Native Plants of Hawai‘i - Solanum nelsonii

Solanum nelsonii

Solanaceae or Nightshade family [Five-minute families]

Synonyms: Solanum laysanense, Solanum nelsonii var. intermedium, Solanum nelsonii var. thomasiifolium

Pōpolo

‘Ākia (on Ni‘ihau)

Nelson's horsenettle

Natural Range & Environment

Solanum nelsonii is a rare and federally-listed endangered shrub endemic to the Hawaiian Islands. Historically, it was found on Kure Atoll (Mokupāpapa), Midway Atoll (Pihemanu), Pearl & Hermes Atoll (Holoikauaua), Laysan (Kauō), Nīhoa (however, see end of this section), Ni‘ihau, Kaua‘i, O‘ahu, Moloka‘i, Maui, and Hawai‘i Island. Today, according to the last USFWS 5-Year Review [2021]: less than ten wild S. nelsonii exist on Hawai‘i Island; less than 25 exist on Moloka‘i; less than 60 exist on Pearl and Hermes Atoll; 783 exist on Midway Atoll; about 130 exist on Kure Atoll; and over 90% of the total population (168,000 to 278,000) exist on Nīhoa. Solanum nelsonii grows in coral rubble or sand in coastal areas from near sea level to 490 feet (149 m) [USFWS;ECOS].

The wild populations of Solanum nelsonii have declined dramatically over the years, presumably due to multiple factors such as grazing by feral ungulates, seed predation by rats, herbivory by nonnative invertebrates, and habitat loss due to competition with invasive plants and wildfires. However, several agencies and organizations such as Lyon Arboretum, Maui Nui Botanical Gardens, Moloka‘i Land Trust, National Tropical Botanical Garden, Plant Extinction Prevention Program, State of Hawai‘i Department of Forestry and Wildlife, The Nature Conservancy of Hawai‘i, Waimea Arboretum, and US Fish & Wildlife Service have been active in restoration efforts (protection and plantings) and seed collection and storage at multiple sites on both the main and Northwestern Hawaiian Islands [USFWS;ECOS].

Recently (2020), the pōpolo found on Nīhoa, formerly considered a variety of Solanum nelsonii, was elevated to the level of species, and is now Solanum caumii. It grows in shallow rocky soil, from 10 to 270 m (33 to 886 ft) [Flora of the Hawaiian Islands].

Solanum nelsonii has small cup-shaped flowers that develop into black pea-sized fruits when ripe, with each containing a few dozen flat tomato-like seeds (Photograph of flower here and in header courtesy of Kyan Russell CC BY-NC 4.0; Photograph of split fruit with seeds courtesy of Forest & Kim Starr CC BY 3.0 US.). There are two primary forms of Solanum nelsonii: a prostrate form seen (or formerly seen) on most of the main Hawaiian Islands, and a more upright form found on some of the Northwestern Hawaiian Islands (Prostrate and upright pōpolo photographs courtesy of Forest & Kim Starr CC BY 3.0 US.). In 2020, the pōpolo found on Nīhoa, formerly considered a variety of Solanum nelsonii, was elevated to the level of species and is now Solanum caumii. One of its distinctive features is that some of the plants have red berries (Photograph courtesy of Danielle Frohlich CC BY-NC 4.0.). A containerized pōpolo (Moloka‘i form) being trained to climb a trellis, and a small one-year-old (but reproductive) pōpolo (MIdway Atoll form) being grown under LED lamps. (The tiny stick-like plant in front of the pōpolo is a Santalum ellipticum seedling, part of a host plant experiment by Koebele.

Description

Solanum nelsonii is yet another example of an endemic Hawaiian species that is morphologically variable depending upon its geography within the Hawaiian Archipelago. Taxonomists, recognizing this variability, have already attempted to divide the species into varieties, and, more recently (2020), have divided the "old" Solanum nelsonii into the "new" S. nelsonii and the "new" S. caumii. We suspect this taxonomic division and revision will continue given the morphological differences with S. nelsonii.

Solanum nelsonii is an endangered trailing or sprawling shrub with a lifespan exceeding five years. Its morphological diversity can best be examined from a geographic viewpoint. S. nelsonii growing on Hawai‘i Island and Moloka‘i (and formerly on most of the other main Hawaiian Islands) are prostrate viny shrubs with small round to oval fleshy light green leaves that are quite fuzzy because of their star-like (stellate) hairs (see photograph above). In contrast, S. nelsonii found on some of the Northwestern Hawaiian Islands (and S. caumii on Nīhau, Ni‘ihau, and formerly on Kaua‘i) are taller (but still less than 3 ft [1 m] tall), more "branchy" than viny, and typically have larger thinner and grayish or bluish green leaves that are also fuzzy. (Taxonomists have divided the S. nelsonii-caumii complex into four intergrading variants. Unfortunately, we have not seen all four types, and, therefore, can only properly describe the complex as three types.)

