i: Cyanea solanacea, Phytolacca sandwicensis, Solanum americanum, S. nelsonii, and S. sandwicense [Flora of the Hawaiian Islands]. Pōpolo is also the Hawaiian name used on Ni‘ihau for a type of surgeonfish (Acanthurus nigroris), also called maiko [Pukui & Elbert 1986].

Etymology

Solanum is of uncertain origin but is the Latin name for some plants in this genus, most likely S. nigrum, which was once used to treat epilepsy [Wagner et al. 1990]. The species name, incompletum, is Latin for "lacking parts" [Gledhill 2008]. It's unclear what lacking parts are being referred to in the name, although the reference may be to the lack of prickles on the species' flowers.

BPK