Native Plants of Hawai‘i - Carex wahuensis subsp. wahuensis

Carex wahuensis subsp. wahuensis

Cyperaceae or Sedge family

Synonyms: Carex nuptialis, Carex wahuensis, Carex wahuensis var. meyeri

No known ancient Hawaiian name

(see Hawaiian Name below)

Oʻahu sedge, Oʻahu carex

Natural Range & Environment

Carex wahuensis subsp. wahuensis is endemic to all the main Hawaiian Islands except Ni‘ihau and Kaho‘olawe [Wagner 1990], although it seems likely it once grew on these two islands as well. It can be found from near sea level to 1,000 m (3,281 ft) [Wagner 1990], often growing in rocky soil in dry and mesic shrublands and forests and infrequently in wetter forests in both sunny and shaded areas.

Ripe (yellow) and unripe fruits (Photograph courtesy of Forest & Kim Starr CC BY 3.0 US.). Several cultivated O‘ahu carex under shade. O‘ahu carex en masse in full sun at the entrance of a Maui high school (Photograph courtesy of Forest & Kim Starr CC BY 4.0). A new variegated form of O‘ahu carex cultivated by Dr. Orville Baldos and his lab at the University of Hawai‘i at Mānoa (Photographs courtesy of Orville Baldos ©.). (Header photograph of inflorescences containing many tiny male (nearest tip) and female flowers courtesy of G.D. Carr ©.)

Description

Carex wahuensis subsp. wahuensis is a grass-like sedge with a lifespan greater than five years. The glossy strap-like leaves can be as much as 40 inches (102 cm) long and are normally dark green. However, recently Dr. Orville Baldos, along with researchers in his lab at the University of Hawai‘i at Mānoa, have been cultivating a variegated form of the O‘ahu carex (see photographs above). Hopefully, we will see more of this beautiful cultivar in future landscapes. (Check out what Dr. Baldos and his colleagues have been doing with native Hawaiian plants at the Sustainable Ornamental Horticulture Lab. For more information on the variegated O‘ahu carex, check out this LICH link.) While carex's leaf margins are not serrated, its edges (like nearly all sedges) are still sharp and, therefore, one should be careful when handling this plant.

When mature, Carex wahuensis subsp. wahuensis begins producing inflorescences atop long spikes. The tiny male and female flowers are segregated, with male flowers at the tip of the inflorescence and female flowers below (see header photograph). The pollinated female flowers develop into small round green fruits (i.e., achenes) that turn light to dark yellow when ripe; the entire ripe inflorescence looks somewhat like a tiny ear of corn. In our experience, mature cultivated plants bloom and produce fruits nearly year-round.

Growth Requirements

General

Carex wahuensis subsp. wahuensis will grow both in sites with daylong full sunlight and sites with daylong shade; leaves will be greener in shadier sites. This makes it a particularly versatile landscape plant. Plant it as an accent, or en masse (about two feet [61 cm] apart) for ground cover, erosion control, or as a low informal hedge to define a walkway or space. It can also be kept as a mobile container plant. Cultivated (non-Hawaiian) Carex species have been used for years outside (and inside) Hawai‘i for landscaping; check these out online for other creative landscape ideas.

Once established in the ground, O‘ahu carex requires little water. However, to keep it looking its best, try to keep the surface soil dry and the subsurface (1 to 2 inches deep) soil moist. The soil can be lava/cinder, clay, or organic dominant and should drain water easily; we have not tried to grow this sedge in sand or coralline soils. O‘ahu carex can tolerate drought, heat, and strong winds, and is somewhat tolerant of salt spray. Actually, this sedge looks great in windy locations where its long leaves will flutter.

Carex wahuensis subsp. wahuensis in the ground generally do not need to be fertilized. For container plantings, we recommend a light application of a balanced controlled-release fertilizer with minor elements every six months or an occasional foliar feeding with a water-soluble inorganic or an organic fertilizer at ¼ to ⅓ the recommended strength. To keep your sedge looking tidy, trim off any dead leaves. We think the spiked inflorescences add to the beauty of this plant, but if you think otherwise, these too can be removed to maintain a more uniform appearance.

Pests and Diseases

Oʻahu carex is rarely attacked by pests, with scale insects on the leaves and mealybugs on the roots being the most common. Young plants are sometimes damaged by snails and slugs, but older plants seem to be more resistant. Refer to the Pests & Diseases page for treatments if you encounter these pests.

Uses

Ancient Hawaiian

None of the ten native Carex species has a (currently) known Hawaiian name. This is somewhat surprising given how common Carex wahuensis and some of the other Carex species are, and that many other native sedges have ancient Hawaiian names. A likely explanation is that the ancient name(s) were lost sometime shortly after 1778 during the period when so many Hawaiians died because of introduced diseases, taking with them their combined knowledge of Hawai‘i's native flora. Without a name, any attributable uses of Carex wahuensis would also lost.

Modern

Special Features and Information

General

The large Sedge family (Cyperaceae) has about 5,500 described species in approximately 90 genera [WikipediA;Cyperaceae]. Carex is the largest genus within Cyperaceae, with about 2,000 species worldwide growing in a diversity of habitats [Wagner et al. 1990]. Of the ten species of Carex native to Hawai‘i, five are endemic [Flora of the Hawaiian Islands].

In addition to subsp. wahuensis, Carex wahuensis has two other recognized subspecies:

Subspecies herbstii is today likely extinct. It is known from a single collection from Moanalua Valley in the Ko‘olau Mountains of Oʻahu at about 365 m (1,198 ft).
Subspecies rubiginosa is found primarily in shrublands and dry forests from 510 to 2,500 m (1,673 to 8,202 ft) on Maui and Hawaiʻi Island, but also grows at a few scattered mesic sites on Kauaʻi, Oʻahu, Molokaʻi, and Lānaʻi from about 240 to 600 m (787 to 1,969 ft) [Wagner et al 1990].

Hawaiian Name

None of the ten native Carex species has a (currently) known Hawaiian name. This is somewhat surprising given how common Carex wahuensis and some of the other Carex species are, and that many other native sedges have ancient Hawaiian names. A likely explanation is that the ancient name(s) were lost sometime shortly after 1778 during the period when so many Hawaiians died because of introduced diseases, taking with them their combined knowledge of Hawai‘i's native flora. Without a name, any attributable uses of Carex wahuensis would also lost. Within this website, we will use the common name, Oʻahu carex, for this species.

Etymology

Carex is the classical Latin name for sedge, possibly from carere, meaning "to be absent," referring to the upper portion of its floral spike, which is composed of all staminate (male) flowers and, therefore, doesn't produce seeds [Wagner et al. 1990]. The species and subspecies name, wahuensis, is an old spelling variation referring to the island of Oʻahu with the suffix -ensis added to Latinize the name [Gledhill 2008].

BPK