The perfect (bisexual) flowers of Solanum nelsonii are not as variable as its growth form or leaves, and are small, cup-shaped, with white (tinged with lavender) to pale or medium purple petals. The flower's anthers are S-shaped and purple or yellow or mixed color. (The S-shaped anthers are unusual for the genus, with some suggesting they are evidence of a different pollinator than the usual solitary bees [Wagner et al. 1990].) Wild plants have been seen flowering in August [USFWS;ECOS], while cultivated plants seem to have a more sporadic blooming period with multiple periods of flowering throughout the year.

Pollinated Solanum nelsonii flowers develop into pea- to marble-sized berries that turn from green to black (or red for S. caumii) when ripe. Within each fruit are a few dozen yellow flattened tomato-like seeds (see photographs above).

Growth Requirements

General

Think tomatoes. Anyone that's grown tomatoes knows it's not that difficult to "grow" tomatoes. However, they also know the hard task is keeping their tomatoes pest-free until harvest. Solanum nelsonii is similar — not too hard to grow, but a challenge to keep free of its many pests. Therefore, while it's possible to grow pōpolo in the ground, we recommend keeping your plant in a container where it will be much easier to avoid and combat any pests.

If possible, try to determine the origin of your pōpolo in order to determine its ultimate growth form. Will it become a viny prostrate shrub (typical of the main islands) or a more upright branching shrub (typical of the northwestern islands)? This will help you make better decisions on how to display your plant (e.g., hanging basket or wine-barrel planter).

Pōpolo does best in a site with full sunlight for all or most of the day, and will languish in constant shade. The media in your container should drain water easily. We like to use a mix of sphagnum peat moss and either sand or coral rubble; however, black cinders or perlite will also do the job. (We have also planted pōpolo in clay-dominant soils where it has grown to maturity; however, as stated above, we recommend not planting your pōpolo in the ground).

Pōpolo naturally grow in dry places. Therefore, do not overwater your plant. We find that pōpolo does best with infrequent watering, such that the surface media is dry to the touch most of the time.

Under the optimum conditions described above, pōpolo grows and matures quickly. Within a year, you can expect a two-foot (61 cm) diameter shrub beginning to flower. If you followed our advice and have your pōpolo in a container, you will want to fertilize it with either a balanced controlled-release fertilizer with minor elements every six months, or with an organic or inorganic water-soluble foliar fertilizer diluted to ⅓ to ½ the recommended strength.

In addition to drought, pōpolo can tolerate extreme heat, strong winds, and salt spray. Typically, pōpolo grow naturally into an aesthetic form, be it viny or branchy, and require little to no pruning. However, if you do prune, cut only a few branches at one time by hand.

Pests and Diseases

Pōpolo is prone to infestations by several sap-sucking pests, including: aphids, mealybugs (both above and below the media), spider mites, scale insects, thrips, and whiteflies. These are often farmed and protected by ants. Given the chance, rats and mice will occasionally eat pōpolo fruits or leaves. Kerin Lilleeng-Rosenberger [2005] has reported seeing pōpolo attacked by leaf miners, black twig borers, and root knot nematodes. Refer to the Pests & Diseases page for ways to deal with each of these pests.

Uses

Ancient Hawaiian

Solanum nelsonii shares the Hawaiian name pōpolo with five other plant species in Hawai‘i: Cyanea solanacea, Phytolacca sandwicensis, Solanum americanum, S. incompletum, 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 nelsonii. 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 nelsonii is in the large Nightshade family (Solanaceae), which has about 98 genera and 2,700 species, with species on every continent except Antarctica. Many members of the family have a high alkaloid content, 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].

Hawaiian Name

Pōpolo is also the Hawaiian name used on Ni‘ihau for a type of surgeonfish (Acanthurus nigroris), also called maiko [Pukui & Elbert 1986].

This species (now Solanum caumii) is referred to as ‘ākia on Ni‘ihau, a name it shares with some native Wikstroemia species [Wagner et al. 1990], perhaps because of its red berries.

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, nelsonii, honors David Nelson, a botanist who traveled with Captain Cook on his third voyage of discovery in 1779 [Evenhuis & Eldredge 2004].

For the new species, Solanum caumii, caumii honors Edward Leonard Caum (1893–1952), the American botanist who collected the type specimen [Flora of the Hawaiian Islands].

BPK